Which Three Criteria Should You Consider When Choosing a Relay?

I’ve had my fair share of selecting relays in the past, and I always found it challenging to cover all aspects. However, after some experience, I’ve narrowed it down to three main criteria that should guide your decision.

1) The primary technical performance of the relay, such as contact load capacity, switching time, mechanical and electrical lifespan, must meet the requirements of the overall system. It's not just about meeting specs—it’s about ensuring reliability over time.

2) The physical structure and dimensions of the relay must be suitable for the application environment. This includes how it’s mounted and whether it fits within the space constraints of the design.

3) Cost-effectiveness is also important. While high-quality components are essential, you should look for a balance between performance and affordability. Sometimes, a slightly more expensive relay can save money in the long run by reducing failures or maintenance.

What Causes Unstable Relay Operation?

Based on my own experiences, I believe unstable relay operation usually comes from three main factors.

1) Unstable power supply. This can be due to: 1) excessive ripple voltage; 2) insufficient voltage; 3) coil resistance deviation. A poor power supply can lead to inconsistent activation and deactivation.

2) Unstable relay parameters. This might happen if the relay is dropped or exposed to strong shocks, or if there’s a short circuit in the coil. These issues can affect the internal mechanics and cause erratic behavior.

3) Relay malfunction. This often occurs due to improper programming or environmental stress, such as extreme temperatures or humidity. It's important to ensure the control logic is well-designed and the operating conditions are within the relay's specifications.

Why Is the Relay Not Releasing?

In my opinion, there are generally two main reasons why a relay fails to release. One is related to the coil itself, and the other is a fault in the circuit.

If the residual voltage at the coil is too high, the relay may not release properly. There are three possible causes for this: 1) energy storage components on the other end of the coil, like capacitors, may retain charge; 2) there could be leakage current or bypass current flowing through the coil; 3) the remaining voltage from a semiconductor circuit might be too high.

Additionally, circuit faults can also prevent the relay from releasing. For example, if the wiring is loose, damaged, or subjected to mechanical shock, or if there are contact problems in the circuit, the relay may not function as intended. Regular inspection and proper installation can help avoid these issues.