Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Mon, 13 May 2002 11:10:48 -0400
Message-ID: <redirect-1236305@logan.com>
From: "Charles R. Patton" <charles.r.patton@ieee.org>
Reply-To: charles.r.patton@ieee.org
MIME-Version: 1.0
Subject: Re: [LML] Re: Diodes
References: <list-1236051@logan.com>
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Walter Dodson wrote:
>  If it won't work turn it around.

I respectfully suggest that the diode is dead, or certainly suspect from
stress, if mis-installed backwards.  Either the fuse/circuit breaker or
diode or both will blow as the diode will provide a short with many
times its current rating to flow, either shorting it (the typical
result) or blowing it open (much rarer).  

Also to be a bit picky, the coil inductance is in henries, not mhos. 
Also on a previous post someone mentioned that the BEMF causes a big
pulse of current.  Not exactly.  Once the current starts flowing, as was
subsequently described correctly, it wants to continue to flow
(described as "inertia").  The consequence of this is that an attempt to
cut the circuit abruptly, such as a switch, causes the coil to generate
a large *voltage* spike in an attempt to keep that same current flowing
-- not a larger current spike.  That voltage spike then can jump the
opening contacts of the switch causing an arc which results in a small
bit of metal being vaporized.  After many hundreds or thousands of
operations the contact has been vaporized all over the inside of the
switch.  

If you draw a circuit and change the conditions on a coil from a voltage
drop, such as an energized coil, to a battery as it attempts to maintain
current flow, you'll see the polarity reverses on the coil.  That is why
the diode works.  In its normal condition it is back-biased, blocking
current flow through itself.  When the circuit is interrupted, and the
coil attempts to maintain current flow, the coil voltage reverses, and
the current that was flowing is now carried by the diode.  The
resistance in the coil and the diode then gradually dissipate the stored
energy (in the magnetic field).  As in many things, the good effects are
not totally free.  There is at least one downside to the diode
suppression approach.  The release time on the relay is delayed, for as
the diode continues to conduct, it is just as if the original current
was flowing, and it will be held in until the current drops below the
drop-out value.  This typically is a few tens of milliseconds.  Not a
problem for starter relays.  But in other applications, this can be a
severe effect, and then MOVs or series capacitor/resistor combinations
are used which cause the drop out to be much faster.

Regards,
Charles R. Patton