As key components in electrical control systems, relays are widely used in automotive, industrial automation, household appliances and new energy sources. However, relays may have various faults during long-term use, resulting in equipment not working properly. This article will systematically introduce the detection methods of relays, including basic detection techniques, professional diagnostic means and the latest patented technology, to help you fully grasp the essentials of relay troubleshooting.
Before in-depth testing, you first need to understand what fault phenomena may occur in the relay:
Contact ablation: high current causes contact oxidation, fusion, and increased contact resistance
Contact sticking: contacts cannot be disconnected due to arc fusion, resulting in continuous energisation of the circuit
Poor contact: contacts are oxidised or worn out, resulting in the circuit being on and off at times
Coil Disconnection: the wire inside the coil is broken, the relay cannot be absorbed.
Coil Short Circuit: the short circuit between turns leads to abnormal resistance and increased absorption current.
Insulation Damage: the coil is short-circuited with the shell, which may lead to leakage.
Armature Stuck: Dust, corrosion, or spring failure causes poor operation.
No Absorption Sound: Coil is normal, but mechanical structure is damaged, unable to produce a "click" sound.
Semiconductor breakdown: output cannot be switched off
Optocoupler failure: control signals cannot be transmitted
Check whether there are cracks, burn marks or deformation of the shell
Observe whether the pins are oxidised and firmly soldered
Shake the relay gently and listen to whether there is any sound of loosening of the internal components.
Coil test
Measure the coil pins (usually marked as 85 and 86) with the resistance gear
Normal: resistance value in tens to hundreds of ohms (depending on the model)
Disconnected: display "OL" or infinite resistance
Short-circuit: resistance value is much lower than the nominal value
Contacts test
Normally closed contact (NC): when not energized, COM and NC should be conductive (close to 0 Ω)
Normally open contact (NO): when not energized, COM and NO should be disconnected (∞) NO: When not energized, COM and NC should be disconnected (∞Ω)
Energized test: After supplying power to the coil, NO should be closed and NC should be disconnected
Energized test method
Apply rated voltage to the coil (e.g., 12V DC)
Listen to whether there is a clear "clacking" sound of suction and closure
Verify whether the contacts are normally switched using a multimeter
Test the suction and release voltages (the normal release voltage should be ≥10% of suction and closure voltages)
Replacement method
When suspecting the relay faults, the most direct way is to Replace the normal relay of the same type and test it.
Judge the contact status by detecting the voltage difference between the input and output of the relay, this method has high detection efficiency and is especially suitable for batch detection occasions.
Observing the coil current waveform and contact switching timing with an oscilloscope can deeply analyse the operating characteristics of the relay and diagnose complex dynamic faults.
Automotive relays
Focus on the current-carrying capacity of the contacts (high currents are easy to burn)
Simulate the vibration of the vehicle during the test to check for intermittent faults
Power relays
Must be load tested to verify stability under high currents
Check whether the arc extinguishing device is intact (to prevent the contacts from burning)
Time relays
In addition to the basic function test, the time-delay accuracy needs to be calibrated
Check whether the timing circuit is normal
Correct Selection: current/voltage margin ≥ 20%
Environmental control: keep dry, dust-free, avoid corrosive gases
Electrical protection:
DC relay coils in parallel with the renewal diode
AC relays with capacitance-absorbing resistive circuits
Regular testing:
Monthly inspection of contact resistance (should be <100mΩ)
Quarterly testing of the suction/release power
Multimeter and Oscilloscope: Simultaneous monitoring of coil current and contact voltage waveforms
Relay Tester: Specially designed for rapid testing of all kinds of relay parameters
Thermal Imager: Detection of abnormal contact hot spots
Relay testing requires a combination of methods, ranging from simple visual inspection and multimeter testing to specialised load testing and proprietary testing techniques. With the development of intelligent testing technology, relay fault diagnosis will be more automated and precise in the future. Mastering these testing skills will not only enable you to quickly troubleshoot equipment, but also prevent potential safety hazards and extend the service life of relays.
For relay detection in complex systems (e.g., new energy vehicles, photovoltaic grid-connected systems), it is recommended to refer to the relevant patented technology, or use special testing equipment to ensure accuracy. When encountering malfunctions that are difficult to judge, professional technicians should be consulted in time to avoid misjudgment leading to greater losses.
