From: jrwalliker@gmail.com
On 17/01/2026 04:58, Bill Sloman wrote:
> On 17/01/2026 4:19 am, john larkin wrote:
>> On Sat, 17 Jan 2026 03:59:00 +1100, Bill Sloman
>> wrote:
>>
>>> On 16/01/2026 11:01 am, john larkin wrote:
>>>> 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.
>>>
>>> This misses the point. The Vishay resistor data showed that - at least
>>> for their surface mount thin film resistors - the heat doesn't get
>>> beyond the resistive track itself for about 300usec.
>>>
>>> If you get the track too hot for any time shorter than that it can melt
>>> (or at least get hot enough to let the atoms move around). For their
>>> resistors, nothing lower than 10k can take 1kV, which equates to a peak
>>> current of 100mA.
>>>
>>> Once you've work out how much resistive area you need to use to work
>>> with any pulse shorter than 300usec, you then need to work out the duty
>>> cycle of your short pulses and make sure that you can dissipate the
>>> average power to ambient without getting the average temperature too
>>> high.
>>
>> I don't expect to have much average power dissipation. The resistor on
>> my prototype is rising about 15c at 180 watts and 1 us/1KHz pulses,
>> according to my thermal imager. Do the math on that.
>
> You'd need to identify the resistor so that I could get the thermal
> resistance of the resistor to ambient before I could do that. If you
> knew what you were talking about you'd know that
>
>>> MELF resistors may have more surface area to dissipate heat over the
>>> long term but the peak short term power dissipation limit is strictly
>>> determined by what is happening in the resistive track itself.
>>
>> But a MELF can have a longer and wider resistance track compared to a
>> planar equivalent.
>
> But most resistors dissipate most of the heat they generate into the
> printed circuit board on which they are mounted. Planar surface mount
> resistors do that a lot better than MELFs.
Do you have any data for that? In both cases the substrate is alumina
which has a high thermal conductivity. MELFs have crimped on end caps
which probably provide good thermal conduction into the pcb pads.
If you are less worried about
> the average power dissipation and just want high peak dissipation, MELFs
> may look attractive, but that long track is likely to be an inductive
> helix, which may be less attractive in your application.
>
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