# Relay Driver Circuit

Posted on Feb 5, 2014

In this project, we will build a relay driver for both DC and AC relays. Since DC and AC voltages operate differently, to build relay drivers for them requires slightly different setup. We will also go over a generic relay driver which can operate from either AC or DC voltage and operate both AC and DC relays. All relays come with a voltage rating

. This is called on a relay`s datasheet its rated coil voltage. This is the voltage needed in order for the relay to be able to operate and be able to open or close its switch in a circuit. In order for a relay to function, it must receive this voltage at its coil terminals. Thus, if a relay has a rated voltage of 9VDC, it must receive 9 volts of DC voltage to operate. So the most important thing a DC relay needs is its rated DC voltage. If you don`t know this, look up what relay you have and look up its datasheet and check for this specification. And the reason why a diode is needed is usually because it functions to eliminate voltage spikes from a relay circuit as the relay opens and closes. The coil of a relay acts an inductor. Remember that inductors are basically coils of wires wrapped around a conductive core. This is what relay coils are as well. Therefore, they act as inductors. Inductors are electronic components that resist changes in current. Inductors do not like sudden changes in current. If the flow of current through a coil is suddenly interrupted, for example, a switch opening, the coil will respond by producing a sudden, very large voltage across its leads, causing a large surge of current through it. From a physics or physical perspective, this phenomen is a result of a collapsing magnetic field within the coil as the current is terminated abruptly. Mathematically, this can be understood by noticing how a...

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