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Recently I helped a gentleman troubleshoot his AWA Battery
Eliminator board. He had completed assembly of the supply and was
experiencing some problems with it. On powering up the board there
were audible crackling sounds emanating from the supply as well as
resistors overheating.
To cut to the chase, the problem was with the heat sink grease or
thermal compound used to mount the high voltage regulator
transistors. (I call it grease because when I started in
electronics in the early 1970s plain, transparent silicon grease
was used on the thermal interface to heat sinks.)
At low AC input voltages everything was fine but as I approached
100V input the crackling began. I was also able to make the
crackling sound come and go by wiggling the heat sinks. There were
no cold solder joints, broken leads or any other mechanical
defects.
Dismounting the series pass transistors, wiping away all traces of
the original silvery grey thermal compound, then remounting with
white Wakefield compound cured the problem.
The only plausible explanation is that the thermal compound broke
down above a certain voltage level.
Doing some research, I found a number of "CPU Cooler" thermal
compounds available on the web. Apparently many of these can have
a silver content to aid in thermal conductivity. Volume
resistivity specs look very good but dielectric strength is not
mentioned.
The "good old" white paste was merely zinc oxide suspended in
silicone grease. The builder did not know what brand or type
compound he used.
Lesson gained is - if building projects where heat sinking of
active devices is required, or repairing existing solid state
equipment - choose your thermal compound carefully. For
applications where all mating surfaces are at ground potential and
no insulators are needed, the new CPU Cooler compounds may be
fine.
For power supply applications where TO-220 series pass transistors
are mounted to heat sinks (the AWA Battery Eliminator, Capacitor
Checker and HV Power Supply boards come to mind) make sure you use
the right (white) grease.
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