From: bill.sloman@ieee.org   
      
   On 24/01/2026 3:36 am, john larkin wrote:   
   > On Sat, 24 Jan 2026 01:56:46 +1100, Bill Sloman    
   > wrote:   
   >   
   >> On 23/01/2026 6:44 am, john larkin wrote:   
   >>> On Thu, 22 Jan 2026 22:45:56 +1100, Bill Sloman    
   >>> wrote:   
   >>>   
   >>>> On 22/01/2026 5:48 am, john larkin wrote:   
   >>>>> On Sun, 18 Jan 2026 15:29:11 +1100, Bill Sloman    
   >>>>> wrote:   
   >>>>>   
   >>>>>> On 18/01/2026 10:33 am, 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@poppyrecor   
   s.invalid.invalid   
   >>>>>>>>>>>>> (Liz Tuddenham) wrote:   
   >>>>>>>>>>>>>   
   >>>>>>>>>>>>>> john larkin  wrote:   
   >>>>>>>>>>>>>>   
   >>>>>>>>>>>>>>> On Thu, 15 Jan 2026 15:18:31 +0000, liz@poppyrec   
   rds.invalid.invalid   
   >>>>>>>>>>>>>>> (Liz Tuddenham) wrote:   
   >>>>>>>>>>>>>>>   
   >>>>>>>>>>>>>>>> john larkin  wrote:   
   >>>>>>   
   >>>>>>    
   >>>>>>   
   >>>>>>> I'll reveal the secret mathematics:   
   >>>>>>   
   >>>>>> The secret you reveal isn't in the mathematics. It's the thermal   
   >>>>>> resistance from the dissipating element to ambient   
   >>>>>   
   >>>>> No; average power is low.   
   >>>>>   
   >>>>> What matters is microsecond-range heat capacity in the resistive   
   >>>>> element. The ceramic substrate may help a little.   
   >>>>   
   >>>> The impulse load ratings for the Vishay thin films that I posted   
   >>>> suggested that for them the thermal mass of the substrate stops   
   >>>> mattering below about 300usec.   
   >>>>   
   >>>> You may have to worry about the skin effect as well, which restricts   
   >>>> very high frequency components to the edge of any track. For higher   
   >>>> resistance metal film resistors the tracks tend to be pretty narrow   
   anyway.   
   >>>>   
   >>>>>>> 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.   
   >>>>>>   
   >>>>>> Why? It's on the data sheet of whatever resistor you happen to be using,   
   >>>>>> which you haven't specified. You may not realise this.   
   >>>>>   
   >>>>> The data sheet of a cheap thickfilm resistor does not characterize it   
   >>>>> for pulsing at 1000x rated power.   
   >>>>   
   >>>> So you probably shouldn't use them for that job.   
   >>>   
   >>> Should be fine at 250 volts per resistor.   
   >>>   
   >>>>>   
   >>>>> Or 2000x, which I'm running now.   
   >>>>   
   >>>> And where you are starting to see changes in resistance with time.   
   >>>   
   >>> After a day of pulsing at 500v, 500 watts, the resistance has dropped   
   >>> about 0.1%.   
   >>>   
   >>> It takes me about a minute a day to run this experiment, so I'll let   
   >>> it run some more.   
   >>   
   >> 0.1% in a day is worryingly high.   
   >   
   > It's within the tempco range of a cheap thickfilm.   
   >   
   > If the resistor is being damaged, we may have fused a few weak spots,   
   > and things may stabilize. So I'll let it keep running.   
   >   
   >>   
   >>> I might try the thinfilm 1206 just for fun.   
   >>   
   >> A prudent engineer would try a part that was specified for some kind of   
   >> short pulse use, rather than one which wasn't at all.   
   >   
   > A sensible engineer will test parts that are available. Maybe you can   
   > find a small surface-mount 499 ohm resistor that is specified to   
   > tolerate kilowatt pulses.   
      
   Why test parts when you can buy something that is guaranteed to work?   
      
   > Being prudent isn't fun, or a good way to invent things.   
      
   I've got three patents and you've got your name on one that was taken   
   out by other people. Being moderately prudent has worked better for me   
   than your slap-dash habits have worked for you.   
      
   > It's good to get crazy and reckless once in a while, destroy things.   
   A lunatic might think that.   
      
   > The final design stages of a sellable product should be very prudent   
   > and reviewed by careful and prissy people.   
      
   You need design reviews at every stage of a design. Other people can   
   sometimes see possibilities that you have missed.   
      
   >>> I should have bought a higher voltage version of this power supply.   
   >>> The mosfet is good for 1500 volts.   
   >>   
   >> All sorts of nasty stuff starts happening at high voltages. Cambridge   
   >> Instruments didn't like photomultipliers where the photocathode was more   
   >> that 1kV away from ground. The glass widow of the photomultiplier tube   
   >> and the glass window of the sample chamber both started conducting   
   >> current by ion migration at that sort of voltage, with a little   
   >> electroluminescence, which was an inconvenient source of noise.   
   >   
   > Numbers under a kv or so aren't too scary on a surface-mount PCB. Much   
   > higher ones are.   
      
   Exactly the point I was making.   
      
   --   
   Bill Sloman, Sydney   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)