20 Jul
2020
20 Jul
'20
2:18 p.m.
On 07/20/2020 10:55 AM, Peter Coghlan via cctalk
wrote:
It looks like the zener is connected between the adjust terminal
of a heatsink mounted LM337T adjustable negative voltage regulator
and the positive connection to one of the fans. Also connected
to the adjust terminal is what looks like a thermistor mounted on
the heatsink of a MBR3045, presumably a switching transistor.
Perhaps the zener didn't end up carrying much current, however if
that is the case, it is a bit strange that it failed.
Regards,
Peter Coghlan.
I was thinking about wiring them to 5V because it was only
marginally
overheating after running for a long time. However, I
started
poking around with the multimeter and discovered a low
resistance
across one of the connectors for the fans. This led me to a
1N759A 12V 400mW zener diode which read about 20 Ohms in both
directions. Looking at the H7821, there was a 1N4742 12V
1W zener
diode in a similar position. It had much more plausable
readings
so I borrowed it and fitted it to the H7822 in place of
the dud
1N759A. The fans are spinning nicely now with about 7.5
to 8V
across each one. This was a lot easier than I was
expecting :-)
Wow, lucky it didn't smoke anything. there must be a series
resistor somewhere that probably got pretty hot.
Jon