12v automotive relay 12v trigger circuit, 24v load circuit?

Thread Starter

kcobean

Joined Jun 23, 2018
4
I'm adding some wiring to a vehicle that is a 24v system. I have a 12v step down providing about 20A of 12v capacity. My question is, can I use 12v on the trigger circuit of the relay but use 24v for the load? I want to keep the load off the switches in my marine switch panel. I've not been able to find anyone doing this. All similar questions I've found seem to revolve around using 24v only with a 12v relay.

Thanks for your help!
 

Thread Starter

kcobean

Joined Jun 23, 2018
4
You can use a 12 relay to switch the 24v if that is what you mean?
Max.
Thanks Max. This is my first attempt at this and I'm a bit out of my depth. I guess what I'm really asking is this: Let's say I'm trying to power a set of flood lights that are designed for 24v. I know that in a 12v automotive relay, it takes 12v on the trigger (is that the right term) to close the load circuit inside the relay. I have a 12v available for that. Can the load circuit to the lights be 24v? If I'm doing a horrible job of explaining this, I can make a schematic. I appreciate your help!
 

MaxHeadRoom

Joined Jul 18, 2013
19,042
The relay coil is switched with 12vdc, the lights are switched by the relay contact using 24vdc, the 12v & 24v commons (-ve's) of each supply can be made common to one another.
Max.
 

Thread Starter

kcobean

Joined Jun 23, 2018
4
Thanks Max, that answers my question. So is it safe to assume that when we talk about relays in general, the voltage 'rating' (if that's the right word) of the relay (12v in this case) is what's required to actuate the relay?

So how do we determine the load limit of the relay? If I buy one that's rated at 40A, is that 40A at either 12v or 24v? I don't have a good understanding of amps to volts and how a change in one affects the other. Hoping to fix that.
 

Reloadron

Joined Jan 15, 2015
4,510
Quality automotive relays normally have a data sheet so here is one such example. Notice in this example there is a rated coil voltage of 12 Volts, also note that there are some "Switching Area" specifications which tell us about the relay contacts and how much current we can expect to switch. Unfortunately you notice the switching voltage is limited to 12 Volts. One problem with doing what you want to do is most 12 volt coil automotive relays have contacts rated for 12 volts. Will they handle and switch 24 Volt lights? Likely yes but I won't be the guy to tell you don't worry about it.

There are other relays, non automotive, which have 12 volt coils and contacts rated for much higher voltages. You need to look at your load (how much current the lamps will need) and work from there leaving a margin.

Ron
 

MaxHeadRoom

Joined Jul 18, 2013
19,042
Thanks Max, that answers my question. If I buy one that's rated at 40A, is that 40A at either 12v or 24v? I don't have a good understanding of amps to volts and how a change in one affects the other. Hoping to fix that.
Relay contacts are generally rated at max operating voltage and rated current, when you go up in voltage it is more of a concern when inductive loads are used, if this is a filament etc load, I don't see any need for concern.
Max.
 
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