Relays are essential components in many electrical and electronic systems. They allow a low power circuit to switch a higher power circuit on and off. Knowing how to properly test a relay is important for verifying that it will operate reliably in your application.
Why Test a Relay?
There are a few key reasons why you should test a new relay before putting it into service:
- Confirm the relay meets the electrical specifications you require
- Check for any defects or damage from manufacturing or shipping
- Ensure the relay operates properly when energized and de-energized
- Determine if the relay contacts have enough load capacity for your application
- Identify any potential failure modes through burn-in testing
While relays are very reliable components, they can still fail over time. Testing helps catch any issues before the relay gets integrated into an assembly where a failure could cause bigger problems.
Visual Inspection
The first step in testing a new relay is a basic visual inspection:
- Check for any cracks, chips or damage to the relay housing
- Verify the coil terminals and contact terminals are straight and undamaged
- Make sure the relay base is clean and free of contamination
- Confirm the relay model number or markings match the specifications you require
This simple check can reveal physical defects that could cause premature failure down the line. If any physical damage is spotted, the relay should be replaced.
Coil Resistance Test
The coil resistance should be measured to confirm it matches the expected value:
- Set a multimeter to the ohms setting
- Connect the probe tips to the coil terminals
- Note the resistance reading – it should be within 10% of the coil rating
This verifies the coil winding is intact and has close to the expected electrical resistance. Any significant deviation could indicate a shorted or open coil. Relays with an improper coil resistance should not be used.
Coil Continuity Test
A coil continuity test can also identify any open or shorted conditions:
- Set a multimeter to the continuity setting – this checks for a complete circuit
- Connect the probe tips to the coil terminals
- The multimeter should beep, flash, or read 0 ohms in the closed state
- An open circuit will not provide continuity
This provides quick feedback that electricity can properly flow through the complete coil circuit. An open coil will prevent the relay contacts from actuating.
Insulation Resistance Test
Checking the insulation resistance verifies there are no conductive paths between the coil and contacts:
- Obtain an insulation resistance tester (megohmmeter)
- Connect one probe to the coil terminal and the other to all the contact terminals shorted together
- Apply a 500V DC test voltage and note the insulation resistance reading
- The resistance should be greater than 100 megohms
Low insulation resistance indicates contamination, moisture, or physical defects allowing current to bypass the open contacts. This could lead to false triggering of the relay.
Dielectric Withstand Voltage Test
Subjecting the relay to high voltage can reveal any insulation breakdown:
- Use a hipot tester capable of slowly ramping up to 1000VAC
- Connect one probe to the coil terminal and the other to all contact terminals shorted
- Gradually increase the test voltage to 1000VAC while monitoring leakage current
- There should be negligible leakage current (less than 1 mA)
If the insulation breaks down, significant current will flow, indicating a failure to isolate the coil and contacts at high voltages. This could lead to false triggering or damage to the coil or contacts.
Contact Resistance Test
Measuring the contact resistance in the closed state verifies the contacts are in good condition:
- Use a digital ohmmeter capable of measuring milliohms
- Connect the probes across a closed set of contacts
- The resistance should be less than 100 milliohms for standard electro-mechanical relays
High contact resistance can cause overheating of contacts and reduce the maximum switching load. This helps determine if the contacts are damaged or contaminated.
Operate Time Test
The operate time indicates how fast the contacts switch from open to closed when the coil is energized:
- Use an oscilloscope to monitor the voltage across closed contacts
- Quickly energize the relay coil with a voltage source
- Measure the time from 10% to 90% of the voltage rise
- Typical operate times range from 2-15 milliseconds
A slower than expected operate time may indicate issues with the relay mechanism or contacts. The operate time could increase further as the relay ages.
Release Time Test
The release time indicates how fast the contacts switch from closed to open when the coil is de-energized:
- Monitor the voltage across closed contacts with an oscilloscope again
- Energize the coil, then quickly remove power to de-energize it
- Measure the time from 90% to 10% of the voltage drop
- Typical release times range from 2-20 milliseconds
A longer than expected release time could prevent a fast-changing signal from breaking the contacts in time. This points to sluggish contact movement upon deactivation.
Contact Bounce Test
When the relay contacts close or open, they may bounce before settling:
- Monitor the voltage across the switching contacts with an oscilloscope
- Operate the relay coil and observe the voltage waveform
- Contact bounce appears as rapid oscillations in the voltage signal
- Typical contact bounce lasts from 1-5ms after switching
Excessive contact bounce can cause issues in timing sensitive circuits. In extreme cases, arcing can occur and damage the contacts.
Mechanical Life Test
Repeated operation of the relay determines how long it lasts mechanically:
- Construct a circuit to cycle the relay coil on and off at its rated voltage
- Monitor the contact resistance periodically during cycling
- Stop when the contact resistance exceeds the maximum rating
- Typical life expectancy is 100,000-1,000,000 cycles
This confirms the relay can withstand mechanical wear to meet the expected lifetime target. Batch testing multiple samples also reveals early life failures.
Load Switching Test
Testing the ability of the contacts to switch a load validates the current and voltage ratings:
- Assemble a circuit with a power supply and load (resistor, motor) in series with the relay contacts
- Energize the coil to close the contacts and allow current to flow through the load
- Start at a low load and slowly increase toward the relay rating, monitoring contact heating
- Stop at any sign of overheating, arcing, or failure to open/close properly
This confirms the contacts can withstand repeated switching at the expected electrical load without damage over time.
Overload Testing
Applying overload conditions reveals the breaking capacity of the contacts:
- Assemble a test circuit with the relay contacts interrupting a current source beyond the relay rating
- Energize the coil to close the contacts, allowing overload current to flow
- Interrupt the control circuit to de-energize the coil and open the contacts
- Repeat this process at increasing overload levels until contact failure occurs
This helps determine the true fault current breaking ability. However, repeated overloading will damage the contacts.
Environmental Testing
Testing relays under various environmental conditions can reveal weaknesses:
- Temperature – Operate the relay across the rated temperature range and check for changes in operate time, contact resistance, insulation resistance, etc.
- Vibration – Subject the relay to vibration while energized to ensure contacts remain properly closed.
- Shock – Apply mechanical shock or drop testing to verify the relay sustains no damage.
- Humidity – Expose the relay to humid environments and inspect for reduced insulation resistance or corrosion issues.
This provides confidence that the relay construction is robust enough for the expected operating environment. Temperature and vibration testing are especially useful.
Burn-in Testing
Running relays for an initial burn-in period helps eliminate early failures:
- Energize relays continuously for 24-48 hours at rated voltage
- Periodically check parameters like coil resistance and contact resistance
- Discard any units that exhibit deviations or excessive drift
- Only proceed with relays that pass the burn-in period
Infant mortality failures will often arise during burn-in testing. This prevents premature failures in the end application.
Common Relay Testing Equipment
Here is some of the equipment commonly used for testing relays:
| Equipment | Typical Models | Purpose |
|---|---|---|
| Multimeter | Fluke 87, AstroAI DM6000A | Measure resistance, continuity, voltage, current |
| Insulation Tester | Fluke 1507, Megger MIT515 | Measure insulation resistance |
| Hipot Tester | Associated Research 7245 | Perform dielectric withstand testing |
| Oscilloscope | Rigol DS1054Z, Siglent SDS1104X-E | Time electrical switching waveforms |
| Timer/Counter | Extech 365515 | Precisely measure operate and release times |
Investing in quality testing tools helps ensure relays are thoroughly evaluated and performing optimally before deployment.
Conclusion
Rigorously testing relays verifies they meet all critical electrical and mechanical parameters for the application. Visual inspection, coil resistance/continuity checks, insulation resistance, high voltage withstand, contact resistance, operate/release timing, contact bounce, load switching, burn-in testing, and environmental stress screening should be considered. Following structured test procedures and using the right equipment helps maximize reliability and prevent field failures.