Coil relay mod with a twist

[MrZig]

Has anyone had their relays fail closed?

I put a 30 amp relay in my Jeep to run an electric fan and every few months it'll stick closed and keep the fan turning when the trigger power source is disconnected. If I tap the relay it fixes this.

I suppose it could just be a faulty relay and the vast majority don't have this problem. Would suck to have it happen on a bike and have it kill your battery or worse.

[p_s]

P_S with three failures you should rethink your relay mounting strategy

They were all hanging loose on zipties. The first two were expensive (relatively) German relays that died after 3 blocks. The 3rd was a cheap Chinese relay & lasted 3k miles and then died on a freeway onramp in Seattle in morning rush hour. If it had failed 2 minutes later, I would have caused major congestion on a major bridge. I bought a new ignition switch after that, which solved the voltage drop problem.

Since November I've been using a relay for my heated grips that I rubber mounted. I'll see if it lasts longer.

[reddirtrider]

Has anyone had their relays fail closed?

I put a 30 amp relay in my Jeep to run an electric fan and every few months it'll stick closed and keep the fan turning when the trigger power source is disconnected. If I tap the relay it fixes this.

I suppose it could just be a faulty relay and the vast majority don't have this problem. Would suck to have it happen on a bike and have it kill your battery or worse.

Relays can fail closed due to arcing that welds the contacts together. Usually it's due to high inductance loads (your motor is one such load). These problems can be minimized by adding a free wheeling diode around the load. The diode can then supply the current the inductor needs when power is removed. Inductors cannot change current instantaniously and breaking their current with a relay will cause an arc.

[MrZig]

Relays can fail closed due to arcing that welds the contacts together. Usually it's due to high inductance loads (your motor is one such load). These problems can be minimized by adding a free wheeling diode around the load. The diode can then supply the current the inductor needs when power is removed. Inductors cannot change current instantaniously and breaking their current with a relay will cause an arc.

So you're saying that when power is shut off, the electromagnetic field that the motor generates sends off a voltage spike through it's wiring? Or perhaps maybe when I'm on the highway and the fan is turned off, yet rushing air is spinning the motor, and I turn it back on. Same concept. I'm probably wrong, but I'm not an electrical engineer. It interests me, though.

Now, because the coil is an inductive method, is there any possibility the coils could create such an effect? I don't think so, but then this is out of my league.

[reddirtrider]

So you're saying that when power is shut off, the electromagnetic field that the motor generates sends off a voltage spike through it's wiring? Or perhaps maybe when I'm on the highway and the fan is turned off, yet rushing air is spinning the motor, and I turn it back on. Same concept. I'm probably wrong, but I'm not an electrical engineer. It interests me, though.

Now, because the coil is an inductive method, is there any possibility the coils could create such an effect? I don't think so, but then this is out of my league.

I'm not sure of the inductances of the coils, so I can't say if they would cause this arc. If you want to be absolutely sure there is no arcing, place a diode from ground to the relay on the coil side. The anode should be ground and the cathode tied to the coil.

What happens when the relay is opened is that the voltage across the coil is reversed and the diode provides the coil the current it demands. The coil cannot immediately stop conducting current, so you need to let it die out on it's own. This is pretty standard stuff when dealing with inductive loads.

You can try the same thing on your motor load and see if it makes any difference.