Relay switching contacts do not open

Holms

Relay switching contacts do not open [Copy link]

 

I used a relay to switch the positive pole of the 24V power supply. After using it for a while, I found that when I disconnected the relay voltage, the relay was electrocuted but not disconnected.

Then after powering on and off the relay several times, the contacts can open and work normally again.

The relay used is APAN3124

What is the reason that the contacts are not opening?

maychang

Are the contacts that do not open carrying a heavy or inductive load? If so, it is possible that the contacts are slightly stuck together.

qwqwqw2088

If there is a quality problem with the relay, you can check whether the contacts melt and stick instantly.

Or there is voltage in other energy storage components at the coil end that has not been released.

btty038

Is it possible that there is still some power left?

Holms

maychang posted on 2019-10-8 19:20 Do the contacts that are not disconnected carry a heavy load or inductive load? If so, it is possible that the contacts are slightly stuck together.

The load side capacitor is about 470uF. Is this considered heavy?

maychang

Holms posted on 2019-10-9 10:55 The load side capacitor size is about 470uF. Is this considered heavy?

Whether the load is considered heavy depends on the contact capacity of your relay.

For capacitive loads, a large current will flow when the contacts close. This large current may cause a small part of the contact to melt and cause adhesion.

chunyang

The contact capacity needs to be increased, especially for cheap relays, the margin needs to be increased.

Holms

chunyang posted on 2019-10-10 13:58 The contact capacity needs to be increased, especially for cheap relays.

I checked the contact capacity of the relay: 3A/30V can be used 100,000 times, but when I use it to switch a working current of less than 100mA and a working voltage of 24v, the contacts become sticky. I am a little confused.

Holms

maychang posted on 2019-10-9 11:02 Whether the load is considered heavy depends on the contact capacity of your relay. For capacitive loads, a large current will flow when the contacts are closed. This large current may...

I checked the contact capacity of the relay: 3A/30V can be used 100,000 times, but when I use it to switch a working current of less than 100mA and a working voltage of 24v, the contacts become sticky. I am a little confused.

maychang

Holms posted on 2019-10-10 18:01 I checked the contact capacity of the relay: 3A/30V can be used 100,000 times, but I use it to switch the working current less than 100mA, 24v working voltage contact...

The method to check whether the contacts are sticking is very simple: if you find that the relay voltage is disconnected, but the relay contact does not disconnect, flick the relay with your fingernail or tap it with tweezers. If the contact disconnects after flicking or tapping, it means that the contact is slightly sticky.

maychang

Holms posted on 2019-10-10 18:01 I checked the contact capacity of the relay: 3A/30V can be used 100,000 times, but I use it to switch the working current less than 100mA, 24v working voltage contact...

Does this happen to just one relay, or do multiple relays experience this "non-disconnecting" situation?

Holms

maychang posted on 2019-10-10 18:30 Is this happening to just one relay, or have multiple relays experienced this "non-disconnecting" situation?

The power supply is connected to four identical loads, so there are four relays. Currently, two of them have been found to have this phenomenon.

As for being able to disconnect with just one tap, I haven't tried it yet. I'll do a test if this happens. Before, I tried to disconnect it by turning it on and off many times.

Now try to connect the inductor at the load line to see if this problem can be solved.

chunyang

Holms posted on 2019-10-10 18:00 I checked the contact capacity of the relay: 3A/30V can be used 100,000 times, but I use it to switch the working current less than 100mA, 24v working voltage contact...

The key is to exceed the transient current limit, you can put the filter capacitor before the contact.

轩焱森

For 24V100mA load, would it be better to use P-MOS, which is cheaper than relays?

轩焱森

Generally speaking, for small currents, you can just connect a small-value resistor in series, calculate the power, and it will be more economical.

zwk34

Is the relay coil connected to the diode?

topwon

Holms posted on 2019-10-10 18:00 I checked the contact capacity of the relay: 3A/30V can be used 100,000 times, but I use it to switch the working current less than 100mA, 24v working voltage contact...

The nominal contact capacity of the relay is generally measured based on a pure resistive load. For capacitive, inductive, incandescent lamps, motors and other loads, the current needs to be derated by dozens of times. If your working current is less than 0.1A and the capacitance of the subsequent stage cannot be reduced, just connect a current limiting resistor in series in front of the capacitor. The resistance and power values need to be calculated. For example, if the space allows the installation of a 2W resistor, then the derated value is half to 1W. According to the maximum current of 0.1A, the maximum series resistance is 100 ohms (the instantaneous resistance power of the power-on = 24^2/100 = 5.76W, because the time is very short, it is not a big problem). The larger the resistance, the smaller the impact current caused by the capacitor charging, and the smaller the damage to the relay contacts, but the greater the power consumed during normal operation, which requires a compromise after experimental testing. For high-current applications, NTC resistors are generally used in series to balance the contradiction between the cold machine startup current (high resistance) and the normal working power consumption (low resistance).

cnc-cut

Connecting a capacitor at both ends of the contact can reduce arcing and the chance of sticking. Pay attention to the withstand voltage.
But this does not increase the capacity of the relay.