How do coil relays work?

When a relays coil is energized, current flow through the coil creates a magnetic field. Whether in a DC unit where the polarity is fixed, or in an AC unit where the polarity changes 120 times per second, the basic function remains the same: the magnetic coil attracts a ferrous plate, which is part of the armature.

How can you tell if a relay coil is bad?

In order to check to see if the relay’s coil is good or bad, one of the best methods is to check its resistance. A relay coil’s resistance should always come with the datasheet of the relay. Relay resistances may be 320Ω, 900Ω, or any of the various resistances. Its resistance tolerance should also be stated.

Is a relay coil an inductor?

So, a relay coil has some inductance, which means that it is an energy storage device. The answer is to stick a diode across the terminals of the coil. The diode is reverse biased, which means that you put it in the direction so that current does not normally flow through it.

Do relays go bad?

When operated at full load, a relay is said to have failed when its contacts fail to operate (often because they weld together) or when contact material erosion results in an unacceptable high path resistance. When asked to hot switch a load, a relay may only be good for 100,000 operations.

How a relay works in a car?

In the most simple terms an automotive relay is an electronically operated switch. The type most commonly used in the automotive industry is an electro-mechanically operated switch. They employ an electromagnet device to mechanically manoeuvre a switch to make or break an electrical circuit.

Why put a diode across a relay coil?

Placing a diode across a relay coil passes the back electro magnetic field and its current through the diode when the relay is energized as the back EMF drives the flyback protection diode in forward bias. Tiny flyback diodes prevent huge flyback voltage from damaging your components.

What is relay coil inductance?

The inductance is measured in Henrys (H). So, the inductance is 90mH with the relay closed and 60mH with the relay open. As the relay closes, the magnetic path changes, and the inductance changes. You could use this to predict the behavior of the relay coil voltage when the relay is energized or de-energized.