[Asterisk-Users] POTS failover relays (was Vonage, PSTN, 911, and hardware question)

[Greg Hill]

On Sun, 10 Oct 2004, Rajeev Sharma wrote:

> Yeah, thanks, I was thinking of doing something similar to that.
> Actually, I was gonna spice a cable in my computer's power supply and
> use that. Why? Because if it's on a UPS, then the switch will throw at
> the same time as the computer looses off. I dunno, I might not even use
> a UPS, just a surge protector, but I'll see. Thanks for the idea.

I was going to suggest this, but didn't because it's (slightly) more
involved. Due to the physics of a relay (that it's constructed with a coil
of wire), there is some inherent inductance. When you try to interrupt the
current to an inductor, the voltage across it spikes. This is considered a
Bad Thing in sensitive electronics like a computer. This isn't really an
issue with the wall-wart power adapter, because there isn't likely to be
anything terribly sensitive in there (and they're cheap to replace in the
event of failure).

In a sensitive computer environment, you should include a reverse-biased
diode in parallel with the relay's coil.  This diode, because it's
reverse-biased in normal circuit operation, won't conduct any current. But
when power is lost, the voltaged induced by the inductor will forward bias
the diode, and the voltage spike will be clamped by the diode rather than
going out into other components via the power bus. Most any common diode
will work fine (1N4148 small-signal diode, 1N4001/2/3/4 rectifier diode,
etc) for this purpose.

Greg