How to Test a Relay – Step by Step!

Relays are electronic devices commonly used today and extremely important, knowing this in this article we will explain what a relay is, what are the applications of a relay and what are the most common types of relays, in addition to explaining step by step how to test a relay!

What is a relay?

The relay is an electromechanical device, whose purpose is to switch an electrical circuit. Its operation is very similar to a common lamp switch, but the difference is that the relay is not activated manually, but electrically, through a coil present inside. In a simple way, when the coil is submitted to an electric current the relay changes its state from “On” to “Off” or vice versa, allowing the switching between two signals that do not mix, leaving the same electrically isolated other.

Relay Applications

The main characteristic of relays is that they can be energized with a very small current, when compared to  the nominal current of the circuit to be controlled, enabling the control of high current circuits, using as part of their composition some semiconductor devices. This is one of the most important factors, which makes relays very common in the industry and in automobiles, in addition to household appliances, especially when there is an electronic control that turns on or off something like, for example, an engine or a lamp.

Relay Types

Relays are classified according to the technology used in their construction. Currently, three generations of relays can be found, each with its own structural variations.

  • Electromechanical relays – First generation
  • Solid State Relays – Second Generation
  • Digital relays – Third generation

There are countless variations of relays, each modified according to its purpose, but in general, its function is basically the same, to turn off or turn on some electrical circuit.

How to Test a Relay?

Necessary material:

  • Digital multimeter
  • Voltage source

Before starting any test, for safety reasons, it is advisable to consult the electrical or manual diagram of the relay.

The pin configuration of the device is usually standardized, but it is wiser to look for the manual (“data sheet”) to obtain more genuine information directly from the manufacturer, some of this information, the number of pins, for example, can usually be found printed on the relay.

Initial procedure

Perform basic visual analysis of the device. A large part of the relays available on the market have their transparent housing, which houses the coil and contacts. By identifying more visible damage, you can limit the possible causes of the problem.

Most current relays have an LED that indicates whether the device is turned on. If the LED is off, even if the relay or the coil terminals are being powered (normally A1 and A2), it can be safely said that the relay is damaged.

Disconnect the relay from the power source. Any and all activities related to electricity must be carried out with all power sources turned off. Particular attention should be paid to the capacitors of the circuit in question, they are capable of storing charge for a considerable period, even after removing the power source.

Testing an electromechanical relay

For an electromechanical relay, it is necessary to determine some requirements for carrying out the test. The part number, also known as “Part Number” or PN, must be printed on the relay housing. The coil operating current and voltage must be determined. This information can also be printed on the packaging, otherwise it can be obtained through the manual. Also check that the control coil is protected by a diode whose purpose is to protect it from damage caused by voltage variations.

Assess the arrangement of the contacts, which will be referenced in the manufacturer’s manual or even printed on their housing. The relays have one or more poles identified in their diagram by a selection switch on the line, connected to one of their pins. Each pole can be classified as normally open (NO) or normally closed (NC).

The diagram will show each of the poles connected to the pin, which indicates a contact “NC”, or not connected, indicating a contact “NO”. Use a digital multimeter to determine the resistance at each pole and its corresponding contact. The “NC” contacts must have a resistance of 0 ohm and the “NO” contacts must have an infinite resistance


Electromechanical Relay Test

Use an independent voltage source, make sure the coil is protected by diode, check that the indicated source is connected with the proper polarity. When energizing it, it will make a small noise, a “pop”. Then use the multimeter to test the resistance of the contacts.

Testing a solid state relay

This type of relay requires special attention, when shorted, there is a high chance of damaging the device. Therefore, the tests must be done with a de-energized component, the ohmmeter must be connected to the normally open (NO) contact terminals.

Testing a Solid State Relay

When a control voltage is applied, the multimeter should exhibit infinite resistance.

Adjusting the multimeter in the “diode test” mode, place the probes at the ends A1 (+) and A2 (-). The device will apply a minimum voltage to conduct the semiconductor, if the component is damaged, the multimeter will display 0 or an open circuit indication. Otherwise, it will show the respective diode voltage.