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

Hi Jeff

The diode is there to reduce radiated electrical noise that the relay
will generate when turned off. This transient can be very intense and
is best handled at the source, thus the diode.

AC relays due to the nature of AC power have to have a solution other
than a diode. Often RC networks and or metal oxide varistors are used
here. Both of these devices could also be found across a DC relay also
to supplement the diode. The diode though is the primary protection
mechanism.

One thing to be aware of is that the diodes will change the time
constant of the circuit. This is important in some usages.

Dave

On Sat, 2004-01-10 at 21:05, washcomp 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?
>
> Jeff
>
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