OT: Condenser Capacitance Value

Any suggestions are welcome.

-VinnieNC

As KT and KEB said, a better solution is an anti-kickback diode. KEB described the correct way to connect it; it must be connected in parallel to the load, not the relay. And it must have a very high current rating, as Gerald said. If your horn draws 3 amps, you might get away with a 1N400x, which is rated at 1 amp. If it draws 10 amps, I doubt it will last. (Of course, you'll never know if the diode is blown, unless you disconnect it and test it!) You can get 3 amp silicon rectifier diodes (e.g. 1N5402) with axial leads that are a better choice.

Really, if you have a sufficiently big relay, you don't need to worry about arcing. After all, your starter relay routinely handles several hundred amps, and you don't see capacitors or diodes hanging off it. Also, I think the current through a horn is basically ac (pulsed dc, to be correct); if you look on most switches and relays you'll find that they have much higher ac ratings than dc, because ac interrupts itself.

A better, more commonly used approach is to use a diode as a "back-emf" suppressor on the load you're switching. Simply connect a rectifier diode, like a 1N4004 or something similar, across the load (not across the contact points), with the banded end connected to the positive side of the load and the non-banded end connected to the negative (grounded) side. If you really want to use a capacitor across the contacts, any disk capacitor in the 0.01 to 0.1 uF range would work, but in general its better to suppress the source rather than try to protect against possible effects.

The inductive "kick" from switching an inductive load is always the opposite polarity of the applied power, so the diode shorts the inductive spike but does not normally conduct.

What you're trying to do isn't quite analogous to a kettering ignition. Here, you want to suppress the effects of the back emf spike. In a kettering ignition, you want to use that spike to make a spark in the cylinder. In addition to limiting wear on the points, the condenser in a kettering ignition is part of a resonant circuit which "rings" when the points open, resulting in a hotter spark than you would get from the collapsing magnetic field in the coil alone.

Do a web search on "back-emf suppression" for more information than you'll ever be able to use. Actually, I seem to recall automative horns coming with suppression already installed, but can't be sure.

By the way, the comment below that a current surge will cause a capacitor to explode is not true. What typically damages a capacitor is a VOLTAGE spike high enough to puncture the dielectric material between the capacitor plates. Once this path is established, how much current flows depends on how much current is available from the source of the voltage spike; if the source impedance is low enough, it may be possible to drive enough current through the capacitor to cause it to overheat and rupture. It is possible to overheat a capacitor by trying to force too much alternating current through it, but this generally requires a sustained current source, not a surge.

Keith

You can get one at Radio Shack. It would be a general purpose 1N4001 up to 1N4007 diode.

Gerald J.

Keith

Gerald J.

I should have added the other details. I am wiring a 12 Vdc car horn through a relay and want to get as much life from the relay contacts as possible.

It's a shame tractors don't have a horn as "standard" equipment!

-VinnieNC

Talk to your electrical supplier and see what they use on there large switches.
Walt

What are the relay contacts powering... a large inductive load?

Typical battery ignition condensor capacity is .25 mfd.