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From: crisp@mips.COM (Richard Crisp)
Newsgroups: sci.electronics
Subject: Re: Using computer to switch high current circuit
Message-ID: <34145@mips.mips.COM>
Date: 4 Jan 90 22:09:01 GMT
References: <1990Jan2.005157.12780@uokmax.ecn.uoknor.edu> <34039@mips.mips.COM>
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Reply-To: crisp@mips.COM (Richard Crisp)
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In Mark Johnson's suggested technique for switching large currents using
a computer to control three relays one important point was not made.
Be sure to include shunting diodes across the solenoids of the relays.
They need to be wired in such a way that they are reverse biased when the relay
is energized (assuming you use DC controlled relays). If you fail to put these
devices in, when the relay is turned off you will get a large inductive "kick"
which will stress the hell out of the device driving the relay possibly 
catastrophically. With the diodes in place, when this inductive "kick" starts,
the polarity of the voltage across the relay's solenoid changes (remember
Lenz's law?) and forward biases the diode thereby limiting the magnitude of the
reverse voltage. You might need a resistor in series if the diode can't handle
the current, the tradeoff is that the reverse voltage gets larger due to 
the IR drop of the resistor. 

This oversight has wasted many transistors used for driving relays.

-- 
Just the facts Ma'am