From: jl@glen--canyon.com   
      
   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   
      
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