How to ohm out a relay

Overview

A relay is a crucial component in many electrical and electronic systems, acting as an electrically operated switch. It allows a low-power circuit to control a higher-power circuit, providing isolation and enabling complex control logic. Understanding how to test a relay using a multimeter, often referred to as 'ohming out' a relay, is essential for diagnosing electrical faults and ensuring proper system functionality. This process involves measuring the electrical resistance of different parts of the relay to determine if they are operating as expected.

What is a Relay?

At its core, a relay consists of an electromagnet (the coil) and a set of switch contacts. When a voltage is applied to the coil, it generates a magnetic field that attracts an armature. This armature, in turn, moves the switch contacts. Relays are categorized by the type of contacts they have: normally open (NO), normally closed (NC), and changeover (CO). NO contacts are open when the coil is de-energized and close when energized. NC contacts are closed when de-energized and open when energized. CO contacts provide both NO and NC connections, switching from NC to NO when energized.

Why Ohm Out a Relay?

Testing a relay is important for several reasons:

• Troubleshooting: If a circuit controlled by a relay is not functioning, the relay itself might be faulty. Ohming it out is a primary diagnostic step.
• Preventative Maintenance: Regularly testing relays in critical systems can help identify potential failures before they occur.
• Verification: After replacing a relay, testing the new one ensures it's functioning correctly and is the correct type for the application.
• Understanding Operation: For hobbyists and students, ohming out a relay is a hands-on way to understand its internal workings.

Tools Needed

The primary tool for ohming out a relay is a digital multimeter (DMM). Ensure your multimeter has a resistance (ohms, $\Omega$) setting. You may also need the relay's datasheet or specifications to know the expected resistance values, although general guidelines can often be applied.

How to Ohm Out a Relay: Step-by-Step

Step 1: Identify Relay Terminals

Relays have several terminals. Typically, you'll find:

• Coil Terminals: These are usually paired and marked with symbols like 'A1'/'A2', '1'/'2', or 'Coil'. The number of turns and gauge of wire in the coil determine its resistance.
• Contact Terminals: These are related to the switch function. Common configurations include:
• Normally Open (NO): Marked with 'NO', '10', '13', '14', etc.
• Normally Closed (NC): Marked with 'NC', '9', '10', '11', '12', etc.
• Common (COM): Marked with 'COM', '8', '5', etc.

Refer to the relay's datasheet or markings on the relay itself for precise identification. If the relay is installed in a socket, you can often identify the terminals by tracing wires or using the socket's labeling.

Step 2: De-energize the Relay

Ensure the relay is not powered. If it's part of a larger circuit, disconnect the power supply to that circuit. For relays removed from a circuit, this is not an issue.

Step 3: Test the Coil Resistance

This is the first and often most critical test.

1. Set your multimeter to the resistance mode ($\Omega$). Start with a range that can measure from a few ohms up to a few kilo-ohms (k$\Omega$).
2. Place the multimeter probes on the two coil terminals.
3. Observe the reading:
   • Expected Reading: A healthy coil will show a resistance value. This value varies greatly depending on the relay, but common values range from around 50 $\Omega$ to several k$\Omega$. For small signal relays, it might be higher, and for power relays, it could be lower. Consult the datasheet if available.
   • 'OL', Infinity, or Very High Reading: This indicates an open circuit in the coil, meaning the coil is broken. The relay will not operate.
   • 0 $\Omega$ or Very Low Reading: This suggests a short circuit within the coil. This is also a fault condition.

If the coil resistance is significantly different from the expected value (e.g., double or half), the coil might be damaged, though it might still function. A reading that is extremely high or 'OL' indicates a definite failure.

Step 4: Test the Contacts

This test verifies that the switch contacts are opening and closing correctly.

Testing Normally Open (NO) Contacts:

1. Keep the multimeter on resistance mode.
2. Place probes on the Common (COM) terminal and the Normally Open (NO) terminal.
3. With the coil de-energized: The reading should be 'OL' (open circuit), indicating the contacts are not making a connection.
4. Energize the coil: Apply the correct voltage (check the datasheet) to the coil terminals. You might need to use a separate power source or carefully energize the circuit if the relay is still installed. Caution: Ensure you know what you are doing when applying voltage.
5. With the coil energized: The reading should drop to near 0 $\Omega$ (or a very low resistance, typically less than 1 $\Omega$), indicating the contacts have closed and are conducting.
6. De-energize the coil: The reading should return to 'OL' as the contacts open.

Testing Normally Closed (NC) Contacts:

1. Place probes on the Common (COM) terminal and the Normally Closed (NC) terminal.
2. With the coil de-energized: The reading should be near 0 $\Omega$ (or very low resistance), indicating the contacts are closed and conducting.
3. Energize the coil: Apply the correct voltage to the coil.
4. With the coil energized: The reading should become 'OL' (open circuit), indicating the contacts have opened and are not conducting.
5. De-energize the coil: The reading should return to near 0 $\Omega$ as the contacts close.

Step 5: Interpreting Results

After performing these tests, you can determine the state of the relay:

• Good Relay: Coil has expected resistance, and contacts switch correctly (low resistance when closed, high/OL when open).
• Bad Coil: Open circuit ('OL') or short circuit (0 $\Omega$) on coil terminals.
• Stuck Contacts: Contacts remain closed ('OL' reading when they should be low, or low reading when they should be 'OL') regardless of coil state.
• High Contact Resistance: Contacts show a low but noticeable resistance (e.g., > 1 $\Omega$) when closed. This can indicate pitting or corrosion on the contact surfaces and may lead to overheating or intermittent operation.
• Intermittent Faults: Sometimes, a relay may pass these tests but still fail under load or vibration. This can be due to internal connections or contact issues not revealed by simple resistance measurements.

Important Considerations

• Relay Type: The testing procedure is generally the same for most electromechanical relays, but specific resistance values and coil voltages will vary.
• Solid State Relays (SSRs): SSRs are tested differently as they use semiconductor devices. They typically don't have a coil resistance to measure in the same way and are often tested by applying control voltage and checking for output voltage.
• Load: Resistance tests are performed with no load connected to the contacts. Testing under load requires a different approach, often involving voltage measurements while the circuit is operational.
• Datasheet: Always try to find the datasheet for the specific relay model you are testing. It provides the most accurate information on coil resistance, coil voltage, and contact ratings.

By following these steps, you can effectively 'ohm out' an electromechanical relay and diagnose its condition, ensuring the reliability of the systems it controls.