From: jeroen@nospam.please   
      
   On 1/18/26 19:53, john larkin wrote:   
   > 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@poppyrecor   
   s.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   
      
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