How to Test a Relay?
3 Answers
Relay (English name: relay) is an electrical control device that causes a predetermined step change in the controlled quantity in the electrical output circuit when the change in the input quantity (excitation quantity) meets specified requirements. It establishes an interactive relationship between the control system (also known as the input circuit) and the controlled system (also known as the output circuit). Typically used in automated control circuits, it essentially functions as an "automatic switch" that uses a small current to control the operation of a larger current. Therefore, it plays roles in automatic regulation, safety protection, and circuit switching within a circuit. Test contact resistance. Use the resistance setting of a multimeter to measure the resistance between the normally closed contact and the moving contact, which should be 0, while 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. Test coil resistance. Use the R×10Ω setting of the multimeter to measure the resistance of the relay coil to determine if there is an open circuit. Measure pull-in voltage and pull-in current. Use an adjustable regulated power supply and an ammeter to supply voltage to the relay while monitoring the current in series. Gradually increase the power supply voltage until the relay pulls in, then note the pull-in voltage and current. For accuracy, repeat the test several times and calculate the average. Measure release voltage and release current. Connect the test setup as described above. After the relay pulls in, gradually reduce the supply voltage until the relay releases again, then note 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 be used normally, posing a threat to circuit stability and reliability.
When testing relays, as someone who frequently tinkers with cars, I usually start by picking a nice day to remove the relay from the vehicle, especially those controlling the headlights. Prepare a small 12V battery and a bulb test setup: connect the battery's positive terminal to the relay's coil terminal and the negative to the ground wire. If the relay is functioning properly, you should hear a click, indicating the coil is energized. Then test the contacts: for example, connect the bulb to the relay's contact and ground—if the bulb lights up, it proves the contacts can conduct current in the closed state; if there's no response, the contacts may be burnt or internally damaged. A multimeter is also very practical—set it to the resistance mode, and the resistance between the coil terminals should typically be around 50-100 ohms; anything significantly higher or lower indicates a problem. This method is simple and reliable. I've fixed similar issues several times this way, saving money and boosting confidence. The key is that it can be done at home, avoiding unnecessary expenses for minor issues.
Safety is the top priority when testing relays, as I've seen others get injured from careless contact with live circuits. Always remember to turn off the ignition switch and wear insulated gloves to prevent short circuits. The operational steps include using a multimeter to check: first, measure the coil resistance, with normal values ranging between 60-150 ohms; second, test contact continuity by using the voltage range to see if it shows conduction or open circuit when switching the relay. Choose a good-quality meter for the job, and avoid using rusty equipment. During testing, avoid contact with exposed wires, especially in high-voltage areas of the vehicle. Common relay failures are due to dust accumulation or overheating, which can affect driving functions like the starting system. Addressing issues promptly can prevent greater hazards. Though the process is simple, practicing safety habits gives me and my family greater peace of mind on the road.
