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Old 19-10-2013, 08:44   #16
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Re: 12V vs 24V relay

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Originally Posted by Lake-Effect View Post
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.
That is a common misconception but is not correct for DC relays. Inductors (aka coils) have initially high impedance which decreases as the field builds up. In the simple DC case the current starts out at zero and builds up to the final steady state value. There can be no initial high current with a simple DC relay.

I suspect you are thinking of AC relays where this is sort of true and was describe in a previous post. When some (not all) AC relays are initially energized the steel armature is not pulled in. So the inductance is initially lower than it will be when the relay is fully closed. In that case there is higher AC current in the coil until the relay closes and then the AC current drops because the inductance of the coil increases. This does not happen naturally in DC relays. Some really big DC relays insert a resistor into the coil to drop the current after the relay closes. The normal DC relays found on small boats don't do that.
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Old 19-10-2013, 10:38   #17
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Re: 12V vs 24V relay

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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.
This is an interesting and elegant solution- especially, as it turns out, that the location of the relay is pretty near the main DC bus, so I should be able to wire the control wires onto the 12v system. Will attempt to do this in the next few days and may come back to this forum for further advice!

To Lake-Effect's point, indeed, this is a temporary solution until I install a propre 24V relay , (I'm a firm believer in KIS, especially on sailboats.)

Thanks again to all who contributed

Jean (aka Bubba Jean from the days when I lived in Texas )
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Old 19-10-2013, 11:03   #18
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Re: 12V vs 24V relay

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Originally Posted by transmitterdan View Post
Inductors (aka coils) have initially high impedance which decreases as the field builds up. In the simple DC case the current starts out at zero and builds up to the final steady state value. There can be no initial high current with a simple DC relay.
You're right... I forgot basic dI/dT into in an inductor.

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Originally Posted by transmitterdan View Post
I suspect you are thinking of AC relays where this is sort of true and was describe in a previous post. When some (not all) AC relays are initially energized the steel armature is not pulled in. So the inductance is initially lower than it will be when the relay is fully closed.
I believe that the DC relays (aka contactors) used to control electric winches and windlasses also have a substantial armature that completes the core loop when closed. Currently pondering whether that would alter the required current. Probably not.

Thanks for the correction.
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Old 19-10-2013, 11:08   #19
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Re: 12V vs 24V relay

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Currently pondering whether that would alter the required current. Probably not.
Not is correct. The DC current is only limited by the resistance of the wire in the coil.
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