From: bill.sloman@ieee.org
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. 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.
--
Bill Sloman, Sydney
--- SoupGate-Win32 v1.05
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