From: jl@glen--canyon.com   
      
   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@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'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.   
      
   I might try the thinfilm 1206 just for fun.   
      
   I should have bought a higher voltage version of this power supply.   
   The mosfet is good for 1500 volts.   
      
      
   John Larkin   
   Highland Tech Glen Canyon Design Center   
   Lunatic Fringe Electronics   
      
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