I have a transistor, driving a normally open reed relay (protecting transistor with flyback diode), driving a solenoid. The solenoid is actuating at about 3 Hz.
The relay is good for a few million actuations. And then, it mysteriously stops working. Replacing the relay works, for another few million actuations. I conclude that the problem is not the transistor or the solenoid.
The nature of the failure is somewhat peculiar. At first, it appears to be a stuck closed failure; the relay will not open when the transistor is not driven. When power is removed from the relay, it then opens. When power is re-applied, the relay stays open, until the transistor drives it, at which point it becomes stuck closed again.
I tested one of the "dead" relays. The coil exhibits the correct resistance, 250 ohm. The reed switch is also open.
I would have expected this to be a failure where the contacts are welded together, but the fact that a "dead" relay is open and that it only sticks after it has first been energized lead me to believe there is something else going on. Does anyone have an explanation for this failure?
EDIT: Addressing some comments
Using the reed relay because that's what we have already, this is a small one-off project. The relay is a Radio Shack relay, SPST 5VDC 20 mA nominal, rated for 500 mA 125 VAC/60VDC. I believe the equivalent relay (OMR-106H) can be found on digikey here http://www.digikey.com/product-detail/en/OMR-106H,V000/PB886-ND/1095211. The DC power supply for the solenoid (12V) shows about 400 mA peak current. The datasheet lists 5 million "operations" at 24V 400 mA. Two relays have now died around 3 million at 12V.
The diode is across the coil, not the switch. The literature I have read indicates that the diode is usually for the protection of whatever drives the reed relay's coil.
Best Answer
Every relay has a life expectancy... they are not infinite. You are saying that at 3 hertz you activate it a few million times, that is a lot of activations for a relay.
As you have not provided a part number, for illustrative purposes, I have used a generic one. The datasheet of your relay should have a graph like this one...
As you can see, in this relay particularly, the maximum life operations it can have no more than a million and a half operations...
That type of failure is normal in relays because the contact material starts wearing away with normal operation, and if you operate at the maximum rated load, as you can see in the table, the relay will have no more than 30,000 operating cycles.
I hope this information is clear enough.