ac clutch relay

Bernie Benz b.m.benz at
Wed Jul 3 08:28:26 EDT 2002

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The supression diode is across the relay contacts.  It is one of the
two black diodes located on the base of the relay, inside.  It should
be inserted such as to block the 12V applied to the clutch.  If in backward,
voltage will bypass the contacts and the clutch will run all the time.

To suppress the induced voltage spike of the collapsing magnetic field the
suppression device must be capable of handling the full coil current at the
acceptable peak spike voltage.  Your "small cap" may thus reduce the
spike from 2KV to 1KV, but not good enough!  A simple diode clamps the
spike at its forward voltage drop, 1.2V for silicon.

Any silicon diode rated at least 50V and 5A will do.  Twofer $.50 at RS.


From: DAK <dak at>
Date: Tue, 02 Jul 2002 21:36:55 -0400
To: Bernie Benz <b.m.benz at>, 200q20v at
Subject: Re: ac clutch relay

Bernie, I thought the flyback diode was on the coil side of the relay? It
shorts the pulse when the magnetic field on the coil collapses (as the relay
is shut off). I know a cap on the output side acts as a filter to dampen the
high frequency energy created as the conacts arc (just before closing, or
just after opening). Granted, I'm no EE, but I was headed that way in the
80's when computer science got my attention. Surely, some type of small
value cap couild handle the EMF generated when that clutch coil
de-engergizes. Perhaps a diode on that as well?


PS. Kevin, am I off base here?

Bernie Benz wrote:

Bad suggestion IMO.  The inductive energy stored in the clutch coil can
never be handled by wimppy ceramic capacitor!  Open up the AC relay and
replace the supression diode, which you will find is either open, causing
much radio noise upon deenergiding, or shorted, in which case the compressor
will not cycle, but run all the time.


From: DAK <dak at> <mailto:dak at>
Date: Mon, 01 Jul 2002 22:54:59 -0400
To: Vincenzo Basile <Bleaf1 at> <mailto:Bleaf1 at>
Cc: 200q20v at
Subject: Re: ac clutch relay

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