On Sat, 1 May 1999 12:37:03 +0200 Tjaart van der Walt <tjaart@WASP.CO.ZA>
writes:

>Remember the usually-forgotten diode between the negative side of
>the=20
>relay coil (on the collector) and GND. This way you guarantee that
>no=20
>excessive - voltages get on the GND line.

How will installing a diode that way keep negative voltages from the
ground line?  It is connected to the ground line in such a way to route
any negative voltage right to it.  Such voltages won't occur in normal
operation anyway.  If they did, the transistor would start to conduct in
"inverted transistor" mode and shunt the voltage to the ground line (and
to the drive resistor, which may be more of a problem).

The original problem was a capacitor on the *output* side of the relay
failing.  This would indicate some problem with the interaction of the
relays and the loads.  Diodes, transorbs, RC snubbers, etc. should be
applied to keep transient voltages from inductive loads being switched
off under control.  Unless the properties of tantalum capacitors are
really necessary, use aluminum ones instead.  They are much more
resistant to damage from overvoltage.   But it's not only a problem of
capacitors failing, as spikes of at least 20-30 V on the 12V supply are
bound to mess up something else.


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