automotive relays 12 / 24

[bagmaker]

Does anyone have experience with standard auto relays switching 12v using a 24v relay?
I want to power a 12v circuit with a 24v relay -using 24v as the switching voltage.
But I am having problems and not sure if the relay has limitations or the problems are elsewhere..

thanks

[Mrcoolabah1au]

Ask T1 he the expert on electrical things

[BruceS]

That should work OK. What sort of relay is it? (Oh! just read it again!)
Check polarity perhaps depending on type?
Is it a 4 pin or 5?

[T1 Terry]

As long as the amp rating is better than the load the relay will work fine. If arcing and or voltage spikes are the problem, switch the negative rather than the positive the same as they do in factory automotive where there are computers in the systems involved.
The old water analogy, water running down the hill in a pipe from the tank, turn the water off suddenly at the bottom of the hill and there is water hammer (voltage spike) or try to put your finger over the pipe after it has been flowing for a while and there is quite a spray before you actually get to stop the flow. Now try turning the water off at the base of the tank, the water flow stops almost immediately but no pressure build up or water hammer. Electron flow to a load is from negative to positive, cut the flow on the negative side and the feed stops so the flow stops and the load is no longer feed. Turn it off at the positive end and the flow has already passed through the load but has no where to go, so it has to back track to balance out the electron flow against the direction of flow creating a voltage spike. The coil in the relay acts a bit like the coil in a car engine, when the coil is saturated the relay pulls closed but when the circuit is broken there is a spark as the field collapse, this is why the points are in the negative side of the circuit and not the positive side. Any load that also has a coil winding as part of the circuit will act the same way.
A rather crude analogy but it puts across the general idea, water analogies do have their limits.
Often a quenching diode is added to take the over voltage back to the input side, but ya gotta make sure you put it in the right way around or it becomes a dead short. Some relays come with the diode already installed so the correct polarity of flow is essential.
Polarity of flow nearly does the head in but look at it as the tank or battery being the supply side so it is the more positive if the positive cable going to the load is interrupted by the relay.... but the opposite way around if it's in the negative cable........ if the relay is for controlling a charging source like solar, then that is more positive than the battery as it is trying to push current into the battery where a load is trying to take it out...... now add in the reverse if the relay interrupts the negative cable....... got a headache yet?


T1 Terry

[brianh]

Water and electics don't mix. try this stand in a bucket of water and grab a live 240v, that will give ya a headacke. :evil
Bri Bri

[T1 Terry]

Water and electics don't mix. try this stand in a bucket of water and grab a live 240v, that will give ya a headacke. :evil
Bri Bri

Depends how long you stay connected to the power, if the bucket of water is on a dry insulating surface, if the supply is floating or grid and if there is a working RCD in the supply... or a working RVD in the floating system...... The RCD will give you a hit and you will know it happened but that's about it, the RVD will cut the supply before you even realise it happened, and insulated barrier between the bucket and the ground will result in nothing at all if you only grab one wire, the RVD would trip if you grabbed 2 wires or you would become an appliance if there was only an RCD protecting the circuit when you grabbed the 2 wires, bucket of water or not...

[jon_d]

The coil will run hot if using 24V on a 12V coil. Might limit the life span.

The contacts have to be derated by 1/2 if using a 12V relay on a 24V circuit. (ie 40amp swtiching becomes 20amp switching)


Just by the 24 volt version.

[bagmaker]

FYI, I bypassed it just for annoying me. Could have been dodgy relay(s).
JD, I was using 24v relay on 24v switching but with a 12v load (and a tiny one at that, just telling my radio to show reverse cam when selected at the gearbox) but I couldnt get the relay(s) to even click.
Interesting

[T1 Terry]

FYI, I bypassed it just for annoying me. Could have been dodgy relay(s).
JD, I was using 24v relay on 24v switching but with a 12v load (and a tiny one at that, just telling my radio to show reverse cam when selected at the gearbox) but I couldnt get the relay(s) to even click.
Interesting

The problem was probably the lack of load didn't have enough grunt to pass across the contact faces. A mini SSR will do the job as they are rated from 3v to 32v on both the control and load side.

T1 Terry

[jon_d]

JD, I was using 24v relay on 24v switching but with a 12v load (and a tiny one at that, just telling my radio to show reverse cam when selected at the gearbox) but I couldnt get the relay(s) to even click.

Ahh, beware if it the gearbox is switching to ground, or to battery. It might be the opposite to how you've wired it up.

Stick a voltmeter on the wire from the gearbox and measure the voltage on it when in forward and reverse.