Electronic – 12V Relay Voltage Surge on contact close

When the relay closes, the charging of the 1000-uF/50V capacitor can create a large inrush current -- assuming that the 24-VDC power supply has a large output capacitor that can supply the inrush current. This inrush current could exceed the relay ratings and weld the contacts.

Some possible solutions are the following:

(1) Use a much smaller capacitor value in place of the 1000-uF cap if you don't need the extra holdup time. The LM340T-5 datasheet only calls for a 0.22-uF capacitor.

(2) Add some series resistance before the 1000-uF capacitor to limit the inrush current. Since the contacts have a 10-amp rating, you only need a few ohms to limit the inrush at 24 volts. You may need to check the pulse-energy rating of the resister because now the resistor will absorb the inrush energy: (24V^2)(1000 uF) / 2 = 0.29 watt-seconds

(3) Use a relay rated for a tungsten load, which can handle a high inrush. The inrush current should still be checked against the relay limits (either measure the inrush or calculate the inrush from the capacitor ESR value).

The single pulse surge rating is usually a 'maximum stress level' parameter that should only occur under adverse conditions and not in normal operation. If you expect it to occur regularly then the diode's reliability may suffer.

I would use a diode rated for at least double the expected surge current. Heating shouldn't be a problem if it only has to operate once every few seconds. The inductive current will probably decay within a few hundred milliseconds of the solenoid being turned off.

A couple of 60A diodes in parallel should do the job. These are readily available from most suppliers. If you can't find any suitable diodes locally then perhaps you could try scavenging some from old PC power supplies.