Message-ID: <001a01bed495$70adb340$2a00a8c0@fat-albert>
From: "Pat Weston" <peweston@teleport.com>
To: <lancair.list@olsusa.com>
Subject: Re: Master relay
Date: Thu, 22 Jul 1999 15:55:33 -0700
Mime-Version: 1.0

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All,

A little clarification around the heat being generated by turning the relay
on and off may be needed, so here it is.

As mentioned earlier, there are two sources of heat (dissipated power) in a
relay, at least the types we're talking about here. The coil and the
contacts. All of the heat comes from the power being dissipated which is the
square of the current through the coil or the contacts multiplied by the
resistance of either the coil or the contacts respectively. There is no
increased power generated from the coil or the contacts due to frequently
energizing and de-energizing the relay.

The heat from the contacts is produced only when the contacts are closed, a
load is applied and a current is flowing, thus if a relay is turned off,
there is no heat from the contacts.

The power dissipated during the transition from off to on and vice-versa is
less than when the relay is on in the stable state. Relays are actuated by
an electromagnet (coil) which is primarily inductive with a resistive
component, these are in series. There is also a capacitive element which is
small enough to be ignored for purposes of this discussion. When a voltage
is applied across the coil, the current rises exponentially and settles to a
value determined by the voltage divided by the resistance of the coil. When
the voltage is removed (and assuming there is a snubbing diode across the
coil the current diminishes in a similar manner to zero. At no time is the
current more than that in the steady state, thus the power (current squared
times coil resistance) greater than when the set up is in the steady state.

Thus a relay being turned on and off dissipates less power than one in a
permanently on state.

So why are there two different types of relays?

Intermittent relays are designed to switch high currents for short
durations, such as starters. So the idea here is to close the contacts with
a high speed to minimize contact bounce (short contact life due to arcing)
and high force to reduce contact resistance (lower heat as described above).
In order to achieve this as much magnetic force as feasible is generated.
The electrical force is a proportional to the product of the current through
the coil and the number of times the wire forming the coil is wrapped around
the core (turns). So get the current high (thick wire and small resistance)
and many turns. The result is a higher power in the coil as the current is
high (the squared part of the formula for power) thus the coil is gets
pretty hot, but this is OK as one doesn't ordinarily crank an engine for 20
minutes continuously. Even if you did the battery would die and
automatically save the relay.

The always on (master) relay on the other hand doesn't require the same
degree of electromotive force and can have much less current through the
coil and hence much, much (the squared part) less heat. One should turn off
as much of the load as possible before actuating the  master relay as the
contacts are not as robust and these relays don't last long when switching
high loads. But once closed, the contacts can carry the specified current.

Hope this clears that all up. The bottom line is... "Don't wire the right
relay into the wrong application."

Pat Weston
"Only three short years to go!"
http://www.teleport.com/~peweston


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