Relays are separate electrical devices (unlike integrated circuits) that are used to allow a low logic electrical signal to control a circuit with higher power. The relay isolates the high power circuit while protecting the low power one thanks to a small electromagnetic coil that acts as a logic criterion. You can learn how to test both the coil and a solid state relay.
Steps
Method 1 of 3: Introduction
Step 1. Check the relay diagram or datasheet
This device typically has a standard pin configuration, but it is best to do some research with the manufacturer to find out more about pin numbers if possible. Generally this data is stamped on the body of the relay itself.
- Information about current strength and potential difference, together with pin configuration and other similar data, are often available on the datasheet and are extremely useful, as they help eliminate most of the errors associated with the test. You can randomly check the pins without knowing their arrangement, but if the relay is damaged, the results are unpredictable.
- Some relays report this information directly on their external structure (based on the size of the relay itself).
Step 2. Perform a basic visual inspection
Many devices have a clear plastic outer shell that holds the coil and contacts. Obvious damage (melted or blackened points) greatly reduce the range of possible anomalies.
Most modern relays are equipped with an LED that "informs" if the device is active. If the light is off and there is control voltage connected to the component or coil terminals (typically A1 [line] and A2 [common]), then you can confidently tell that the relay is damaged
Step 3. Disconnect the electricity source
Any intervention on the electrical components must be carried out after disconnecting the energy source, including the batteries or the system. Pay special attention to capacitors, as they can accumulate a considerable charge for a long time even after removing the source of electrical current. Do not attempt to discharge them by shorting them.
You should always consult the municipal ordinances before proceeding to work on the electrical system and, if you are unsure, let a professional take care of it. Generally, interventions on circuits with low potential difference should not be subject to any legislation, but it is always worth staying safe
Method 2 of 3: Test the Coil
Step 1. Find the relay coil parameters
The manufacturer's serial number should be stamped on the outer body of the element. Consult the datasheet to determine the voltage and current strength of the control coil. This data is typically printed on large components.
Step 2. Check if the control coil is protected by a diode
A diode is typically installed around the pole to protect the logic circuit from damage caused by voltage spikes. This element is represented in the wiring diagrams as a triangle with a crossbar at one of the corners. The bar is connected to the current input port - or positive terminal - of the control coil.
Step 3. Determine the relay connection configuration
For this purpose you can consult the manufacturer's data sheet, but in other cases the information is printed directly on the larger components. The relays can have one or more poles, which are indicated in the wiring diagrams as individual line switches connected to one of the terminals of the relay itself.
- Each pole has a normally open (NO) or normally closed (NC) contact. The diagram indicates these types of contacts as connections to the relay contacts.
- The diagram shows the terminal in contact if it is normally closed, or a contactless terminal if it is of the normally open type.
Step 4. Verify the unexcited condition of the relay terminals
To do this you must use a digital multimeter that allows you to measure the resistance between each pole of the device and the corresponding NC and NO terminal. All NC terminals should have a resistance of 0 ohms with the corresponding pole. All NO terminals should report infinite resistance with the corresponding pole.
Step 5. Energize the relay
Use an independent source of potential difference that respects the coil capacity. If this is protected by a diode, make sure the power source is connected in the right polarity. You will hear a "click" when the relay is energized.
Step 6. Check the excitation conditions of the relay terminals
Use a digital multimeter to detect the resistance between each pole and the corresponding NO and NC terminals. All NC terminals should report infinite resistance with the corresponding pole, while all NO contacts should report a resistance value of 0 ohms.
Method 3 of 3: Test the Solid State Relay
Step 1. Use an ohmmeter to check the solid state relays
When one of these devices short out, it almost always gets damaged. For this reason the relays should be checked with an ohmmeter by connecting them to the NO terminals when the control power source is turned off.
The relay should be open, set to OL, and then closed when the control current is applied (the internal resistance of the ohmmeter is 0.2)
Step 2. Use a multimeter set to "diode" mode to confirm the results
You can be sure that the relay is damaged by having a multimeter set to "diode" which connects to terminals A1 (positive) and A2 (negative). The instrument will apply a small potential difference to activate the semiconductor, so that the values can be read on the display. In this way it is possible to control the transistor (usually of the NPN type) from the base (p) to the emitter.
If the relay is damaged, the instrument will report a value equal to 0 or the OL overload symbol; a relay in good condition, on the other hand, will report a value of 0.7 for silica transistors (the material of which almost all transistors are made) or a value of 0.5 for germanium transistors (which are particularly rare, but not unusual)
Step 3. Make sure the solid state relay stays cool
This relay model is easy to test, inexpensive to replace, and will last a long time if kept at the right temperature. New relays are typically included in the DIN rail and mounting block packages.
