How to Test a RelayArticle

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Relays are discrete devices (as opposed to integrated circuits) that are used to allow a low power logic signal to control a much higher power circuit. The relay provides isolation of the high power circuit from the low power logic circuit to protect the low power circuit. The relay accomplishes this by providing a small electromagnetic coil for the logic circuit to control. When the electromagnetic coil is energized, it causes a high power capacity magnetized switch to activate the high power circuit. An example of this is a relay that provides a large amount of current to the headlights of an automobile when the relay controlling the headlights is activated by the low power logic circuitry in the dashboard of the car. Use these tips to learn how to test a relay.

Edit StepsFind the Characteristics of the Relay to Be Tested

1. 1

Determine the coil requirements of the relay. The manufacturer's part number should be listed on the case of the relay. Look up the applicable data sheet and determine the voltage and current requirements of the control coil. This also may be printed on the case of larger relays.

2. 2

Find out if the control coil is diode protested. A diode around the pole is often used to protect the logic circuitry from damage due to noise spikes. The diode will be shown on drawings as a triangle with a bar across 1 corner of the triangle. The bar will be connected to the input, or positive connection, of the control coil.

3. 3

Assess the contact configuration of the relay. This will also be available from the manufacturer's data sheet, or may be printed on the case of larger relays. Relays may have 1 or more poles, indicated in drawings by a single line switch connected to a pin of the relay. Each pole may have a normally open (NO) and or normally closed (NC) contact. The drawings will indicate these contacts as connections with a pin on the relay. The relay drawings will show each pole as either touching the pin, indicating a NC contact, or not touching the pin, indicating a NO contact.

Test the Relay1. 1

Remove the relay from any contact with circuitry. Most larger relays are socketed and may easily be removed.

2. 2

Test the de-energized condition of the relay contacts. Use a digital multimeter (DMM) to test the resistance between each pole of the relay and the corresponding NC and NO contacts for that pole. All NC contacts should read 0 ohms to the corresponding pole. All NO contacts should read infinite resistance to the corresponding pole.

3. 3

Energize the relay. Use in independent voltage source appropriate for the rating of the relay coil. If the relay coil is diode protected, make sure that the independent voltage source is connected with the proper polarity. Listen for a click when the relay is energized.

4. 4

Check the energized condition of the relay contacts. Use a digital multimeter (DMM) to test the resistance between each pole of the relay and the corresponding NC and NO contacts for that pole. All NC contacts should read infinite resistance to the corresponding pole. All NO contacts should read 0 ohms to the corresponding pole.