From: jl@glen--canyon.com   
      
   On Sun, 18 Jan 2026 19:43:30 +0100, Jeroen Belleman   
    wrote:   
      
   >On 1/18/26 16:55, 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@poppyrecord   
   .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   
   >   
   >I've been abusing Welwyn SMD resistors in a similar manner. Never a   
   >problem. Good to know.   
   >   
   >On the other hand, I've tried to use 10W Radiall SMA attenuators   
   >with ns kW-level pulsed power, and they'd die, even though the   
      
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