On 07/20/2020 10:55 AM, Peter Coghlan via cctalk wrote:
>
> 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.
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.
Jon
