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QMX - smoke - another C107/Q108 failure

 

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That's true, but it wasn't really the question, as I recall it. The question was about what made the lower voltage segment on the LIN_REG_EN line. Since I don't know the microprocessor, I can't comment on what Hans could have done, only comment on what is observed!

73, Paul -- AI7JR

On 8/29/23 12:35, Chris wrote:

On 29/08/2023 20:20, Paul - AI7JR wrote:

Until the microprocessor comes up, LIN_REG_EN will be pulled up to +12 by R101. Once it's up and initialized, it will be fed by the microprocessor output from it's 3.3 volt supply.

Does that fit what's observed?

Yes, but typically modern processors have a variety of control methods that can be configured, one of which is an 'open collector' style where it only pulls the line low and leaves it to float high. Hans seems to have decided to pull it high.

Chris, G5CTH

Paul -- AI7JR

On 8/29/23 11:54, Chris wrote:
Nothing.? The Linear continues to produce 3.3V relative to it's 'GND' pin. if the supply produced by the SMPS is more then it provides nothing.? All this is confused by D102 andD103 which prevent the linear getting a voltage on its output which it can't cope with.

Chris, G5CTH

On 29/08/2023 19:40, Jonathan Burchmore wrote:
What is the mechanism that reduces the voltage on LIN_REG_IN during that half-way state from the supply voltage to 3.3V? ?It seems to me that the supply voltage passes through Q103 & Q105 (on the 5V board), then R101 on the 3v3 board and is directly connected from there to PD7 on the processor.

Jonathan KN6LFB



 

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What we observe is that the LIN_REG_EN line goes from floating high to being pulled to the 3.3V MCU high state which as far as the V_IN line is concerned is low.? Hans has done this in his setup code. The effect is to turn the SMPS on without turning the Linear off. No one other than Hans can have done this or can say why.

There are two logic Off levels here, for the FETs in the positive supply lines off corresponds to close to the positive line and on is a lower voltage while for the FETS connected to the GND line and the MCU logic low is 0V and logic high is +3.3 V.

In most places the transition between these is managed using additional FETs such as Q101 for LIN_REG_EN and the linear supply, Q106 for the PWM_5V signal and Q107 for the PWM_3V3 signal but in the case of Q111 and Q110 this doesn't happen, they can both be on at the same time.

Chris, G5CTH

On 29/08/2023 20:41, Paul - AI7JR wrote:

That's true, but it wasn't really the question, as I recall it. The question was about what made the lower voltage segment on the LIN_REG_EN line. Since I don't know the microprocessor, I can't comment on what Hans could have done, only comment on what is observed!

73, Paul -- AI7JR

On 8/29/23 12:35, Chris wrote:
On 29/08/2023 20:20, Paul - AI7JR wrote:

Until the microprocessor comes up, LIN_REG_EN will be pulled up to +12 by R101. Once it's up and initialized, it will be fed by the microprocessor output from it's 3.3 volt supply.

Does that fit what's observed?

Yes, but typically modern processors have a variety of control methods that can be configured, one of which is an 'open collector' style where it only pulls the line low and leaves it to float high. Hans seems to have decided to pull it high.

Chris, G5CTH

Paul -- AI7JR

On 8/29/23 11:54, Chris wrote:
Nothing.? The Linear continues to produce 3.3V relative to it's 'GND' pin. if the supply produced by the SMPS is more then it provides nothing.? All this is confused by D102 andD103 which prevent the linear getting a voltage on its output which it can't cope with.

Chris, G5CTH

On 29/08/2023 19:40, Jonathan Burchmore wrote:
What is the mechanism that reduces the voltage on LIN_REG_IN during that half-way state from the supply voltage to 3.3V? ?It seems to me that the supply voltage passes through Q103 & Q105 (on the 5V board), then R101 on the 3v3 board and is directly connected from there to PD7 on the processor.

Jonathan KN6LFB




 

Those are all good reasons the LTM8078 won't be the best choice for this particular application, I really just picked it by going on the Analog Devices web site and filtering for a dual-output regulator with integrated inductors. There are tons of parts out there, some more cost effective and/or practical than others. My basic point is that switching regulators are a problem that has been solved and packaged into off-the-shelf components by companies with more engineering resources than the entire amateur radio community combined. There's very little reason to roll your own solution unless you have very unique needs.

I don't think that "it's a single source part" is a strong argument in the context of a product that contains an ARM CPU, which I can personally attest to being frustratingly unobtainable during the semiconductor shortage. I also see an ADC from Texas Instruments and a DAC from Cirrus Logic, also not generic parts. At least the power supply boards are modular. If Analog Devices' foundry burned to the ground and they stopped manufacturing switching regulators overnight, it's not a huge deal to just spin up a different power supply board that occupies the same area and produces the same voltages, without needing to touch the design of the main PCB.

As far as BOM cost, if every dollar of BOM adds two dollars to retail price (this is my formula for things I build and sell), I'd have paid an extra $20 for a QMX with that extra $10 of BOM in the power supply... And spent several fewer hours troubleshooting my dead radio. That time is worth so much more than $20 to me. I actually ended up deciding it was more worthwhile to spend $100 on another kit than gamble on the possibility that the same event that killed my IC403 didn't also damage other parts in less obvious ways, forcing me to spend even more time (and shipping costs from the local Digikey warehouse) troubleshooting. Take my $20 instead, please.

>Of all the QMX I have yet seen, other than the Q103/Q104 Drain short (manufacturing problem) I have yet to see a failure that is not attributable to shorts, damaged components or other construction errors.

I do remember reading about at least one other person who killed their QMX by toggling their power supply between 6 and 12 volts, in order to toggle their transmit power. This is similar to my own scenario, where I forgot to raise the current limit on my power supply, which went into current limit when I keyed the radio into a dummy load, then came out of limit when I released the key, putting a similar voltage transient into the power supply input. Should I have been more diligent about making sure my current limit was set correctly? Yes. Is halving and doubling your input voltage a relatively extreme condition to subject a power supply to? Also yes. However, simply switching the thing on is a power supply transient from 0 to 12 volts, so a transient between 6 and 12 should be well within what it should be expected to tolerate. I don't think there is a single DC-powered device in my house that would be harmed by subjecting it to voltage transients within its specified input range. So I would have to disagree that this phenomenon should be categorized as a "constructor error."
 

The N-Channel FET Q101 will conduct ON if the Gate Threshold voltage is nominally 1.7V. That is with +12V, +9V, or +7.5V feeding the 78M33 regulator. This in turn causes Q102 to conduct and the 78M33 comes alive. This will also?happen if LIN_REG_EN is at the Q101 Gate Threshold voltage? .....a voltage that must be supplied by pin PD7 of the processor after it's been powered up with 3.3V and initialized. I don't know how happy PD7 is with a pull-up to 12V but at least the current is limited by R101. By definition this happens later and it looks like about t=80ms after pushing the ON button. So it looks like LIN_REG_EN from PD7 of the processor just keeps the 78M33 regulator ON until t=250ms causes the switch over to the SMPS. Since PD7 can now control LIN_REG-EN the processor controls the switch timing to the SMPS. I assume PD7 has it's own internal pullup to 3.3V ? ....or is Open Collector

One thing that you want to make sure of is that LIN_REG_EN actually goes below the Gate Threshold of Q101 or <<0.8V or it won't turn OFF.?No "floating" levels allowed, they drift. You won't see the result because the STMP 3.3V is slightly higher than the 78M33 3.3V and D103 is back biased.....but it can still supply current through the "dot OR".?

73 Kees K5BCQ
 

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I do remember reading about at least one other person who killed their QMX by toggling their power supply between 6 and 12 volts, in order to toggle their transmit power. This is similar to my own scenario, where I forgot to raise the current limit on my power supply, which went into current limit when I keyed the radio into a dummy load, then came out of limit when I released the key, putting a similar voltage transient into the power supply input. Should I have been more diligent about making sure my current limit was set correctly? Yes. Is halving and doubling your input voltage a relatively extreme condition to subject a power supply to? Also yes. However, simply switching the thing on is a power supply transient from 0 to 12 volts, so a transient between 6 and 12 should be well within what it should be expected to tolerate. I don't think there is a single DC-powered device in my house that would be harmed by subjecting it to voltage transients within its specified input range. So I would have to disagree that this phenomenon should be categorized as a "constructor error."

Which was exactly my second point.

I don't think there is a single DC-powered device in my house that would be harmed by subjecting it to voltage transients within its specified input range. So I would have to disagree that this phenomenon should be categorized as a "constructor error."

That you know of... And I didn't categorize it as construction error, either! :-) It's a condition *I* wouldn't have thought to address, honestly.

It could still be a problem for an off the self switching supply. Probably not a problem for a linear regulator, because there's no window of vulnerability but nowhere near as efficient with the power consumption.

I refuse to throw stones at this glass house!

Paul -- AI7JR

On 8/29/23 17:40, Stephan Ahonen KE0WVA wrote:

Those are all good reasons the LTM8078 won't be the best choice for this particular application, I really just picked it by going on the Analog Devices web site and filtering for a dual-output regulator with integrated inductors. There are tons of parts out there, some more cost effective and/or practical than others. My basic point is that switching regulators are a problem that has been solved and packaged into off-the-shelf components by companies with more engineering resources than the entire amateur radio community combined. There's very little reason to roll your own solution unless you have very unique needs.

I don't think that "it's a single source part" is a strong argument in the context of a product that contains an ARM CPU, which I can personally attest to being frustratingly unobtainable during the semiconductor shortage. I also see an ADC from Texas Instruments and a DAC from Cirrus Logic, also not generic parts. At least the power supply boards are modular. If Analog Devices' foundry burned to the ground and they stopped manufacturing switching regulators overnight, it's not a huge deal to just spin up a different power supply board that occupies the same area and produces the same voltages, without needing to touch the design of the main PCB.

As far as BOM cost, if every dollar of BOM adds two dollars to retail price (this is my formula for things I build and sell), I'd have paid an extra $20 for a QMX with that extra $10 of BOM in the power supply... And spent several fewer hours troubleshooting my dead radio. That time is worth so much more than $20 to me. I actually ended up deciding it was more worthwhile to spend $100 on another kit than gamble on the possibility that the same event that killed my IC403 didn't also damage other parts in less obvious ways, forcing me to spend even more time (and shipping costs from the local Digikey warehouse) troubleshooting. Take my $20 instead, please.

>Of all the QMX I have yet seen, other than the Q103/Q104 Drain short (manufacturing problem) I have yet to see a failure that is not attributable to shorts, damaged components or other construction errors.

I do remember reading about at least one other person who killed their QMX by toggling their power supply between 6 and 12 volts, in order to toggle their transmit power. This is similar to my own scenario, where I forgot to raise the current limit on my power supply, which went into current limit when I keyed the radio into a dummy load, then came out of limit when I released the key, putting a similar voltage transient into the power supply input. Should I have been more diligent about making sure my current limit was set correctly? Yes. Is halving and doubling your input voltage a relatively extreme condition to subject a power supply to? Also yes. However, simply switching the thing on is a power supply transient from 0 to 12 volts, so a transient between 6 and 12 should be well within what it should be expected to tolerate. I don't think there is a single DC-powered device in my house that would be harmed by subjecting it to voltage transients within its specified input range. So I would have to disagree that this phenomenon should be categorized as a "constructor error."
 

On Tue, Aug 29, 2023 at 02:24 PM, Jonathan Burchmore wrote:
I did a poor job of stating my question. ?I agree with your assessment of the voltage regulator output. ?My question was about the LIN_REG_EN signal. ?If you look at the trace below, channel 2 is LIN_REG_EN as measured on the 3v3 board connector, i.e. downstream of R101 and Q101. ?It is initially around 7.5V, then drops to 3.3V, then to 0 when the SMPS comes online. ?I'm trying to understand the mechanism that pulls LIN_REG_EN FROM ~7.5V to 3.3V. ?Or for that matter why it is initially at 7.5V instead of the supply voltage of 9V.



Secondly, since I was measuring the voltage right at the connector, it seems that the processor pin would also have been exposed........

Jonathan,

What you are seeing is the V_IN level when you first push ON or turn ON the Power Supply. This is goes through R101 to the gate of Q101 and LIN_REG_EN. You definitely exceed the 1.7V nominal Gate Threshold of Q101 and it turns on powering up the 78M33. After about 80ms of 3.3V the processor is initialized and comes alive. The processor now drives LIN_REG_EN .....also....with it's own 3.3V supply (probably has an internal pull-up to its 3.3V). This 3.3V LIN_REG_EN also exceeds the Gate Threshold of Q101 which keeps it ON and keeps the 78M33 going. When the processor decides it's time to enable the SMPS it drops LIN_REG_EN to 0V (or at least <<0.8V ......the Gate threshold voltage of Q101 is not met and it turns OFF turning OFF the 78M33 and turning ON the SMPS.

LIN_REG_EN has to STAY at that 0V level to keep the 78M33 turned OFF.?

I don't know where the 7.5V you mentioned came from. I also don't know how happy pin PD7 was about being connected to 12V or 9V and then being initialized.

73 Kees K5BCQ
 

I suspect that, as Chris alluded to, Hans has PD7 configured in push-pull rather than open collector, so it's sinking the (minimal) current to drop LIN_REG_EN down to 3.3V when the processor is driving it high.

I'm still confused why we don't see the full supply voltage on LIN_REG_EN before the processor boots up.

It does appear that LIN_REG_EN, once it is pulled down to 0V after the SMPS initialization, stays there.

Jonathan KN6LFB
 

Good morning folks and happy Wednesdays...?

I will put several replies in ONE post. One long post.?

Jonathan:?

> I suspect that, as Chris alluded to, Hans has PD7 configured?
> in push-pull rather than open collector, so it's sinking the?
> (minimal) current to drop LIN_REG_EN down to 3.3V when?
> the processor is driving it high.

Yes, correct. PD7 is configured push-pull during the boot up. Before it is activated push-pull and initially set to high (3.3V) the LIN_REG_EN signal is pulled high by R101 (100K).?

> I'm still confused why we don't see the full supply voltage?
> on LIN_REG_EN before the processor boots up.

LIN_REG_EN is connected directly to a processor GPIO pin (PD7) via a 100K resistor. This is a "5V tolerant" GPIO pin. Stuff goes on inside the pin. There is ESD protection circuitry, for one, which might be visualized simplistically?as a diode to the supply rail; however there being more to it since "the supply rail" in this case is a higher potential as it's a 5V tolerant pin. A similar thing occurs at pin PD4. I did measure the voltage there vs supply voltage. See attached.?

> It does appear that LIN_REG_EN, once it is pulled down?
> to 0V after the SMPS initialization, stays there.

Yes it does. Once the processor thinks everything is OK with the SMPS initialization, it sets LIN_REG_EN to low (ground). The signal stays low, in order to keep the linear regulator off, the 47-ohm "dummy load" R114 off, and the SMPS routed through to the 3.3V supply rail.?

Kees:

> I also don't know how happy pin PD7 was about being?
> connected to 12V or 9V and then being initialized.

One can debate that, in conjunction with a detailed study of the datasheet, an exercise which to me at least, is ambiguous and inconclusive. Or one can calculate the worst case current through that 100K resistor. Or one can observe the evidential reality which does indicate that the port pin PD7 (and indeed PD4) is not particularly unhappy about the experience.?

Chris:

> In most places the transition between these is managed?
> using additional FETs such as Q101 for LIN_REG_EN?
> and the linear supply, Q106 for the PWM_5V signal and?
> Q107 for the PWM_3V3 signal but in the case of Q111?
> and Q110 this doesn't happen, they can both be on at?
> the same time.

No, you have misunderstood the circuit. There is no scenario where the linear supply and SMPS can both be on at the same time.?

1) PD4 is not yet initialized "high", LIN_REG_EN is pulled towards Vin by 100K resistor R101. Then
a) Q101 is on, pulling the?gate of Q102 to ground, Q102 is ON and the 78M33 is powered.?
b) Q110 is on, putting the 47-ohm resistor R114 "dummy load" across the 3.3V buck converter output.
c) Q111 is off since its gate is higher than its source.?

2) PD4 is initialized "high", LIN_REG_EN is now set to 3.3V and the pin sinks some uA from R101.?
a) Q101 is on, pulling the?gate of Q102 to ground, Q102 is ON and the 78M33 is powered.?
b) Q110 is on, putting the 47-ohm resistor R114 "dummy load" across the 3.3V buck converter output.
c) Q111 is off since its gate is at 3.3V, which is higher than its source - the source is at 0V initially, rising to 3.3V as the SMPS is brought up.?

3) PD4 is set "low" when the processor thinks the SMPS is ready; LIN_REG_EN is now 0V.
a) Q101 is off; the gate of Q102 is pulled to Vin by 10K R102, so Q102 is OFF and the 78M33 is off.?
b) Q110 is off, the dummy load is therefore disconnected.?
c) Q111 is on, since its source is at 3.3V and its gate is at 0V, comfortably exceeding its Vgs.?

At no point can the SMPS and linear regulator both be active at the same time. Diode D103 prevents 3.3V trying to go back into the unpowered 78M33.?

Tony:

> The 1N4148 diode in series with the linear 3.3V output is a?
> necessary evil to prevent backward current flow once the?
> 3.3V SMPS rail comes up. However, it destabilizes the linear?
> 3.3V rail as noted by the variation in voltage as the?
> processor is executing its boot code.?

"Destabilizes" is probably a little strong. It doesn't destabilize anything. It permits some variation in the 3.3V rail, which is harmless and a small band of operating voltage in the context of the processor working fine right down to something like 1.7V. A diode doesn't have a 0.6V drop, it has a drop which depends on the current through it; in this case I believe as the processor current varies as it boots up, the drop is varying and that's why we see some small variation in Vdd during this time. But it's a narrow band and we are far from the minimum operating voltage of the processor.?

> It is advisable to have a substantial capacitance on the?
> processor’s power pins to minimize voltage. This should be?
> at least 47uF - 100uF to ensure that the processor doesn’t
> experience an instantaneous dip below its minimum spec?
> voltage which could cause the boot process to hang or?
> go off into never-never land.

When I make my one-off homebrew projects I often chuckle to myself, "you can never have too many capacitors or too much shielding". However - when you want to make a commercial product which has a very high performance to price ratio - you then consider how much shielding you really need, and how many capacitors you really need.?

I believe the capacitors in the QMX supply (2.2uF and 0.1uF) are consistent with the recommendations of the STM32F446 datasheet and the 78M33 datasheet. No higher capacitance is needed for stability or reliable operation. One could, if one was a follower of Muntz, probably take lots of capacitors out of QMX and it still work reliably and without performance deterioration.?

> What happens on one unit is not representative of 1000’s?
> of units so I hope the processor voltage during bootup is?
> being tested on every unit prior to shipment.?

No! There is absolutely NO need! Everything is fine. The design is not anywhere near marginal.?

Paul:

> The thing I found difficult was that both PS boards must be present?
> (and working) to bring up the microprocessor. (The 3.3 volt supply?
> is dependent on +12 from PCB1...) It would have been nice to be?
> able to bring up the 3.3 volt board, then the 5 volt.

There's really three things going on.?
  • Reverse polarity protection and soft power switch
  • 5V SMPS
  • 3.3V SMPS (and initial 3.3V linear regulator)
I chose to put them on two boards, just because it let me reduce my overall PCB size. 6-layer PCBs are expensive so doing this was a cost-saving, because otherwise when you snapped out a larger SMPS PCB with all the power stuff on it, it would waste a lot of unused space on the PCB panel. Right now all we waste is a little square with the QRP Labs logo printed on it, which I hope you all have on your keyrings!

So the way it worked out, there was best use of space having the soft power switch on the 5V board. The 5V supply doesn't have to work properly, for the 3.3V supply to work and the processor to boot up and allow you access to the diagnostics screen.

> In software, we used to call a unique, elegant, piece of code?
> a "Neat Hack", a sort of backhanded compliment to the coder.
> Hans, this unit is a Neat Hack in the realm of electronic (and?
> software) engineering. Lots of promise here-- You Go Guy! ;-)

Thanks :-)?

> Clearance between Q103/Q104, leading to solder bridges?
> and other challenges as people reworked the board. (I know?
> that was the root cause of?my?problems, anyway!)

Yes. Production problem. Easy to solve in future PCB revisions. And easy to fix now we know about it. I did another 50 boards last night, very quick and easy.?

> The impact of rapid changes (e.g., voltage spikes) in input
> voltage as power comes up on V_IN. Maybe the best fix?
> for this would be to wait until the input supply stabilizes for?
> a bit, before engaging the switching supplies?

I don't think there's a problem during power up. There has been a problem for a couple of people when there was a sudden large increase in voltage during normal operation. This is really the ONLY problem with the concept. But even this can be substantially mitigated by firmware, and is really quite unusual operating scenarios.?

Kees:

> That tells me that if you power ON again about 1-2 seconds?
> after powering OFF, the 5V VCC rail is still at about 1.7V.?
> Question is does the subsequent Power ON try to drive?
> VCC above 5V ?

No. When you power on, the firmware looks at the voltage it is seeing on the 5V ADC sensing input. If the voltage is above 5V it will not enable PWM. If the voltage is below 5V, but above zero, such as your 1.7V example, then the rail will just be gently brought up to 5V from there. No problem.?

Stephan:

> I don't think that "it's a single source part" is a strong?
> argument in the context of a product that contains an ARM?
> CPU, which I can personally attest to being frustratingly?
> unobtainable during the semiconductor shortage. I also?
> see an ADC from Texas Instruments and a DAC from?
> Cirrus Logic, also not generic parts.?

Yes, all that is correct... these are all non-generic parts, yes. And yes during the semiconductor shortage STM32 were hard to get (at no point, unobtainable, at least the things I needed). For me, the semiconductor shortage was a reason to TRY to avoid non-generic parts where possible. When finding STM32 and such was a fight, it seemed inadvisable to take on more unnecessary fights if possible. At the time when I did the design for the buck converter in the QDX PIN diode supply circuit, my first thought was certainly to look at SMPS ICs. However it was frustrating that so many were out of stock and unobtainable everywhere, and furthermore many are designed for higher current capability and don't work very efficiently at 50mA. So in the end I thought it would be fun and practical with discrete components!

> As far as BOM cost, if every dollar of BOM adds two dollars?
> to retail price (this is my formula for things I build and sell),?
> I'd have paid an extra $20 for a QMX with that extra $10?
> of BOM in the power supply... And spent several fewer?
> hours troubleshooting my dead radio. That time is worth?
> so much more than $20 to me.?

Yes, I understand; but your view is molded by your particular experience and hindsight in your specific case. Had you happened to have been in the fortunate majority who didn't suffer this type of failure then your view about increasing the kit price $20-$30 might be very different.?

To frivolously take it further - if someone made mistakes because the board is so compact, they might say they would have preferred a slightly larger radio and been happy to carry the extra grams and pay the increased materials costs on the PCB and enclosure. If someone had trouble with the connectors to the SMPS they could say they would prefer a larger board and it all on one board. If someone had trouble winding toroids, they could say they want a larger toroid in the kit and would be happy with the larger size and costs. Many people, if they had a problem, would have been happy paying more to have avoided it; many other people have other views about how the kit could have been a little different, to suit their own desires and circumstances better. In the end, it has to be acknowledged that you can never please all the people all the time, and have to try and strike a balance somewhere. Which is often very difficult indeed.?

> > Of all the QMX I have yet seen, other than the Q103/Q104 Drain?
> > short (manufacturing problem) I have yet to see a failure that is?
> > not attributable to shorts, damaged components or other >?
> > construction errors.
>
>?I do remember reading about at least one other person who?
> killed their QMX by toggling their power supply?
>?between 6 and 12 volts, in order to toggle their transmit?
> power. This is similar to my own scenario, where I?
>?forgot to raise the current limit on my power supply,?

Yes you are right, it is not only constructor?error. There have been a few cases such as this, that exposed a vulnerability. I think with my firmware changes I was able to very substantially mitigate that vulnerability but the risk is still there if extreme voltage fluctuations suddenly take place.?

> I don't think there is a single DC-powered device in my house
>? that would be harmed by subjecting it to voltage transients?
> within its specified input range. So I would have to disagree?
> that this phenomenon should be categorized as a?
> "constructor error."

Yes, I agree; these improbable scenarios are not constructor error.?

On the one hand, in my defence, there's always the question of how far one should go in order to protect the device. All the QRP Labs transceivers include reverse polarity protection. This means they are not harmed even by voltages OUTSIDE their specified input range. On one view, which has even been expressed in this group, you should be able to connect anything you like, to any port of a device, and it should not harm it. But to accomplish that would really increase circuit complexity and cost significantly. And one could argue that as radio hams, we are supposed to be somewhat technically competent, so we can perhaps be expected to take more care over not shorting the PTT output for example, or providing a reasonable SWR and antenna match. So, how far you go with protection is an interesting and difficult question.?

On the other hand, I am, and should be, always looking for ways to improve the designs... and we can see in other designs such as QDX, there were minor alterations like the addition of the AO3407 reverse polarity MOSFET, then when that turned out to be a little more delicate than its datasheet suggested, it was changed to AOD403. Etc. IF possible, it's always nice to try to make the transceivers more robust. Use of a SMPS IC would be one way. But maybe there are also other ways that can keep the current low cost, "neat hack", discretes version! Minor tweaks that could cost nothing or nearly nothing, but improve the robustness.?

Summary:
  1. All the 'scope screenshots in this thread, I consider normal.
  2. The 3.3V rail narrow band of voltage fluctuation during boot up is also, I consider, normal and far from any marginal operating scenario, there are no problems here.
  3. The capacitors choice and placement is I believe, compliant with the STM32 datasheet recommendations and 78M33 recommendations.
  4. The operation of the LIN_REG_EN signal (starting at 7.5V, then 3.3V, then Grounded) is also correct, and there are no scenarios where the linear and SMPS are switched on at the same time.?
73 Hans G0UPL
 

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Good morning Hans,

I was already impressed by your technical abilities, but this is another powerful example of communication and literary skills! ?Top!?
Hans Merz / Pa3AAW

Op 30 aug. 2023 om 08:13 heeft Hans Summers <hans.summers@...> het volgende geschreven:

?
Good morning folks and happy Wednesdays...?

I will put several replies in ONE post. One long post.?

Jonathan:?

> I suspect that, as Chris alluded to, Hans has PD7 configured?
> in push-pull rather than open collector, so it's sinking the?
> (minimal) current to drop LIN_REG_EN down to 3.3V when?
> the processor is driving it high.

Yes, correct. PD7 is configured push-pull during the boot up. Before it is activated push-pull and initially set to high (3.3V) the LIN_REG_EN signal is pulled high by R101 (100K).?

> I'm still confused why we don't see the full supply voltage?
> on LIN_REG_EN before the processor boots up.

LIN_REG_EN is connected directly to a processor GPIO pin (PD7) via a 100K resistor. This is a "5V tolerant" GPIO pin. Stuff goes on inside the pin. There is ESD protection circuitry, for one, which might be visualized simplistically?as a diode to the supply rail; however there being more to it since "the supply rail" in this case is a higher potential as it's a 5V tolerant pin. A similar thing occurs at pin PD4. I did measure the voltage there vs supply voltage. See attached.?

> It does appear that LIN_REG_EN, once it is pulled down?
> to 0V after the SMPS initialization, stays there.

Yes it does. Once the processor thinks everything is OK with the SMPS initialization, it sets LIN_REG_EN to low (ground). The signal stays low, in order to keep the linear regulator off, the 47-ohm "dummy load" R114 off, and the SMPS routed through to the 3.3V supply rail.?

Kees:

> I also don't know how happy pin PD7 was about being?
> connected to 12V or 9V and then being initialized.

One can debate that, in conjunction with a detailed study of the datasheet, an exercise which to me at least, is ambiguous and inconclusive. Or one can calculate the worst case current through that 100K resistor. Or one can observe the evidential reality which does indicate that the port pin PD7 (and indeed PD4) is not particularly unhappy about the experience.?

Chris:

> In most places the transition between these is managed?
> using additional FETs such as Q101 for LIN_REG_EN?
> and the linear supply, Q106 for the PWM_5V signal and?
> Q107 for the PWM_3V3 signal but in the case of Q111?
> and Q110 this doesn't happen, they can both be on at?
> the same time.

No, you have misunderstood the circuit. There is no scenario where the linear supply and SMPS can both be on at the same time.?

1) PD4 is not yet initialized "high", LIN_REG_EN is pulled towards Vin by 100K resistor R101. Then
a) Q101 is on, pulling the?gate of Q102 to ground, Q102 is ON and the 78M33 is powered.?
b) Q110 is on, putting the 47-ohm resistor R114 "dummy load" across the 3.3V buck converter output.
c) Q111 is off since its gate is higher than its source.?

2) PD4 is initialized "high", LIN_REG_EN is now set to 3.3V and the pin sinks some uA from R101.?
a) Q101 is on, pulling the?gate of Q102 to ground, Q102 is ON and the 78M33 is powered.?
b) Q110 is on, putting the 47-ohm resistor R114 "dummy load" across the 3.3V buck converter output.
c) Q111 is off since its gate is at 3.3V, which is higher than its source - the source is at 0V initially, rising to 3.3V as the SMPS is brought up.?

3) PD4 is set "low" when the processor thinks the SMPS is ready; LIN_REG_EN is now 0V.
a) Q101 is off; the gate of Q102 is pulled to Vin by 10K R102, so Q102 is OFF and the 78M33 is off.?
b) Q110 is off, the dummy load is therefore disconnected.?
c) Q111 is on, since its source is at 3.3V and its gate is at 0V, comfortably exceeding its Vgs.?

At no point can the SMPS and linear regulator both be active at the same time. Diode D103 prevents 3.3V trying to go back into the unpowered 78M33.?

Tony:

> The 1N4148 diode in series with the linear 3.3V output is a?
> necessary evil to prevent backward current flow once the?
> 3.3V SMPS rail comes up. However, it destabilizes the linear?
> 3.3V rail as noted by the variation in voltage as the?
> processor is executing its boot code.?

"Destabilizes" is probably a little strong. It doesn't destabilize anything. It permits some variation in the 3.3V rail, which is harmless and a small band of operating voltage in the context of the processor working fine right down to something like 1.7V. A diode doesn't have a 0.6V drop, it has a drop which depends on the current through it; in this case I believe as the processor current varies as it boots up, the drop is varying and that's why we see some small variation in Vdd during this time. But it's a narrow band and we are far from the minimum operating voltage of the processor.?

> It is advisable to have a substantial capacitance on the?
> processor’s power pins to minimize voltage. This should be?
> at least 47uF - 100uF to ensure that the processor doesn’t
> experience an instantaneous dip below its minimum spec?
> voltage which could cause the boot process to hang or?
> go off into never-never land.

When I make my one-off homebrew projects I often chuckle to myself, "you can never have too many capacitors or too much shielding". However - when you want to make a commercial product which has a very high performance to price ratio - you then consider how much shielding you really need, and how many capacitors you really need.?

I believe the capacitors in the QMX supply (2.2uF and 0.1uF) are consistent with the recommendations of the STM32F446 datasheet and the 78M33 datasheet. No higher capacitance is needed for stability or reliable operation. One could, if one was a follower of Muntz, probably take lots of capacitors out of QMX and it still work reliably and without performance deterioration.?

> What happens on one unit is not representative of 1000’s?
> of units so I hope the processor voltage during bootup is?
> being tested on every unit prior to shipment.?

No! There is absolutely NO need! Everything is fine. The design is not anywhere near marginal.?

Paul:

> The thing I found difficult was that both PS boards must be present?
> (and working) to bring up the microprocessor. (The 3.3 volt supply?
> is dependent on +12 from PCB1...) It would have been nice to be?
> able to bring up the 3.3 volt board, then the 5 volt.

There's really three things going on.?
  • Reverse polarity protection and soft power switch
  • 5V SMPS
  • 3.3V SMPS (and initial 3.3V linear regulator)
I chose to put them on two boards, just because it let me reduce my overall PCB size. 6-layer PCBs are expensive so doing this was a cost-saving, because otherwise when you snapped out a larger SMPS PCB with all the power stuff on it, it would waste a lot of unused space on the PCB panel. Right now all we waste is a little square with the QRP Labs logo printed on it, which I hope you all have on your keyrings!

So the way it worked out, there was best use of space having the soft power switch on the 5V board. The 5V supply doesn't have to work properly, for the 3.3V supply to work and the processor to boot up and allow you access to the diagnostics screen.

> In software, we used to call a unique, elegant, piece of code?
> a "Neat Hack", a sort of backhanded compliment to the coder.
> Hans, this unit is a Neat Hack in the realm of electronic (and?
> software) engineering. Lots of promise here-- You Go Guy! ;-)

Thanks :-)?

> Clearance between Q103/Q104, leading to solder bridges?
> and other challenges as people reworked the board. (I know?
> that was the root cause of?my?problems, anyway!)

Yes. Production problem. Easy to solve in future PCB revisions. And easy to fix now we know about it. I did another 50 boards last night, very quick and easy.?

> The impact of rapid changes (e.g., voltage spikes) in input
> voltage as power comes up on V_IN. Maybe the best fix?
> for this would be to wait until the input supply stabilizes for?
> a bit, before engaging the switching supplies?

I don't think there's a problem during power up. There has been a problem for a couple of people when there was a sudden large increase in voltage during normal operation. This is really the ONLY problem with the concept. But even this can be substantially mitigated by firmware, and is really quite unusual operating scenarios.?

Kees:

> That tells me that if you power ON again about 1-2 seconds?
> after powering OFF, the 5V VCC rail is still at about 1.7V.?
> Question is does the subsequent Power ON try to drive?
> VCC above 5V ?

No. When you power on, the firmware looks at the voltage it is seeing on the 5V ADC sensing input. If the voltage is above 5V it will not enable PWM. If the voltage is below 5V, but above zero, such as your 1.7V example, then the rail will just be gently brought up to 5V from there. No problem.?

Stephan:

> I don't think that "it's a single source part" is a strong?
> argument in the context of a product that contains an ARM?
> CPU, which I can personally attest to being frustratingly?
> unobtainable during the semiconductor shortage. I also?
> see an ADC from Texas Instruments and a DAC from?
> Cirrus Logic, also not generic parts.?

Yes, all that is correct... these are all non-generic parts, yes. And yes during the semiconductor shortage STM32 were hard to get (at no point, unobtainable, at least the things I needed). For me, the semiconductor shortage was a reason to TRY to avoid non-generic parts where possible. When finding STM32 and such was a fight, it seemed inadvisable to take on more unnecessary fights if possible. At the time when I did the design for the buck converter in the QDX PIN diode supply circuit, my first thought was certainly to look at SMPS ICs. However it was frustrating that so many were out of stock and unobtainable everywhere, and furthermore many are designed for higher current capability and don't work very efficiently at 50mA. So in the end I thought it would be fun and practical with discrete components!

> As far as BOM cost, if every dollar of BOM adds two dollars?
> to retail price (this is my formula for things I build and sell),?
> I'd have paid an extra $20 for a QMX with that extra $10?
> of BOM in the power supply... And spent several fewer?
> hours troubleshooting my dead radio. That time is worth?
> so much more than $20 to me.?

Yes, I understand; but your view is molded by your particular experience and hindsight in your specific case. Had you happened to have been in the fortunate majority who didn't suffer this type of failure then your view about increasing the kit price $20-$30 might be very different.?

To frivolously take it further - if someone made mistakes because the board is so compact, they might say they would have preferred a slightly larger radio and been happy to carry the extra grams and pay the increased materials costs on the PCB and enclosure. If someone had trouble with the connectors to the SMPS they could say they would prefer a larger board and it all on one board. If someone had trouble winding toroids, they could say they want a larger toroid in the kit and would be happy with the larger size and costs. Many people, if they had a problem, would have been happy paying more to have avoided it; many other people have other views about how the kit could have been a little different, to suit their own desires and circumstances better. In the end, it has to be acknowledged that you can never please all the people all the time, and have to try and strike a balance somewhere. Which is often very difficult indeed.?

> > Of all the QMX I have yet seen, other than the Q103/Q104 Drain?
> > short (manufacturing problem) I have yet to see a failure that is?
> > not attributable to shorts, damaged components or other >?
> > construction errors.
>
>?I do remember reading about at least one other person who?
> killed their QMX by toggling their power supply?
>?between 6 and 12 volts, in order to toggle their transmit?
> power. This is similar to my own scenario, where I?
>?forgot to raise the current limit on my power supply,?

Yes you are right, it is not only constructor?error. There have been a few cases such as this, that exposed a vulnerability. I think with my firmware changes I was able to very substantially mitigate that vulnerability but the risk is still there if extreme voltage fluctuations suddenly take place.?

> I don't think there is a single DC-powered device in my house
>? that would be harmed by subjecting it to voltage transients?
> within its specified input range. So I would have to disagree?
> that this phenomenon should be categorized as a?
> "constructor error."

Yes, I agree; these improbable scenarios are not constructor error.?

On the one hand, in my defence, there's always the question of how far one should go in order to protect the device. All the QRP Labs transceivers include reverse polarity protection. This means they are not harmed even by voltages OUTSIDE their specified input range. On one view, which has even been expressed in this group, you should be able to connect anything you like, to any port of a device, and it should not harm it. But to accomplish that would really increase circuit complexity and cost significantly. And one could argue that as radio hams, we are supposed to be somewhat technically competent, so we can perhaps be expected to take more care over not shorting the PTT output for example, or providing a reasonable SWR and antenna match. So, how far you go with protection is an interesting and difficult question.?

On the other hand, I am, and should be, always looking for ways to improve the designs... and we can see in other designs such as QDX, there were minor alterations like the addition of the AO3407 reverse polarity MOSFET, then when that turned out to be a little more delicate than its datasheet suggested, it was changed to AOD403. Etc. IF possible, it's always nice to try to make the transceivers more robust. Use of a SMPS IC would be one way. But maybe there are also other ways that can keep the current low cost, "neat hack", discretes version! Minor tweaks that could cost nothing or nearly nothing, but improve the robustness.?

Summary:
  1. All the 'scope screenshots in this thread, I consider normal.
  2. The 3.3V rail narrow band of voltage fluctuation during boot up is also, I consider, normal and far from any marginal operating scenario, there are no problems here.
  3. The capacitors choice and placement is I believe, compliant with the STM32 datasheet recommendations and 78M33 recommendations.
  4. The operation of the LIN_REG_EN signal (starting at 7.5V, then 3.3V, then Grounded) is also correct, and there are no scenarios where the linear and SMPS are switched on at the same time.?
73 Hans G0UPL

<Port Pin PB4 vs Supply voltage.png>
 

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On 30/08/2023 04:40, Jonathan Burchmore wrote:
I suspect that, as Chris alluded to, Hans has PD7 configured in push-pull rather than open collector, so it's sinking the (minimal) current to drop LIN_REG_EN down to 3.3V when the processor is driving it high.

I'm still confused why we don't see the full supply voltage on LIN_REG_EN before the processor boots up.

LIN_REG_EN is connected to a MCU pin through a 100K resistor. It's quite possible the MCU is taking some current even when inactive, this will lower LIN_REG_EN relative to the supply.? I'm sure that the spec for the MCU allows this sort of thing, subject to a maximum current.

Chris, G5CTH


It does appear that LIN_REG_EN, once it is pulled down to 0V after the SMPS initialization, stays there.

Jonathan KN6LFB


 

Hi Stephen. I was probably a bit harsh in responding to you. Your idea is a good one, and there might be a great part out there that reduces parts count at low cost and is repairable by the average purchaser. A second source would be a big plus. That would be a winner.? I have been bitten every which way in the points I brought up in my previous design life. A single source component is my biggest issue.? Here's an example I experienced.? Many aerospace companies build custom processor boards using industry standard chips.? A new high performance/low power Power PC processor chip had been developed by a a particular company, who had been in business for many years.? All the aerospace companies started designing processor boards around that chip.? When the boards designs were just about complete by many companies, In a surprise move, Apple Computer bought out the entire design team to make processor chips for Apple products!? The chip's availability became unobtanium. All the aerospace companies were left holding the bag.? This caused massive amounts of wasted development costs, destroyed schedules, and beginning all over in these design efforts.

In the aerospace industry, things had to work reliably, and cost was not a big issue, within reason.? We used BGAs all the time but sometimes had sourcing issues and BGAs had to be removed from defective boards to use on new boards.? This was an expensive process, as it required re-balling the BGA with new solder balls before it could be re-used on a new board.? A few companies do this. BGAs are out of the question in the QMX type application unless you are willing to scrap the board when the BGAs fail. Even so, they would have prohibitive manufacturing costs as this is a relatively low volume endeavor.

Best of luck in your search.
-Steve K1RF?


------ Original Message ------
From "Stephan Ahonen KE0WVA" <stephan.ahonen@...>
Date 8/29/2023 8:40:41 PM
Subject Re: [QRPLabs] QMX - smoke - another C107/Q108 failure

Those are all good reasons the LTM8078 won't be the best choice for this particular application, I really just picked it by going on the Analog Devices web site and filtering for a dual-output regulator with integrated inductors. There are tons of parts out there, some more cost effective and/or practical than others. My basic point is that switching regulators are a problem that has been solved and packaged into off-the-shelf components by companies with more engineering resources than the entire amateur radio community combined. There's very little reason to roll your own solution unless you have very unique needs.

I don't think that "it's a single source part" is a strong argument in the context of a product that contains an ARM CPU, which I can personally attest to being frustratingly unobtainable during the semiconductor shortage. I also see an ADC from Texas Instruments and a DAC from Cirrus Logic, also not generic parts. At least the power supply boards are modular. If Analog Devices' foundry burned to the ground and they stopped manufacturing switching regulators overnight, it's not a huge deal to just spin up a different power supply board that occupies the same area and produces the same voltages, without needing to touch the design of the main PCB.

As far as BOM cost, if every dollar of BOM adds two dollars to retail price (this is my formula for things I build and sell), I'd have paid an extra $20 for a QMX with that extra $10 of BOM in the power supply... And spent several fewer hours troubleshooting my dead radio. That time is worth so much more than $20 to me. I actually ended up deciding it was more worthwhile to spend $100 on another kit than gamble on the possibility that the same event that killed my IC403 didn't also damage other parts in less obvious ways, forcing me to spend even more time (and shipping costs from the local Digikey warehouse) troubleshooting. Take my $20 instead, please.

>Of all the QMX I have yet seen, other than the Q103/Q104 Drain short (manufacturing problem) I have yet to see a failure that is not attributable to shorts, damaged components or other construction errors.

I do remember reading about at least one other person who killed their QMX by toggling their power supply between 6 and 12 volts, in order to toggle their transmit power. This is similar to my own scenario, where I forgot to raise the current limit on my power supply, which went into current limit when I keyed the radio into a dummy load, then came out of limit when I released the key, putting a similar voltage transient into the power supply input. Should I have been more diligent about making sure my current limit was set correctly? Yes. Is halving and doubling your input voltage a relatively extreme condition to subject a power supply to? Also yes. However, simply switching the thing on is a power supply transient from 0 to 12 volts, so a transient between 6 and 12 should be well within what it should be expected to tolerate. I don't think there is a single DC-powered device in my house that would be harmed by subjecting it to voltage transients within its specified input range. So I would have to disagree that this phenomenon should be categorized as a "constructor error."
 

Thank you, Hans, for your detailed reply.

For my own part, I've been reassured as I measured my 9V QMX build. ?I posted the scope screenshots in the hope of sharing that reassurance with others and to deepen my understanding of the QMX's design. ?I've definitely been successful in the latter.

Jonathan KN6LFB
 

Hans,

What a complete detailed response and education for everyone. I am impressed with your bandwidth and expertise relative to this wonderful QMX product.

73 Kees K5BCQ
 

Hi Kees
?
What a complete detailed response and education for everyone. I am impressed with your bandwidth and expertise relative to this wonderful QMX product.

Hi hi, bandwidth and expertise? You should ask my family, how often I showered, shaved, even SPOKE to them in the last few months! It's by far the biggest project I've ever done. So yeah I know every resistor and every line of code! I could probably write it all out from my head with very few errors!

73 Hans G0UPL
 

>In the end, it has to be acknowledged that you can never please all the people all the time, and have to try and strike a balance somewhere. Which is often very difficult indeed.

I suppose you know your target market better than I do, and it's presumptuous of me to think otherwise, I apologize.

>I think with my firmware changes I was able to very substantially mitigate that vulnerability but the risk is still there if extreme voltage fluctuations suddenly take place.

I think I remember reading somewhere that the control loop operates at a frequency of 1 khz?

If this is the case then the software physically cannot respond quickly enough. According to SPICE, if input voltage steps from 6 to 12 volts, with a 330u series inductor and 470u output capacitor, 1 ms is enough time for the output voltage of the 5v regulator to rise from 5v to 11v before the CPU even has an opportunity to figure out that something is wrong.

to the power supply circuit would help. At a cost of three extra parts (including a series resistor on the PWM_5V line, not pictured), a voltage excursion now immediately shuts down PWM, much faster than the CPU can react. Total BOM cost of $.10 or so?
 

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I'm curious about what you propose, but that link doesn't show anything. :-(

Paul -- AI7JR

On 8/30/23 18:17, Stephan Ahonen KE0WVA wrote:

>In the end, it has to be acknowledged that you can never please all the people all the time, and have to try and strike a balance somewhere. Which is often very difficult indeed.

I suppose you know your target market better than I do, and it's presumptuous of me to think otherwise, I apologize.

>I think with my firmware changes I was able to very substantially mitigate that vulnerability but the risk is still there if extreme voltage fluctuations suddenly take place.

I think I remember reading somewhere that the control loop operates at a frequency of 1 khz?

If this is the case then the software physically cannot respond quickly enough. According to SPICE, if input voltage steps from 6 to 12 volts, with a 330u series inductor and 470u output capacitor, 1 ms is enough time for the output voltage of the 5v regulator to rise from 5v to 11v before the CPU even has an opportunity to figure out that something is wrong.

to the power supply circuit would help. At a cost of three extra parts (including a series resistor on the PWM_5V line, not pictured), a voltage excursion now immediately shuts down PWM, much faster than the CPU can react. Total BOM cost of $.10 or so?

 

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In this line of emails does it mean one should not turn the power ON and OFF in rapid succession or even semi quickly because it takes a few extra milliseconds for the power supply controls to do their job? ?IE turn on, let it run a few seconds before turning off….. about the time needed to smell or see the magic smoke. ?

Dave K8WPE?

David J. Wilcox’s iPad

On Aug 30, 2023, at 10:33 AM, Hans Summers <hans.summers@...> wrote:

?
Hi Kees
?
What a complete detailed response and education for everyone. I am impressed with your bandwidth and expertise relative to this wonderful QMX product.

Hi hi, bandwidth and expertise? You should ask my family, how often I showered, shaved, even SPOKE to them in the last few months! It's by far the biggest project I've ever done. So yeah I know every resistor and every line of code! I could probably write it all out from my head with very few errors!

73 Hans G0UPL
 

Hello Dave

I don't think that is necessary.?

With a lot of effort, messing around with a poor intermittent 12V power supply connection, while simultaneously holding the left knob (power on/off) pressed, I was able to kill the 5.6V zener. But I don't think it is a major risk. You really had to try hard, to do it.

73 Hans G0UPL



On Thu, Aug 31, 2023 at 12:43?PM David Wilcox K8WPE via <Djwilcox01=[email protected]> wrote:
In this line of emails does it mean one should not turn the power ON and OFF in rapid succession or even semi quickly because it takes a few extra milliseconds for the power supply controls to do their job?? IE turn on, let it run a few seconds before turning off….. about the time needed to smell or see the magic smoke. ?

Dave K8WPE?

David J. Wilcox’s iPad

On Aug 30, 2023, at 10:33 AM, Hans Summers <hans.summers@...> wrote:

?
Hi Kees
?
What a complete detailed response and education for everyone. I am impressed with your bandwidth and expertise relative to this wonderful QMX product.

Hi hi, bandwidth and expertise? You should ask my family, how often I showered, shaved, even SPOKE to them in the last few months! It's by far the biggest project I've ever done. So yeah I know every resistor and every line of code! I could probably write it all out from my head with very few errors!

73 Hans G0UPL
 

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Thanks Hans,

A friend of mine recovering from two strokes built the QMX before joining this group (showing all the issues) and it worked to his satisfaction. He is sending it to me as a gift as he doesn’t like digital modes. ?I have the kit I purchased from you (unbuilt yet) at Xenia and was just wondering…. I don’t turn things on and off quickly as that will cause more problems with computers than it’s worth but just had to ask.

Dave K8WPE since 1960

David J. Wilcox’s iPad

On Aug 31, 2023, at 5:49 AM, Hans Summers <hans.summers@...> wrote:

?
Hello Dave

I don't think that is necessary.?

With a lot of effort, messing around with a poor intermittent 12V power supply connection, while simultaneously holding the left knob (power on/off) pressed, I was able to kill the 5.6V zener. But I don't think it is a major risk. You really had to try hard, to do it.

73 Hans G0UPL



On Thu, Aug 31, 2023 at 12:43?PM David Wilcox K8WPE via <Djwilcox01=[email protected]> wrote:
In this line of emails does it mean one should not turn the power ON and OFF in rapid succession or even semi quickly because it takes a few extra milliseconds for the power supply controls to do their job?? IE turn on, let it run a few seconds before turning off….. about the time needed to smell or see the magic smoke. ?

Dave K8WPE?

David J. Wilcox’s iPad

On Aug 30, 2023, at 10:33 AM, Hans Summers <hans.summers@...> wrote:

?
Hi Kees
?
What a complete detailed response and education for everyone. I am impressed with your bandwidth and expertise relative to this wonderful QMX product.

Hi hi, bandwidth and expertise? You should ask my family, how often I showered, shaved, even SPOKE to them in the last few months! It's by far the biggest project I've ever done. So yeah I know every resistor and every line of code! I could probably write it all out from my head with very few errors!

73 Hans G0UPL
 

On Wed, Aug 30, 2023 at 10:09 PM, Paul - AI7JR wrote:

I'm curious about what you propose, but that link doesn't show anything. :-(

Damn. I'm on my phone right now and about to get on a flight so this will have to do -

* Change value of D108 to 5V
* Disconnect Anode of D108 from ground, instead connect to the base of an NPN transistor
* Connect NPN emitter to ground, connect collector to PWM_5V
* Add a resistor from NPN base to emitter (value tbd, likely 1k)
* Add a resistor in series from PWM_5V to to microcontroller (also value tbd, likely 1k or less)
* Repeat for 3V3 rail

Now, whenever the 5V rail exceeds 5.6 volts (zener voltage + NPN Vbe), the NPN turns on, pulling PWM_5V low until the fault clears.

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