From: bill.sloman@ieee.org   
      
   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@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'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.   
      
   > 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.   
      
   > 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.   
      
   --   
      
   Bill Sloman, Sydney   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)