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

Jan:

That was my suspician, but the suggestion was made "for all relays", the
information I found on the web was ambiguous, and it's been more than 30
years since I've had to think about this sort of thing (I guess the grey
cells drift to other uses)

Thanks and regards,
Jeff

-----Original Message-----
From: JanRwl@... [mailto:JanRwl@...]
Sent: Saturday, January 10, 2004 10:57 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: Re: [CAD_CAM_EDM_DRO] Relay question about diodes across the coil

In a message dated 1/10/2004 8:07:06 PM Central Standard Time,
jeff@... writes:
Question is: Only on D.C. relays or is this supposed
to be done on A.C. relays as well?

Jeff:
The diode "across the coil" is for DC relays ONLY, and this absorbs the
stored energy in the winding when the DC is disconnected (aka "spark quench"

diode). If you put a diode across the coil of an AC-fed relay, it would be
a "short
circuit" for half the cycle, and would either "blow" immediately, or heat up

and possibly do damage, and prevent that AC relay from functioning, and
probably ruin the circuit "feeding" that AC-coil, and on-and-on!

If you have a good-sized 12 VDC relay (or similar coil), connect up a
power-source (even a 12 V. "lantern battery") and complete the circuit
intermittently
by simply touching the wires together which would complete the circuit.
Maybe best in a dark room. Watch the point where you touch the wires
together.
See a spark, even if tiny? Now, connect a diode across the relay's coil
(making sure you don't connect it in the direction that would "short" the
power-source!), and "touch 'em again". No spark! See?


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