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

On Sat, 10 Jan 2004 23:39:16 -0500, you wrote:

>
>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.

Ok, point of clarification. What the relay controls is not relevant,
not to this discussion. What is *controlling* the relay is relevant,
since that's what's driving the coil. The coil is what produces the
back EMF that may or may not damage the device driving it.

The snubber that has been mentioned by others will protect the
contacts of the relay driving this relay (if there's one). These have
been around for quite a while. A typical snubber can be a resistor
and capacitor in series. Don't remember the values.

Harvey

>
>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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