The Stovebolt Site Forums The Shop Area The Electrical Bay Please explain this to me

I'm looking at the horn relay and diagram for my 54 truck. How is it that the relay only has 3 connectors? I would have expected at least 4. Since there is only one power wire coming in to the relay, then there must be a resistor or something inside the relay so that the horn button--presumably connected to the coil inside the relay--is a low current circuit. Is that right? If both the horn button and horn are directly connected to the same power, then what would be the point of the relay? I'm having a minor issue with my horn and I just want to understand the relay internals before I do some testing. Also, everything appears to be the original 6V wiring. Thanks!

well what i can see is one wire comes from the battery, one to the horn, and one to the horn button. thats it Peter

What still isn't making sense to me is that SPST relays are supposed to have two circuits: the higher power circuit that is being switched and the coil circuit that does the switching. So if you have two circuits you need 4 wires. The horn relay has three wires, with only one being power The other two wires lead to ground via the horn and horn button. So if the single power wire is used in both of the circuits in the relay, then what is the point of the relay? That is why I was thinking there must be an internal resistor or something to prevent the contacts in the horn button from getting burned out. Does that make sense? Sorry if I'm just totally missing something here!

Greg,

Everyone is right on the money. You are right on the money too...there is a internal jumper you don't see that powers one side of relay coil.

The third terminal that has the small wire to the horn button provides a ground for the relay coil when you TOOT TOOT.

1-Power in (from batt).
2-Switched power out (to horn).
3-Ground for relay coil (thru horn btn).

This is an example, not a actual numbering sequence.

Stuart

"So if the single power wire is used in both of the circuits in the relay, then what is the point of the relay?"

The purpose of the relay is to prevent the high current draw for the horn from passing through the horn button.


Without the relay your horn button has to be capable of handling the current your horn is drawing. With the relay your horn button only has to handle what the relay itself uses, the relay handle the main load.

so, it's kind of like a solenoid?

I figured there had to be some sort of jumper in there to create the powered-side of the low-current coil circuit, otherwise there would have to be a fourth terminal on the relay. That internal jumper must be/have a resistor in order to lower the current headed to the horn button. Thanks for entertaining my curiosity!

gjberche,

The wire headed to the horn button is the ground side of a very small relay coil. Not only does this coil take little to energize it, being on the ground side, I think most of the inrush is taken up by/through the small windings of the coil.

No big zaps with this little guy! This is the classic "small load switching a big load" kind of deal...just like your TV remote or your garage door opener.

Stuart

The coil is the resistor you are talking about.

The relay has 3 wires. 1 supply (battery +), 1 load (horn), and 1 switch (horn button, ground).
The relay is grounded by the horn switch and then the supply and load or connected.