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Just about to finish my QMX+ build and I have a voltage question.? The kit was built for 12 volts.? I have a 100 amp/hour lithium iron battery for vhf base station emmcom backup.? At full charge, its just north of 14 volts.? Would it be wise to install a buck converter in the battery box to limit voltage out to 12 volts for the QMX+?? ? Thanks in advance - I really don't want to fry this little gem.
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Jim, W7EZN? ? 73!
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Yes you should limit the voltage to 12V. A buck converter or just simply put some diodes in series will do the job.
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73, de BD8BTE
op: Lun Yang
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YES!
Mandatory.
Roger
8P6RX
Get
Just about to finish my QMX+ build and I have a voltage question.? The kit was built for 12 volts.? I have a 100 amp/hour lithium iron battery for vhf base station emmcom backup.? At full charge, its just north of 14 volts.? Would it be wise to install a buck converter in the battery box to limit voltage out to 12 volts for the QMX+?? ? Thanks in advance - I really don't want to fry this little gem.
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Jim, W7EZN? ? 73!
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Thank you everyone for such prompt responses.? A buck converter is on its way!
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Jim, W7EZN? ? 73!
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I built my QMX+ for 9v, then installed an LM7809 inside so it will accept 10-30vdc input. That solves the over-voltage problem (that killed my first QMX) but doesn't allow for adjusting the power output as discussed here.
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In hindsight, I like the adjustable buck-boost idea to solve both problems.
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"I built my QMX+ for 9v, then installed an LM7809 inside so it will accept 10-30vdc input. That solves the over-voltage problem (that killed my first QMX) but doesn't allow for adjusting the power output as discussed here." ? It generates slight RF noize. Which is annoying.?
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Subject: [QRPLabs] QMX+ with 100 amp/hour lithium iron battery;
10.04.2025, 21:13, "Tom - AJ8X via groups.io" <tom.metty@...>:
I built my QMX+ for 9v, then installed an LM7809 inside so it will accept 10-30vdc input. That solves the over-voltage problem (that killed my first QMX) but doesn't allow for adjusting the power output as discussed here.
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In hindsight, I like the adjustable buck-boost idea to solve both problems.
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Hope you’ve got your fault protection sorted.?
Prospective short circuit will be 10kA and an auto fuse ain’t going to stop that arcing ..
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On 10/04/2025 18:28, Petrov Sergey via groups.io wrote: It generates slight RF noize. Which is annoying. Do you mean the 7809 is producing RF noise? If so, then suspect the decoupling capacitors. A 7809 should be noise-free. 73 Alan G4ZFQ
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I used 7812 and there was slight noise. I returned to one diod instead and everything is ok now. 09:32, 11 April 2025, "Alan G4ZFQ via groups.io" <alan4alan@...>:
On 10/04/2025 18:28, Petrov Sergey via groups.io wrote:
?It generates slight RF noize. Which is annoying.
Do you mean the 7809 is producing RF noise?
If so, then suspect the decoupling capacitors.
A 7809 should be noise-free.
73 Alan G4ZFQ
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Sent from Yandex?Mail for mobile
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The 78/79 hundred families of regulators expect bypassing on both input and output sides of the regulator.Look at the data sheet for recommended values. Typically, 3.3uf and 0.1 or 0.01uf on the output. Short lead lengths are also recommended. without these, stray inductance is going to cause the regulators to oscillate.
Rick K8BMA
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I am using a Drok buck converter from Amazon. DROK DC Buck Converter:
It works great, and no noise. I set it at 11.9 volts and just leave it there. I put anderson powerpole connectors on each side input and output: https://www.amazon.com/dp/B0CNZ1V5CL?ref=ppx_pop_mob_ap_share
I predict you will like this solution.
73
Braden Glett KD8ZM?
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On Fri, Apr 11, 2025 at 08:22 AM, Richard Barnich wrote:
The 78/79 hundred families of regulators expect bypassing on both input and output sides of the regulator.
I always parallel a 100- or 1000pF cap with the bulk bypass for additional RF bypassing/filtering.? Those larger values do not work effectively at higher frequencies.
73, Don N2VGU
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The suggestion on adding a low value (100-1000 pF) capacitor in parallel with the 20-1000 uF bypass capacitors if the best answer. It has to do with the frequency response of the capacitors and their ability to nibble away at the high frequency component.
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With linear regulators you need to have a few volts higher on the input when compared to the output so the regulator has something to regulate and drop voltage. Not too much because that voltage drop during regulation is converted to heat. For example; If you had a 18 volt input and a 12 volt regulated output at 2 amps that 6 volts of drop, multiplied by 2 amps would be 12 watts of (thermal) that would need a substantial heat sink to dissipate.
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The other way is bad too, a 12 volt regulator with a 12.5 volt supply. The 78xx series regulates best when the input voltage is > ~2.5V up to ~10V. Below that differential and the regulator cannot function correctly and you would be better off with just a diode string.
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Tisha Hayes
AA4HA
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Tisha,
I wanted to make sure I'm understanding what you are saying.
As stated earlier, I am using a Drok to regulate from a 12v LiFePo Bienno 3 Ah (actually 13v as I measure it), down to 11.9v. I hear you saying I might be losing more in heat from the Drok than the rig is putting out in power when transmitting (a lot more, in fact!). Yet, I don't feel any detectable heat coming from the Drok.
Is there something I've misunderstood?
TY 73
Braden Glett KD8ZM?
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IF you getting noise form 78xx parts its your fault.
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The input need minimally 1uf cap with short leads and the
output not less than .1uf.? ?The 78xx part are linear regulators
and if there is noise its because the bypassing is poor and
its oscillating.
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I've seen that in many cases, used wrong it does bad things.
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Allison
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Post online only,?
direct email will go to a bit bucket.
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Braden,
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Tisha is saying that linear regulators burn the difference between Vin and Vout as heat.
Your DROK is a switcher, not a linear regulators, that's a good solution if it doesn't create too much noise.
About half of the posts in this thread are about linear regulators.
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Tisha's advice on linear regulators is good.
1)? Linear regulators should have caps on input and output to keep them stable, or they may oscillate and cause noise.
2)? Also check the datasheet for the dropout voltage, some require that Vin be a couple volts greater than Vout.
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Some linear regulators have a dropout of less than half a volt.
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Don ND6T descripted an effectively zero dropout circuit he had seen in SPRAT:
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Jerry, KE7ER
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On Fri, Apr 11, 2025 at 04:02 PM, Braden Glett wrote:
Tisha,
I wanted to make sure I'm understanding what you are saying.
As stated earlier, I am using a Drok to regulate from a 12v LiFePo Bienno 3 Ah (actually 13v as I measure it), down to 11.9v. I hear you saying I might be losing more in heat from the Drok than the rig is putting out in power when transmitting (a lot more, in fact!). Yet, I don't feel any detectable heat coming from the Drok.
Is there something I've misunderstood?
TY 73
Braden Glett KD8ZM?
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On Thu, Apr 10, 2025 at 02:52 PM, Bruce Akhurst wrote:
Hope you’ve got your fault protection sorted.?
Prospective short circuit will be 10kA and an auto fuse ain’t going to stop that arcing ..
It can be as simple as a fusible link.? Go down 4 wire size numbers, so for example #16AWG link in #12AWG conductor, a foot long, connected in series with the main 12AWG line with compression crimp splices NOT solder.?
The link including splices and at least an inch beyond (I cover about 2-3 inches), is covered with fire sleeve.?
I use an 18 inch long sleeve for a 12 inch link; the idea is to contain and cool the hot gases if the link fuses so leave room.? Close each end of the sleeve with a zip tie or flat lacing tape.? Extra style points for two ties 1/2 inch apart.
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More folks should be aware of the advantages of lacing tape over zip ties in applications like this, especially where someone has cut the zip tie surplus with a diagonal cutter, leaving two flesh-shredding hooks exposed.? A tool like this will properly tension and cut the tie with a blunt end, save time and avoid much bloodshed: ?
Lacing tape:
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Of course an appropriate fuse is generally needed to protect against nuisance overloads as well; they are easier to replace than a link.
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Let's be safe out there.
73, Don N2VGU
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Jerry, thank you. That helps a lot. I live on a 20 acre horse ranch where there is absolutely no neighborhood RF noise, and even in that low-noise environment, I cannot detect any noise from the Drok. I can't take credit for choosing the Drok, someone on this forum suggested it as a good regulatory to use, and they were correct, apparently!
Thanks again and 73!
Braden Glett KD8ZM?
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On Sat, Apr 12, 2025 at 08:41 AM, Braden Glett wrote:
I live on a 20 acre horse ranch where there is absolutely no neighborhood RF noise, and even in that low-noise environment, I cannot detect any noise from the Drok.
I keep a supply of 100pF and 1000pF NP0 leaded capacitors handy.? It only takes a second to tack-solder a couple onto a board or slip them into terminal block connections to help with RF bypassing power supply outputs.
It is easy to add leads to chip caps, which is essentially what the epoxy-dipped caps are, if that's what you've got.
73, Don N2VGU
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Tom - AJ8X
Bruce Akhurst
Braden Glett
Donald S Brant Jr
Tisha Hayes, AA4HA