From: bill.sloman@ieee.org   
      
   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@poppyrecords   
   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.   
   >   
   > Or 2000x, which I'm running now.   
      
   And where you are starting to see changes in resistance with time.   
      
   --   
   Bill Sloman, Sydney   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)