Simple guide to getting the best reliability out of power relaysWe often hear of problems in industry where relays seem to be failing even though the relay rating would suggest that they are adequately rated. The failure of these relays is generally due to one or more of the following reasons:
- Incorrect contact material choice. There are many different contact metalurgies which are suitable for different load types. Read the relay data sheet very very carefully. For open frame or ventilated relays, gases in the environment also can affect contact performance.
- Not taking into account start up current levels. Motors, lamps and capacitive loads can have inrush currents many many times greater than the steady state running current. The poor relay contact has to deal with this when the contacts close. For example, it is not unusual for incandescent lamp loads to draw 20 times the steady state current at turn on. Again read the relay data very carefully and check what the approvals bodies (UL, VDE etc) rate them for specific load types.
- Relay drive. How you drive the coil of a relay can have a direct bearing on the lifetime of the contact. Ideally we want to turn the coil on and off in the fastest posible way to minimise arcing of the relay contacts. While turning a relay on is not usually a problem, care has to be taken with turning the relay off if driven from electronics. For instance, we recently ran an in-house test where we had two identical 12 Volt relays switching identical DC lamp loads. Both were driven electronically. One relay had a diode across the relay coil to protect the electronic output from the back EMF of the relay coil, while the other had a 36 Volt transient suppressor fitted (electronic output was rated for 45 Volts). The relay with the diode failed after about 50,000 cycles while the other relay kept going and going and going, as was still going at 300,000+ cycles. This is because the diode delayed the demagnetization of the relay coil, causing the relay contacts to part more slowly, causing more arcing.
Relays can also suffer from too little current. Click here for more...