Question about relays

[Rusty67]

Ok, I was thinking about this while I was driving to school this morning. I don't want to start a huge debate about this I'm just wondering if there is any difference or if one way is "better" then the other.

Is it better to wire a relay so that the power goes into the device it controls first and the relay closes the circuit for the ground or the other way around ?
So is this better:

Power--------|Electrical Device|--------|Relay|------|Ground

Or this:

Power--------|Relay|--------|Electrical Device|------|Ground

Does it make any difference at all ? I've seen instructions saying to wire things both ways and I'm wondering what the deal is.

 

[Bosscat]

It makes no difference, the relay is a short for the main power, its irrelevant which direction the voltage or current flows

 

[rbohm]

it is better for the relay to control the power rather than the ground imo. i realize that there is little difference electrically.

 

[ARTTII]

+1 on the "it makes no difference"


it is one in the same, by design, principle

 

[CraigMBA]

it is better for the relay to control the power rather than the ground imo. i realize that there is little difference electrically.

This is almost correct.

If you are trying to use a relay as a low amperage switch to turn something on or off, you can interrupt the power or ground with the same effect. Zero difference. An open circut is an open circut.

BUT-

The reason most people use a relay is they have a big current device they want to switch on/off and they don't want to use a big current switch. In that case, you absolutely switch the power side.

 

[67Dylan]

This is almost correct.

If you are trying to use a relay as a low amperage switch to turn something on or off, you can interrupt the power or ground with the same effect. Zero difference. An open circut is an open circut.

BUT-

The reason most people use a relay is they have a big current device they want to switch on/off and they don't want to use a big current switch. In that case, you absolutely switch the power side.

X1

 

[Rusty67]

I figured as much... but it would be just my luck if I didn't ask then when I go to rewire my car that I do it the wrong way (if there was a wrong way) and my car would catch on fire, ignite the fuel tank and turn into a rolling... flaming... pile of useless rust. But that is just MY luck lol. Thats why I think of these things randomly, too many bad experiences from not asking questions.

 

[Tim65GT]

The advantage of putting the relay on the power side of the circuit is simplicity. If the relay is in the ground circuit, it will involve more wiring and make troubleshooting more difficult.

It should go: Battery -> Fuse -> Relay -> Load -> Ground

If it's: Battery -> Fuse -> Load -> Relay -> Ground,
then -
If you have a shorted relay or wiring on the ground circuit between the load and relay, it will feed the component until it burns up or the battery goes dead.

That's a big if, and it could also happen on the low current side of the relay circuit.

Does it make any difference at all ? I've seen instructions saying to wire things both ways and I'm wondering what the deal is.

If I may ask. what instructions?

On your application, was there an advantage to putting the relay in the ground wire?

 

[Rusty67]

I think it was the electric fan I put on my buddies Cougar. The instructions said to wire it so that the relay controlled the ground. I didn't like it that way so I did it the other way around. I tested it and it worked fine and didn't give it a second thought since that was the way I was accustomed to doing it.

 

[bnickel]

i always wire relays so that the positive circuit goes through the realy before the switch. like on a headlight circuit in an an early stang, you want the power to go through the relay first becuase it can carry a greater load than the stock headlight switch. it really doesn't make a lot of difference but it's my opinion that the higher amp rated part (usually the circuit relay) take the power first

 

[Jester67]

The advantage of putting the relay on the power side of the circuit is simplicity. If the relay is in the ground circuit, it will involve more wiring and make troubleshooting more difficult.

It should go: Battery -> Fuse -> Relay -> Load -> Ground

If it's: Battery -> Fuse -> Load -> Relay -> Ground,
then -
If you have a shorted relay or wiring on the ground circuit between the load and relay, it will feed the component until it burns up or the battery goes dead.

That's a big if, and it could also happen on the low current side of the relay circuit.

I agree it is always better to control the power than the ground. Just remember almost every thing on a car is a ground sourse.

 

[ForceFed70]

BUT-

The reason most people use a relay is they have a big current device they want to switch on/off and they don't want to use a big current switch. In that case, you absolutely switch the power side.

I fail to see your logic. The choice between using a relay on the power or ground side of the circuit has nothing to do with the current draw.

Now, if you are using a relay to take the load off of something like the factory headlight switch/wiring, then yes it matters which side of the circuit you are switching.

Most people, along with OEM manufacturers use a relay to switch the positive side of the circuit. This is done for 2 reasons:
1) Intuitively it's "simpler". Most people better understand the concept of switching the "power" rather than the ground. Electrically it's the same circuit.
2) This is the main reason. 90% of all automotive electrical short circuits involve a short to ground. Usually a wire chafed on a piece of sharp body metal, etc. Because of this, you want to keep the amount of "live" wires down to a minimum. Having a relay on the positive side of the circuit decreases the amount of electrically charged wiring and components and reduces the chance of a short circuit.


Most of the time, it comes down to what you are trying to achieve, or the particular circuit. But in a simple "single load single circuit" situation it doesn't really make a difference.

Now, if you were asking how to throw the relay (switch + to the relay coil or the negative) this would be a whole different ball of wax...in most cases the ground side of the relay coil is switched in order to control the relay.

 

[mustangman1974]

I think it was the electric fan I put on my buddies Cougar. The instructions said to wire it so that the relay controlled the ground. I didn't like it that way so I did it the other way around. I tested it and it worked fine and didn't give it a second thought since that was the way I was accustomed to doing it.

Also the difference between some electrical fans pushers and pullers.

 

[Bosscat]

If you burn something, normally its the wires that fry, because the load was too big. The relay contacts will normally open if the load is too great for the relay.

If I wire a 40amp fan, per say, I will run two 25 amp relays in parallel, because I have a bunch of them around . Keep the wiring as short as possible for the load wire.