From: jl@glen--canyon.com   
      
   On Sun, 18 Jan 2026 07:55:37 -0800, john larkin    
   wrote:   
      
   >On Sun, 18 Jan 2026 00:48:06 +0100, Jeroen Belleman   
   > wrote:   
   >   
   >>On 1/18/26 00:33, john larkin wrote:   
   >>> On Sat, 17 Jan 2026 15:58:01 +1100, 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   
   >>>   
   >>> I'll reveal the secret mathematics:   
   >>>   
   >>> 180 watts at 0.1% duty cycle is 0.180 watts.   
   >>>   
   >>> 15c divided by 0.18 is 83 watts per degree C.   
   >>>   
   >>> Please keep this confidential.   
   >>>   
   >>>   
   >>>   
   >>> John Larkin   
   >>> Highland Tech Glen Canyon Design Center   
   >>> Lunatic Fringe Electronics   
   >>   
   >>Degrees per watt.   
   >>   
   >>Jeroen Belleman   
   >   
   >Oops. You knew what I meant.   
   >   
   >The 83 K/W is reasonable, considering that measuring a 1206 temp with   
   >a cheap thermal imager isn't an exact science.   
   >   
   >After several days pulsing at 300 volts, 180 watts, the thickfilm   
   >resistance was bobbling around in the 5th decimal place. Boring.   
   >   
   >I stepped it up to 350v, about 250 watts. That's 1000x the part's DC   
   >power rating. I'll check that now and then.   
   >   
   >   
   >   
   >John Larkin   
   >Highland Tech Glen Canyon Design Center   
   >Lunatic Fringe Electronics   
      
   4 days at 350 volts, still no resistance change.   
      
   I guess I'll try 500. That will be 500 watts peak.   
      
      
   John Larkin   
   Highland Tech Glen Canyon Design Center   
   Lunatic Fringe Electronics   
      
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