From: bill.sloman@ieee.org   
      
   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 more that 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 work  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.   
      
   MELF resistors may have more surface area to dissipate heat over the   
   long term but the peak short term power dissipation limit strictly   
   determined by what happening in the resistive track itself.   
      
   > That would be interesting, to find some papers somewhere that discuss   
   > the benefits of melfs vs regular surfmount parts.   
   >   
   > Menawhile, I'm seeing unexpected goodies blasting a cheap thickfilm   
   > 1206. So we can probably use 1206 thickfilms, and go to thinfilms or   
   > even thinfilm melfs on the same PCB if we have any problems.   
      
   Why not read the resistor data sheet. If they don't specify the   
   performance for short pulses, the implication is that they aren't   
   designed to cope with pulses current.   
      
   > I don't think I have any melf resistors here. It would be interesting   
   > to break one in half and measure the ceramics, re thermal   
   > conductivity.   
      
   Not for your application. The ceramic has no effect on the short pulse   
   limit.   
      
   --   
   Bill Sloman, Sydney   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)