From: jl@glen--canyon.com
On Thu, 15 Jan 2026 23:01:38 +0000, John R Walliker
wrote:
>On 15/01/2026 18:15, john larkin wrote:
>> On Thu, 15 Jan 2026 17:51:59 +0000, liz@poppyrecords.invalid.invalid
>> (Liz Tuddenham) wrote:
>>
>>> john larkin wrote:
>>>
>>>> On Thu, 15 Jan 2026 15:18:31 +0000, liz@poppyrecords.invalid.invalid
>>>> (Liz Tuddenham) wrote:
>>>>
>>>>> john larkin wrote:
>>>>>
>>>>>> I need something like 1.5K resistance across a 750 volt pulse. Pulse
>>>>>> widths will be below 1 us.
>>>>>>
>>>>>> Three 1206's in series, 499r each, would work. Peak power dissipation
>>>>>> per resistor will be 125 watts at 250 volts. I think that's OK but I
>>>>>> want to test it.
>>>>>>
>>>>>> Here's the tester. The DUT (device under torture) will go across the
>>>>>> gap on the left.
>>>>> f
>>>>>> I have both regular thickfilm resistors and some thinfilms to test. I
>>>>>> theorize that the thinfilms will hold up better.
>>>>>
>>>>> Would a non-inductively-wound wirewound resistor work well enough? You
>>>>> would have plenty of mass to average-out the pulse energy.
>>>>
>>>> WWs are great for pulse overload, not so great for PCB density. The
>>>> best would be to use three (or two, or one) surface-mount 1206
>>>> thickfilm that we have in stock.
>>>>
>>>> I could stand a micohenry or so parasitic inductance. The 1.5K will in
>>>> fact be in series with a small inductor.
>>>
>>> There's your answer; make the resistor and the inductor one and the same
>>> component. For a small investment in suitable machinery this gives you
>>> total security of supply, quality control and an edge over any
>>> competitor who can't make things but just buys them in (or tries to copy
>>> your design without realising what that component really does).
>>>
>>> Vertical integration was the cornerstone of nearly all the successful
>>> electronics firms. (Philips even owned the sand quarries to supply the
>>> sand to make the glass to make the valves and light bulbs.)
>>>
>>> Experiment with winding a number of turns of resistance wire on a former
>>> in one direction, then winding some more in the opposite direction. The
>>> ratio between the two sets of turns can be adjusted to give the required
>>> inductance and the total number of turns gives the resistance. The
>>> former could be a small piece of heatproof material shaped like a dog's
>>> bone to retain the wire, with a notch to catch the wire and prevent it
>>>from unwinding at the reversal point.
>>
>> Yikes. That would be a huge diversion from getting a product done.
>>
>> I found one paper that shows that thinfilms are tougher than
>> thickfilms, but thinfilm MELFs are even better. That makes sense.
>>
>
>When I visited the factory of a smart meter manufacturer I noticed that
>they used melf surface mount resistors for mains voltage sensing. There
>were several in series.
>John
Makes sense. For a given pcb footprint, they have about pi times the
surface area to work with, for a correspondingly bigger conductor
area. The cooling might be even better.
That would be interesting, to find some papers somewhere that discuss
the benefits of melfs vs regular surfmount parts.
Menawhile, I'm seeing unexpected goodies blasting a cheap thickfilm
1206. So we can probably use 1206 thickfilms, and go to thinfilms or
even thinfilm melfs on the same PCB if we have any problems.
I don't think I have any melf resistors here. It would be interesting
to break one in half and measure the ceramics, re thermal
conductivity.
John Larkin
Highland Tech Glen Canyon Design Center
Lunatic Fringe Electronics
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