From: bill.sloman@ieee.org   
      
   On 16/11/2025 4:02 am, Gerhard Hoffmann wrote:   
   > Am 15.11.25 um 17:03 schrieb john larkin:   
   >   
   >>   
   >> I tested a bunch of high-meg surface-mount cermets for VCR and   
   >> excess/shot noise. That was hard, specifically the noise part.   
   > Cermet is usually a pseudonym for thick film. That can be   
   > easily much worse than carbon composition, esp. for 1/f noise,   
   > preferably when there is DC across it.   
   > Thin film rulez. I use Susumu SMDs as default (DK).   
      
   For very high resistances, thin film isn't practical.   
      
   A single atom thick layer of pretty much any metal has a pretty low   
   resistance - high value thin film resistors have very long, very narrow   
   convoluted tracks.   
      
   Thick film inks are pretty predictable mixtures of metal oxides and   
   glass, and - unlike carbon composition - they haven't got a negative   
   temperature coefficient and don't form hot channels.   
      
   One of my colleagues early on, was an intrinsic safety expert and his   
   parlour trick was setting up a hot channel in a 10k carbon film 0.25W   
   resistor and pushing an amp through it for some ten minutes.   
      
   It still measured 10k after he'd turned the current off but there was a   
   narrow dark line along the surface where the paint had got hot above the   
   hot channel.   
      
   You needed lots of volts across biggish capacitor to form the hot   
   channel, but he'd got that worked out.   
      
   --   
   Bill Sloman, Sydney   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)