From: jl@glen--canyon.com   
      
   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.   
      
   >   
   >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.   
      
      
   John Larkin   
   Highland Tech Glen Canyon Design Center   
   Lunatic Fringe Electronics   
      
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