How to Test a Relay

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Relays are discrete (as opposed to integrated) devices that enable a low-power logic signal to drive a much higher-power circuit. By providing a tiny electromagnetic coil for the logic circuit to regulate, the relay separates the high power circuit and helps to safeguard the lower power circuit. It is possible to learn how to test both coil and solid-state relays.

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Getting Started

1. Perform a simple visual examination of the relay. A translucent plastic shell encases the coil and contacts in many relays. Visible damage (melting, blackening, etc.) will assist in narrowing down the problem.
Most contemporary relays include an LED that indicates whether or not they are in the active state (ON). If the light is not on and you have control voltage to the relay or coil terminals (usually A1[line] and A2 [common]), you may fairly presume that the relay is faulty.

2. Remove the power supply. All power sources, including batteries and line electricity, should be unplugged before doing any electrical work. Be particularly cautious with capacitors in the circuit, since they may retain a charge for an extended period of time after the power source is removed. To discharge, do not short the capacitor terminals. Before undertaking any electrical repair, verify your local regulations, and if you feel dangerous, leave it to the pros. Extra low voltage work is often exempt from this criterion, although it is still critical to remain safe.

3. Refer to the relay diagram or data sheet for further information. Relays have fairly common pin designs, however it is preferable to look for data sheets from the manufacturer to learn more about the number of pins. These are usually written on the relay.

  • Information on current and voltage ratings, pin layouts, and other information is sometimes accessible in datasheets and will be essential in testing, eliminating most mistakes. It is feasible to test pins at random without knowing the pin arrangement, but the results may be unexpected if the relay is broken.
  • Depending on the size of the relay, this information may also be written directly on the body of the relay.
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Testing Coil Relays

1. Determine the relay’s coil needs. The component number of the manufacturer should be printed on the relay’s casing. Look up the relevant data sheet and establish the control coil’s voltage and current requirements. This information may also be written on the casing of bigger relays.

2. Determine if the control coil is diode protected. A diode wrapped around the pole is often used to shield the logic circuitry from noise spikes. On drawings, the diode will be shown as a triangle with a bar across one corner. The bar will be attached to the control coil’s input, or positive connection.

3. Examine the relay’s contact setup. This information may also be obtained from the manufacturer’s data sheet, or it may be printed on the casing of bigger relays. Relays may contain one or more poles, which are represented in drawings by a single line switch linked to a relay pin.

  • Each pole may have one or both ordinarily open (NO) and normally closed (NC) contacts. These contacts will be shown in the drawings as connections to a relay pin.
  • The relay drawings will show each pole contacting or not touching the pin, signifying an NC contact or a NO contact.

4. Check that the relay contacts are de-energized. Test the resistance between each relay pole and the matching NC and NO contacts using a digital multimeter (DMM). To the appropriate pole, all NC connections should register 0 ohms. To the appropriate pole, all NO connections should read infinite resistance.

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5. Activate the relay. Use an independent voltage supply adequate for the relay coil’s rating. If the relay coil is diode protected, ensure that the independent voltage source is properly polarized. When the relay is turned on, listen for a click.

6. Examine the relay contacts for energized status. Test the resistance between each relay pole and the matching NC and NO contacts using a digital multimeter (DMM). To the appropriate pole, all NC connections should read infinite resistance. To the appropriate pole, all NO connections should register 0 ohms.

Testing Solid-State Relays

1. Check solid-state relays using an ohmmeter. Solid-state relays nearly invariably fail when they begin to short. When the control power is turned off, solid-state relays should be tested with an ohmmeter across the normally open (N.O.) terminals.
When control power is provided, the relays should be open, switched to OL, and closed (0.2, the internal resistance of the ohmmeter).

2. To validate your results, use a multimeter in diode-test mode. You may confirm that the relay is defective further by using a multimeter in diode test mode and testing between A1(+) and A2 (-). The meter will apply a modest voltage to the semiconductor to cause it to conduct and display the voltage on the screen. This will test the transistor (usually NPN) from the base(P) to the… emitter.
The meter will read 0 or OL if the relay is defective, but 0.7 for a silicon transistor (which practically all of them are) or 0.5 for a germanium transistor if the relay is okay (which are relatively rare but not unheard of).

3. Maintain the temperature of SSRs. Solid-state relays are simple to diagnose, inexpensive to replace, and endure a long time if kept cold. New relays are often available in DIN rail packages and block mountings.
There is also an unique sort of relay known as an SCR, which comes in two varieties for heating wires, IR lamps, and ovens, and is often used for precise process temperature control. This is essentially a fast switch on a much faster switch that can turn on and off and often fails due to temperature variations.

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How do you test a relay using a multimeter?

Connect the leads of the multimeter across the coil terminals of the relay. The multimeter should read between 40 and 120 for a regular coil. If the coil is broken, i.e., open, the meter reads out of range and the relay must be replaced.

Will a bad relay still click?

If your starting relay has failed, the electrical signal from the battery will never reach the beginning motor. As a consequence, no matter how many times you turn the key, your engine will not start. When you turn your automobile, a malfunctioning relay often generates an audible clicking sound.

How many Ohms should a relay have?

50-120 ohms
Anywhere between 50 and 120 ohms is OK. Out of range or open indicates a faulty electromagnet coil winding and the need for a replacement relay. Keep the multimeter set to ohms or continuity.

How can you tell if a relay fuse is blown?

It is rather easy to test a relay fuse using a multimeter. Set the multimeter to Ohms and measure resistance by touching the leads to the magnetic coil pins. It should range from 50 to 120. If it is outside of that range or indicates that it is open, you have a faulty electromagnet coil.

What causes a relay to fail?

Contamination and mechanical wear of the internal switching components are the two most prevalent failure modes of relays, as mentioned below: a. Early life failures are often caused by contamination.

How do you check for a short circuit with a multimeter?

Preparation and Safety for Finding a Short Circuit with a Multimeter
Select Continuity or Resistance on the Multimeter.
Check the Multimeter’s Function.
Find and identify the circuit component.
Insert the Probe Tips into the Circuit.
Examine the Multimeter’s Display.

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