How To Test a Relay

Relays allow a low power logic signal to be used to control a much higher power circuit.The relay helps protect the lower power circuit by providing a small coil for the logic circuit to control.You can learn how to test relays.

Step 1: Refer to the data sheet or relay schematic.

Relays have fairly standard pin configurations, but it is best to search for the data sheets to find out more about the number of pins from the manufacturer.The relay will usually have these printed on it.Information on current and voltage ratings, pin configurations, and other information can be found in the datasheets.If the relay is damaged, the results may be unpredictable.The relay’s information may be printed directly on the body of the relay.

Step 2: Look at the relay in a basic way.

A clear plastic shell is used for many relays.Damage such as melting and blackening will help narrow down the issue.If they are in the active state, most modern relays have an indicator.You can assume that relay is bad if the light is off and you have control voltage to the relay or coil terminals.

Step 3: The power source should be disconnected.

All power sources should be disconnected for any electrical work.Capacitors in the circuit can hold a charge for a long time after removing the power source.Do not short the terminals.It is best to check your local laws before doing any electrical work, and if you feel unsafe, leave it to the professionals.Extra low voltage work won’t fall under this requirement, but it’s still important to be safe.

Step 4: Determine the coil requirements for the relay.

On the case of the relay, the manufacturer’s part number should be listed.Determine the voltage and current requirements of the control coil by looking up the applicable data sheet.This may be printed on the larger relays.

Step 5: You can find out if the control coil is protected.

The logic circuitry can be damaged due to noise spikes.There is a bar across one corner of the triangle.The control coil will have a positive connection to the bar.

Step 6: Determine the contact configuration of the relay.

This may be printed on the case of larger relays or from the manufacturer’s data sheet.Relays may have one or more poles, indicated in drawings by a single line switch connected to a pin of the relay.The pole may have a normally open or normally closed contact.The drawings show the connections between the contacts and the pin on the relay.Each pole will be shown in the relay drawings as either touching or not touching the pin.

Step 7: The relay contacts have been de-energized.

To test the resistance between the relay pole and the corresponding NC and NO contacts, use a digital multimeter.The corresponding pole should be read by all NC contacts.All contacts should read the resistance to the pole.

Step 8: The relay should be rejuvenated.

You can use an independent source for the rating of the relay coil.If the relay coil is protected, make sure the independent source is connected with the correct polarity.When the relay is on, listen for a click.

Step 9: The relay contacts need to be checked.

To test the resistance between the relay pole and the corresponding NC and NO contacts, use a digital multimeter.All NC contacts should read the resistance to the pole.No contacts should read 0 to the pole.

Step 10: Check solid-state relays with an ohmmeter.

Solid-state relays will almost always fail when they start to short.Solid-state relays should be checked with an ohmmeter.The control power is off.When control power is applied, the relays should be open, switched to OL, and closed.

Step 11: You can use a multi-meter to confirm your findings.

You can confirm that the relay is bad by taking a multi-meter, putting it in a diode test and checking across A1(+) and A2(-).The meter will read the voltage on the screen if it is applied too little.This will check the NPN transistor from the base to the emitter.If the relay is good, the meter will read 0.7 for a germanium transistor, which is relatively rare, but if it’s bad, it will be 0 or OL.

Step 12: Keep them cool.

Solid-state relays are cheap to replace and last a long time if they stay cool.Blocks and rail packages are usually where new relays come from.An SCR that comes in two flavors for heating wires and IR lamps and ovens is a special type of relay.This is a fast switch that can turn off and on quickly, which can fail due to temperature fluctuations.