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Old 17-10-2013, 04:38   #1
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12V vs 24V relay

Question for the electrically proficient:

I have two 12V batteries connected in series to supply 24V to three 24V electric winches. Each winch is controlled by a relay rated for 24V.

One of the relays failed recently, and as I had on hand a similar relay, but rated for 12V, I replaced the failed 24V one with the one rated for 12V, half expecting the thing to burn out… (at that time, I had no other alternative).

Much to my surprise, everything worked out just fine: the winch operates normally, and there does not seem to be undue heat coming from the 12V relay. Granted, these winches are rarely run for a long time, other than by quick bursts, so I may not be witnessing what is really happening in the 12V relay’s inner workings.

My question is: what am I missing? Can I keep this jury rigged set up or am I just asking for trouble? (the relays are spring loaded, magnet type)

Thanks for your insights
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Old 17-10-2013, 04:44   #2
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Re: 12V vs 24V relay

probably ok for intermittant use,as the relay coil does not have time to heat up,if it was in the constantly on position i would think the coil would burn out at the higher voltage
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Old 17-10-2013, 22:22   #3
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Re: 12V vs 24V relay

as above. the coil is taking double the rated current. in short burts may be ok. but certainly not for longer use. I would replace it as soon as possible.
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Old 17-10-2013, 22:35   #4
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Re: 12V vs 24V relay

It is also possible to use a 12V relay in a 24V circuit by inserting a resistor of the proper resistance and wattage in series with the coil. Drop 12V across the resistor and that leaves 12V for the relay coil. This trick only works well with DC relays and not AC relays. One only needs to know the current draw of the 12V coil.

The resistance can be calculated by R=12/I where I is the normal current of the 12V coil. The necessary wattage needed is approximately I*24. Assuming the 12V relay coil required 0.1A then the resistor would be 120 ohms and 2.5 watts.
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Old 18-10-2013, 08:56   #5
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Re: 12V vs 24V relay

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Originally Posted by transmitterdan View Post
It is also possible to use a 12V relay in a 24V circuit by inserting a resistor of the proper resistance and wattage in series with the coil. Drop 12V across the resistor and that leaves 12V for the relay coil. This trick only works well with DC relays and not AC relays. One only needs to know the current draw of the 12V coil.

The resistance can be calculated by R=12/I where I is the normal current of the 12V coil. The necessary wattage needed is approximately I*24. Assuming the 12V relay coil required 0.1A then the resistor would be 120 ohms and 2.5 watts.
There is no reason it would not work with AC relays too.
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Old 18-10-2013, 11:56   #6
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Quote:
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There is no reason it would not work with AC relays too.
Yes there is. AC relay coils demand more current until they close. Then the current drops. The resistor can prevent the initial closure and the relay becomes unreliable.

DC relay coils draw the same current all the time.
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Old 18-10-2013, 12:27   #7
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Re: 12V vs 24V relay

New here so be gentle please
There are two types of relays, well more actually maybe but here we are concerned with intermittent and continuous duty ones, Intermittent, think starter relay on your car is designed for very high amp loads for a short time, if used for continuous use as mentioned the coil will over heat and burn out.
A continuous duty relay is designed for much lower loads, but continuous ones of course, if you try to use it for high amp intermittent use , you will burn out the contacts.
I would assume a winch solenoid is intermittent, the 12V may handle it, and it may not, there is more to the design of a relay than the voltage the coil will handle, the failure mode for the 12V relay may very well be welding the contacts shut, leaving the winch stuck on of course.
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Old 18-10-2013, 13:46   #8
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Re: 12V vs 24V relay

There isn't much, if any, difference in the electrical contacts of a 12V and 24V relay of the same style. The difference will be in the coil itself. It is rare that failure of the coil could "weld" the contacts shut.

This cases involves a common situation where one has a relay with a coil voltage designed for something less than the control voltage that is available. I was just offering a way to use that lower voltage coil with a higher control voltage. The assumption is that the relay is identical in every other way to its 24V brethren. We are not talking about every possible relay type but rather a simple control relay.
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Old 18-10-2013, 14:06   #9
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Re: 12V vs 24V relay

Perhaps I was mis-understood, Possibly welding the contacts shut is from using a continuous duty solenoid where a much higher rated intermittent one is called for, coil voltage in this would be irrelevant.
Most available starter looking relays available at auto parts stores etc are almost always the intermittent type and those if tried to be used as a continuous duty one will usually fail by the coil burning out and the piece of equipment powered just fails to function.
If you have ever been around general aviation aircraft, you would know that most starter relays and battery relays look identical, the are the single post Ford type, only way to tell which is which is by the part number.
Only point being really that if the incorrect relay is used that a failure mode exists that could be more dangerous / problematic that the winch just failing to operate, the incorrect relay needs to be replaced with the correct one, even though it appears to be working fine.
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Old 19-10-2013, 04:59   #10
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Re: 12V vs 24V relay

Thanks to all for your extremely helpful insights.

Here's a picture of the relay and its wiring diagram: as you can see it is a 3 pole relay, with 3 connectors.

I like the solution of adding a resistance in series, but I'm baffled as to where I should install it (again, sorry for my ignorance!)?

many thanks

jean

Click image for larger version

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Name:   harken <a title=wiring.jpg Views: 2141 Size: 6.8 KB " style="margin: 2px" />
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Old 19-10-2013, 07:32   #11
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Re: 12V vs 24V relay

Jean,

You have posted a picture of a 4-wire reversing relay but a diagram for a 3-wire system. Does you relay have 4 big wires and 3 small wires or does it have 3 big wires and 3 small wires?

If it is a 4 wire relay then the resistor can replace the wire that now goes between the small C terminal (the one in the middle of the 3 small terminals) and the big ground wire just above it. But that wire isn't shown in the diagram you posted.

If there are only 3 big wires on the reversing relay then the resistor goes between the small fuse and the 2 foot switches.
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Old 19-10-2013, 07:47   #12
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Re: 12V vs 24V relay

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or does it have 3 big wires and 3 small wires?

.
Sorry the picture is inaccurate (got it from the internet in a hurry, and did not check it thoroughly).

The relay has 3 big wires on top (for the battery cables) and 3 small wires at the bottom (for the controls from the foot bellows)
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Old 19-10-2013, 07:55   #13
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Re: 12V vs 24V relay

transmitterdan: thanks for the reco! The wiring diagram is correct and corresponds to the actual set up on the boat.

I will proceed to install a resistor of appropriate size between fuse and foot switch as you indicated and will measure voltage drop to make sure

Quick question: off the top of your head, how hot is the resistor likely to get, assuming continuous operation- say for 1 minute?
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Old 19-10-2013, 07:57   #14
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Re: 12V vs 24V relay

Then you may have another choice that does not require a resistor. The small fuse that sends control power to the 2 foot switches could be moved over from the 24V to the 12V bus of your DC system. Then the big wires would still be 24V but the small windlass control wires would be 12V.

If you have a diagram of how your boat is actually wired it would be helpful in determining which is the best solution.
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Old 19-10-2013, 08:18   #15
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Re: 12V vs 24V relay

n-thing the recommendation to put in the right 24v relay when it's convenient to do so, if only to keep the system as straightforward as possible for some time in the distant future, when you (or the next owner) have to figure out why there's a 12v relay in there.

I sort of disagree with the resistor idea, because all inductors (the relay coil) have initially high currents before the field builds up and current reaches its steady final value. A series resistance will limit the maximum value of that initial current, which limits that initial relay "pull"... which might be critical in some relays, especially those that rely on the armature to complete the magnetic path.

But if it works it works, and everything in the winch system, relays included, are intended for fast intermittent operation, so perhaps there's enough tolerance in the whole system that these quick-fixes are successful.
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