How to Test a Relay?
2 Answers
Testing a relay involves the following methods: 1. Measure the coil resistance: Use a multimeter set to the R×10Ω range to measure the resistance of the relay coil, which helps determine if the coil is open. The resistance of the relay coil is closely related to its operating voltage and current. The operating voltage and current can be calculated based on the coil resistance. 2. Measure the contact resistance: Use the resistance range of the multimeter to measure the resistance between the normally closed contact and the moving contact, which should be 0; the resistance between the normally open contact and the moving contact should be infinite. This helps distinguish between the normally closed and normally open contacts. 3. Measure the pull-in voltage and current: Use an adjustable regulated power supply and an ammeter. Apply a voltage to the relay and monitor the current in the power circuit with the ammeter. Gradually increase the power supply voltage until the relay pulls in, then record the pull-in voltage and current. For accuracy, repeat the test several times and calculate the average. Measure the release voltage and current: Connect the test as described above. After the relay pulls in, gradually decrease the power supply voltage until the relay releases, then record the voltage and current. Repeat the test several times to obtain the average release voltage and current. Generally, the release voltage is about 10-50% of the pull-in voltage. If the release voltage is too small (less than 1/10 of the pull-in voltage), the relay cannot function properly, which may threaten circuit stability and reliability.
Testing a relay requires a multimeter and a 12V power supply. First, unplug the relay and identify the pins—the case usually has a circuit diagram indicating the coil and contact positions. Set the multimeter to the resistance mode and measure the coil terminals; typically, 50-120 ohms is normal, while an infinite reading means the coil is burnt. Then, connect the 12V power supply to the coil pins and listen for a click sound—if heard, it means the magnet is functional. Maintain the power-on state and test the contacts: the normally open (NO) contacts should switch from open to closed, while the normally closed (NC) contacts should do the opposite. If the contact state doesn’t change after power is applied, or if the resistance between contacts exceeds 1 ohm, it indicates internal spring corrosion or burnt contact points. In routine car repairs, relay contact sticking is common, especially in high-current starter relays. In such cases, testing confirms the issue, and replacing the relay is the easiest solution.
