RE: [CAD_CAM_EDM_DRO] Relay question about diodes across the coil

Harvey:

Thanks, that clears it up. I didn't think it would work on A.C.
It probably doesn't matter much in my case anyway. The relays are
mechanical and generally controlling non-electronic stuff, which should be
pretty immune to small spikes. I'll look into the MOV's anyway.

Jeff

-----Original Message-----
From: Harvey White [mailto:madyn@...]
Sent: Saturday, January 10, 2004 10:47 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: Re: [CAD_CAM_EDM_DRO] Relay question about diodes across the coil

On Sun, 11 Jan 2004 02:05:50 -0000, you wrote:

>Back when I was a kid learning about relay circuits on the job
>(1960's), most of the control circuits I got involved in used low
>voltage A.C. and I don't ever remember seeing a diode. I just found
>out that it's good practice to put a diode across the coil of relays
>to keep them from chattering due to the current stored in the coil
>inductance. Question is: Only on D.C. relays or is this supposed
>to be done on A.C. relays as well?

The diode dumps the back EMF which produces a current spike that can
ruin (by back-biasing) the semiconductors driving the relay.

With an AC relay, there is no way to put diodes across the relay to
accomplish this. It is possible to put an MOV (Metal Oxide Varistor,
often called a surge suppresser) across the coil, which can help. If
the circuit doing the switching is switching at the point of the
waveform where the voltage is zero, then this is not needed. It is
only needed if switching is happening at any other place on the
waveform.

Harvey


>
>Jeff
>



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