Logic level relay controller

Posted on Feb 1, 2013

The schematic below illustrates 4 methods of controlling a relay with a digital logic signal. Figure (A) can probably be used in most cases where the relay coil requires 100 mA or less and the input current is 2 milliamps or more. The resistor value (R) is determined from the input voltage and the available current. For example, a 5 volt input signal supplying 2 milliamps would require (5-.7)/.002 = 2150 ohms, or a 2.2K standard value. If the transistor has a minimum current gain of 50, there will be 100 mA of current available for the relay coil.

Logic level relay controller
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The following table shows various resistor values that can be used to obtain various relay coil currents assuming a transistor current gain of 50 such as the 2N3053. 74XX refers to standard TTL logic, 74LSXX refers to low power TTL logic, 74HC is high speed CMOS and CD40XX is the older CMOS devices. The currents given are approximate values and may not be correct for all devices but should be close. Input Voltage Input Current Relay coil current Standard Resistor 4 74LSXX .0004 20 milliamps 8.2K 4 74XX .0008 40 milliamps 4.3K 4 74SXX .001 50 milliamps 3.3K 5 74HCXX .004 200 milliamps 1K 6 74HCXX .004 200 milliamps 1.3K 5 CD40XX .0003 15 milliamps 13K 12 CD40XX .0006 30 milliamps 18K Figure B can be used when the input voltage is the same as the relay coil voltage. The voltage on the emitter of the transistor will be about 0.7 volts less than the input, so a 12 volt relay would operate on 11.3 which should be close enough. No resistor is needed since the emitter follower configuration presents a high impedance at the input. The input current will be the relay coil current divided by the transistor gain. For example a 120 ohm relay coil will draw 100 mA at 12 volts...

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