# 12 volt relay at 24 volts at 20 amps

I have an automotive relay http://www.radioshack.com/graphics/uc/rsk/Support/ProductManuals/2750001_DS_EN.pdf
I think it says it can switch 60 amps at 12 volts.

Can I use this relay to control a 24 volt 20 amps system? I am controlling 2 large 24 volt motors that each have about a 10 amp stall. So it is not a continuous 20 amps, 20 amps peak. Will the relay survive?

Edit: One more question. Would this in series with the relay and the load be a good idea? (I have the 20 amp version) http://www.snapaction.net/pdf/mb1_insert_page.pdf

The specifications say that the contacts are good for 14VDC 60A for purely resistive loads (like a lightbulb) and 14VDC 40A for an inductive load like a motor.

You should probably get a relay that is rated for at least 24VDC and at least 20A. You can probably find one with a 5V coil, thus saving the bother of having a 12V supply just for the relay coils.

Something like this: http://www.digikey.com/product-detail/en/T90S5D12-5/PB113-ND/44920

Coil: 5V 185mA
Contacts: 20A 28VDC (Max)

what would happen if I used the 12 volt relay? I would like to learn more. does the relays max current flow go in half if the voltage is doubled?

arduinoPi:
what would happen if I used the 12 volt relay? I would like to learn more. does the relays max current flow go in half if the voltage is doubled?

With higher voltage comes more sparking and more risk that the contacts will burn out or weld together. The 12V relay will probably work for a while. If nothing bad happens to your motor if the relay welds closed then give it a try.

It probably OK - twice the voltage means double the arcing power available for the same current,
but you are using 1/3 the rated max current, so that should compensate I think (although its not
really that simple).

The isolation between the contacts is 500V so there's no problem per se with the voltage, just
the arcing/sparking damage as the contacts open.

DC is always more demanding to switch than AC since there are no zero-crossings to help
quench an arc.

Switching large DC voltages at high power requires oil-filled relays or high-pressure inert gas
switch gear due to this difficulty - once an arc is struck it takes a much lower voltage to keep
it going (so long as there is enough power to maintain it - a spark is a low-resistance channel
of ionised gas).

thanks for the posts. I just have one more question. Would I be able to use it in between the 2 lead acid batteries? (on the wire connecting the ground of one battery to the positive of the other battery) would that be 12 volts or is it 24 volts?

arduinoPi:
thanks for the posts. I just have one more question. Would I be able to use it in between the 2 lead acid batteries? (on the wire connecting the ground of one battery to the positive of the other battery) would that be 12 volts or is it 24 volts?

Are you seriously telling us that you are playing around with lead acid batteries and don't know the difference between series vs parallel circuit connections...?

Do you not understand how dangerous this lack of knowledge can be?

We are talking potential fire and explosions here...

For what its worth I am using a similar (probably the same) relay from Radio Shack on a RC Lawnmower. Power source is two 24V batteries, however the contacts are closed with a 12V lined center tapped from the battery bank since I noticed the relay getting hot when switched closed with 24V powering the coil (could also add a small resistor in series).

The motors are controlled with a Sabertooth 2x25 and are pulling around 15 amps each at full throttle......that is 30-40 amps at 24 V through the relay, I have not experienced any issues yet

Keep in mind that in my setup the contacts are only closed at power on and opened at power off and are not used for switching motors on and off.

mech_eng:
Keep in mind that in my setup the contacts are only closed at power on and opened at power off and are not used for switching motors on and off.

That is what mine would be for. Is it okay to do this?

So basically if the relay does not experience any sparks from closing or opening on high current it is alright? As in relay switching on motor controllers that use 40 mA
and then having the motor controllers give the current? (with out shutting down the relay until motors are 100% off by motor controllers)

would this setup be okay?

arduinoPi:
thanks for the posts. I just have one more question. Would I be able to use it in between the 2 lead acid batteries? (on the wire connecting the ground of one battery to the positive of the other battery) would that be 12 volts or is it 24 volts?

Interesting question. I'm not sure if this is better or worse (in terms of relay contact stress) than closing the circuit at one of the motor terminals, but in either case you will be closing/opening a 24V circuit.

It should work as long as you switch light loads, however if you need your setup to be absolutely reliable for whatever reason (safety, etc) then get a relay rated for 24 V.

If you switch 20A there will be a spark! Its a question of how quickly it quenches and how well
the contacts withstand the heat (relay and switch contacts use special heat-resistant alloys to
reduce the damage).

well I don't need to switch 20 amps. I only need to power on the motor controllers (about 40 Ma) the relay would then stay on continuously and 20 amps would flow through the relay that was already closed. So the relay would not switch 20 amps but 20 amps would pass through the relay. Is it okay to do this as long as I avoid high current sparks?

arduinoPi:
well I don’t need to switch 20 amps. I only need to power on the motor controllers (about 40 Ma) the relay would then stay on continuously and 20 amps would flow through the relay that was already closed. So the relay would not switch 20 amps but 20 amps would pass through the relay. Is it okay to do this as long as I avoid high current sparks?

Just assume it’ll be switching 20A, because at some point that will happen (in the real world). If you can avoid
it switching off at full current you’ll extend the contact life a lot, which is great.

I have a similar question, I have a relay with a 12 volt coil and I want to switch on 48 volt at only a couple of amps. I am thinking using a diode on the load side (contacts) will help with arcing at the points maybe a condenser like the used on cars when the had points ignition.

A diode across the relay contacts won't help with inductive kickback, it has to be across the load. What kind of load is the relay switching?

What I am trying to do is use a relay in place of key switch of an 48 volt electric golf cart.

Without knowing knowing how the key switch is connected in the circuitry, it's hard to advise. The switch could be between battery + and load OR between load and ground. do you have a schematic diagram?

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