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I'm looking at the schematic for the HP 3048A option K23 DC blocking
filter. I've attached a simplified version of this schematic for
convenience. Note that I've omitted the circuitry related to reverse
polarity (switch and diode protection). But, I don't need that to
address my question.

The manual states that the 51.1ohm R1 acts as a fuse protecting the DC
blocking filter (C3-16) from a large input DC voltage (K23 is rated for
+/- 30 V). C3-16 are rated for 50 VDC. How does this fuse action work
and how can we ensure it's reliable?

The description of R1 in the manual is:

RESISTOR 51.1 1% .05W TF TC=0+-100

I believe TF stands for thin film. So, the power rating of the resistor
is pretty low. Maybe that gives a hint. There's no DC current path
through R1. But, the startup ramp of a DC voltage is AC. Broadly, it
seems like we need to identify the scenario(s) in which there is a
voltage across R1 creating a short-duration power dissipation that
greatly exceeds R1's power rating. Because there's no DC current path,
we need to rely on a large, short-duration excess of power rather than a
steady excess of power to blow R1.

Current can either flow through C1 or L1 out of R1. C1 will present a
low impedance, but only at high frequencies (|Z_R1|=|Z_C1| at
663MHz). L1 will slow the voltage rise across C3-16. Presumably, the
goal is for R1 to blow open before the voltage across C3-16 exceeds the
voltage rating. If I make the initial ramp rate very fast I can greatly
exceed the 1.6V R1 rating, but only for a very short time. How can I be
sure to destroy the resistor? And, if I destroy it, how can I be sure it
will blow open? The NASA reliability design handbook (MIL-HDBK-338B)
gives the failure mode distribution for film resistors as (p. 7-198):

| failure mode | probability |
| <l> | <c> |
|------------------+-------------|
| open | 0.59 |
| parameter change | 0.36 |
| short | 0.05 |

Wouldn't it be easy to bypass the protection and exceed the capacitor's
voltage rating by supplying a ramp that's just slow enough that C1 looks
like a high-impedance?

There's a limiter across the output, but no proper fuse to blow if one
of these diodes starts to conduct. It seems like the 11848 or 3561 input
is pretty exposed in this case.

Thoughts? Maybe R1 isn't just a normal resistor?

Thanks
Matt

Matt Huszagh <huszaghmatt@...> writes:

I'm looking at the schematic for the HP 3048A option K23 DC blocking
filter. I've attached a simplified version of this schematic for
convenience. Note that I've omitted the circuitry related to reverse
polarity (switch and diode protection). But, I don't need that to
address my question.

The manual states that the 51.1ohm R1 acts as a fuse protecting the DC
blocking filter (C3-16) from a large input DC voltage (K23 is rated for
+/- 30 V). C3-16 are rated for 50 VDC. How does this fuse action work
and how can we ensure it's reliable?

The description of R1 in the manual is:

RESISTOR 51.1 1% .05W TF TC=0+-100

I believe TF stands for thin film. So, the power rating of the resistor
is pretty low. Maybe that gives a hint. There's no DC current path
through R1. But, the startup ramp of a DC voltage is AC. Broadly, it
seems like we need to identify the scenario(s) in which there is a
voltage across R1 creating a short-duration power dissipation that
greatly exceeds R1's power rating. Because there's no DC current path,
we need to rely on a large, short-duration excess of power rather than a
steady excess of power to blow R1.

Current can either flow through C1 or L1 out of R1. C1 will present a
low impedance, but only at high frequencies (|Z_R1|=|Z_C1| at
663MHz). L1 will slow the voltage rise across C3-16. Presumably, the
goal is for R1 to blow open before the voltage across C3-16 exceeds the
voltage rating. If I make the initial ramp rate very fast I can greatly
exceed the 1.6V R1 rating, but only for a very short time. How can I be
sure to destroy the resistor? And, if I destroy it, how can I be sure it
will blow open? The NASA reliability design handbook (MIL-HDBK-338B)
gives the failure mode distribution for film resistors as (p. 7-198):

| failure mode | probability |
| <l> | <c> |
|------------------+-------------|
| open | 0.59 |
| parameter change | 0.36 |
| short | 0.05 |

Wouldn't it be easy to bypass the protection and exceed the capacitor's
voltage rating by supplying a ramp that's just slow enough that C1 looks
like a high-impedance?

There's a limiter across the output, but no proper fuse to blow if one
of these diodes starts to conduct. It seems like the 11848 or 3561 input
is pretty exposed in this case.

Thoughts? Maybe R1 isn't just a normal resistor?

I looked up the R1 part # in the Keysight part finder, which brings me
to . The description
there is: "Resistor-Fixed 51.1 Ohm +-1PCT 0.5W TC+-100 thin film
THT". So, it does look to be a normal resistor, not a fuse resistor, for
instance. Additionally, the part finder indicates that the 0.05 W power
rating in the manual may have been a typo, since this indicates 0.5
W. It seems a bit strange that this design would have relied on a
resistor for its fuse characteristics when there are other parts
designed specifically for this purpose.

Matt

Vishay sells fusible resistors. For example, the CMF fusible series:



This is metal film and comes in a half-watt rating. I expect what makes
this resistor fusible is that it's essentially guaranteed to fail open
and does so with predictable fusing times (see the fusing time plots in
the datasheet). I have no idea how the fusing times of this resistor
compare with those of the original part, but it seems like a good fit.

Matt

"Matt Huszagh via groups.io" <huszaghmatt@...> writes:

Matt, what is the HP part number for R1?

Vladan

"pianovt via groups.io" <pianovt@...> writes:

Matt, what is the HP part number for R1?

0757-0394.

I looked this up on the Keysight part detail a while back:



It gives the discription:

Resistor-Fixed 51.1 Ohm +-1PCT 0.5W TC+-100 thin film THT

and doesn't mention anything about the fusing nature of it. The
description also differs from the manual:

RESISTOR 51.1 1% .05W TF TC=0+-100

in terms of power rating. 0.05W seems low, so I'm inclined to believe
the part detail.

Matt

Part number 0757-0394 is a regular metal film RN50 resistor. It's rated for 0.05W MIL (125C). It is definitely not a 0.5W part.
I don't have a complete schematic for this circuit, so I can't give an opinion on how the protection works. I would keep the circuit as designed. Whoever designed it probably spent enough time making sure it works.

Vladan

"pianovt via groups.io" <pianovt@...> writes:

Part number 0757-0394 is a regular metal film RN50 resistor. It's rated for 0.05W MIL (125C). It is definitely not a 0.5W part.
I don't have a complete schematic for this circuit, so I can't give an opinion on how the protection works. I would keep the circuit as designed. Whoever designed it probably spent enough time making sure it works.

Thanks for looking that up for me Vladan. I'll stick with the
RN50. Vishay sells these too. FYI I've attached the schematic and an
equivalent schematic (the same excluding the switchable polarity), in
case you're interested.

Out of curiosity, do you have a better way to look up this sort of info?
My two methods are the keysight part detail and a text search of all my
documents (via recoll). This part number showed up in a few other
service manuals, including the 11848 and 8753, but it's listed in those
as a 1/8W part. This isn't the first time I've noticed service manuals
disagree about what exactly a part is.


Matt

I think the designer was worried about the inrush current rather than the capacitor voltage. You have 13 * 100uF in parallel, which will make the ESR of the combination extremely small. If someone applied 30V from a constant voltage source, there would be an inrush current so large as to burn up a trace on the board.

RN50 resistors are rated 50mW at 125C. For commercial applications, you can assume that this is equivalent to 125mW at 70C.

If in doubt, trust the service manual rather than the Keysight parts identification on the web.

Vladan

Physically small resistors can serve well as fuses at low voltages - as long as the R element properly disintegrates, it's OK. 30 VDC is borderline into arc-sustaining range, where the device length (between the terminals) needs to be enough to clear after the element is gone. AC is easier to handle since the arc is quenched near zero crossings.

There are all sorts of true "fusible" resistors for use at line voltage and such, that are designed to burn out properly, and are safety agency rated.

Ed

Not being familiar with this item I suspect that protection is paramount for the attached downstream equipment.? There are obviously protection diodes in the circuit.? One item overlooked in the “equivalent” circuit diagram is the presence of diode CR3 which gives a unidirectional DC path through the device depending upon the direction of switch S1.? Bidirectional clamping action for excessive voltages is performed by diodes CR1 & CR2.

?

Given that the description of this item is a “DC Blocking Filter” I doubt if CR3 is pulled into service under normal operation and would only be called on in the event a DC voltage were applied to the input.? If it is a somewhat standard diode then any voltage higher than the standard junction forward voltage would turn it on thereby passing the DC on to either CR1 or CR2 to conduct when the voltage level reaches their forward conducting point thereby blowing R1.

?

Use of a film resistor as a fuse is common in many pieces of equipment.? One more common usage is to meet FCC Part 68 requirements for terminal equipment connected to the public switched telephone network.? It is cheap and protects well.? If it is stated as having a 0.05W rating then I would believe it to be a good fast-acting device in this circuit.

Greg

The value (51.1 ohms) and power rating strongly implies it's a? 'sacrificial' device (ie: a fuse).
It's a common design trick used when protection is required for 'gross', but infrequent overload event(s).

Kit
VK1LL

On Friday 08 July 2022 01:06:46 pm Greg Muir via groups.io wrote:

Use of a film resistor as a fuse is common in many pieces of equipment.

Yup.

Several years ago I acquired a stereo receiver, a nice unit except that there was no output to the speakers. A bit of investigation showed that there were 1 ohm quarter watt resistors connected between the emitters of the output transistors and the speaker terminals. All of these on both left and right channels showed slight charring, and when measured proved to be open. Replacing them gave normal operation. It's given me good service ever since...

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