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Relays can handle a lot of power. However, for certain power sensitive designs you would like to reduce the power needed to hold a relay closed. The circuit below performs such a task.
It uses a single CD4093 quad NAND gate. When the “on” logic input signal is detected, the relay is first pulsed on for about 500ms.
This is sufficient time to insure the relay is fully closed. After that initial pulse the relay is then driven with a square wave signal, whose duty cycle can be adjusted. The signal duty cycle can be adjusted from about 10% to 90%. In most cases a 50% duty cycle will hold the relay closed.
This reduces the average DC current required by the same factor, which means a 4:1 reduction in power. The circuit can operate over a wide 3v to 15v range.
Content By http://www.discovercircuits.com/DJ-Circuits/relay-pwm.htm
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