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Old 13-01-2011, 12:40   #1
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240vac DPDT Switches Suitable for 12vdc ?

Pardon my ignorance, AGAIN.
I am rewiring my masthead nav light & light actuated anchor LED lights. The instructions from Bebi.com say to use a double pole / double throw / centre off switch at mast base. I have FINALLY got my head around DPDT but the only switches I can find are all rated for 240v AC even at automotive stores when I find a sales person that has a clue about what I am talking about.

Can they be used or do I have to continue to search for a DC DPDT switch. Now that's a mouthful
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Old 13-01-2011, 12:48   #2
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Those lights draw very little amperage. Any DPDT switch would work OK.

You can find lots of DPDT toggle switches in electronics stores, maybe Radio Shack, or online from Mouser or other electronics supply houses.

You can also find them on eBay...there are 488 current listings for "DPDT toggle switches":
http://shop.ebay.com/?_from=R40&_trk...All-Categories

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Old 13-01-2011, 12:58   #3
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have used standared dual pole 240 v0lt breakers for the last 12 years,in continious use on board for all the 12v and 24v dc systems,same as you would with a domestic a/c system,not very pretty but available throughout the 220/240v world and circuit boxes cheap and watertight and easy to find, never had a failure yet.
also contact slots on the breakers nice and big for upto 6mm crosssection wire
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Old 13-01-2011, 14:51   #4
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Strider, a 240v rated switch (ac/dc doesn't really matter here) is simply rated 20x beyond what you need. No worry.

But you can find plenty of 12VDC rated switches at any auto parts store, used for fog lights and auxiliary electrical gizmos. I'd avoid the cheapest "Made in China" junk, because I've literally seen some of them fall apart on installation.

Most will be SPST or DPST but there should be some DPDT in there. If not, there's always Radio Shack, where the electronic parts are usually hidden in flatfiles in the back room now. You have to ask for them and show ID to prove you're over 21. Oh, wait, wrong back room.<G>
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Old 13-01-2011, 15:09   #5
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Maybe I'm missing something. But please clarify a 240Volt DPDT adapted to a 12V system. I'm sure it will work but just curious.
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Old 13-01-2011, 15:16   #6
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"clarify a 240Volt DPDT adapted to a 12V system."
What's to clarify?

If you need a 3/8" line and someone gives you a 5/8" line to use instead, what's unclear? The thicker line works, it is just more than you need.

A 240-volt rating just means the switch is overrated for the job. BFD.

DPDT has nothing to do with the ratings either. If it is "DPDT center off" as opposed to plain DPPT, it probably us being used to reverse polarity over two wires, allowing them to replace three wires with two separate switches, or some other purpose beyond a simple on/off. Again having nothing to do with voltages.
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Old 13-01-2011, 18:54   #7
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Weeell................MAYBE!

There are things to consider. For example, what is the load that the switches will interrupt and at how high a current? DC is a very hard current to interrupt where as AC is much easier. Consider what an AC waveform looks like. The wave form is a sine wave starting at a phase angle between 0-360 degrees depending on where it is in the cycle when switched. At 0, 180 and 360 (0 again) degrees, the waveforms magnitude is 0 volts. In other words, the waveform crosses zero twice each cycle.

Why is this important??? If an arc occurs during the cycle at the time of current interruption inside the switch do to an inductive load it has a better chance to extinguish at the times where the waveform is at 0 magnitude.

Elementary circuit theory teaches that voltage cannot be changed instantaneously in a capacitor but our immediate concern is that current cannot be instantaneously changed in an inductor. Yes, these conditions apply unless there are infinite magnitudes of voltage/current changes that is beyond our interest here.

If the load that is being interrupted is inductive, the switch may ARC INTERNALLY and that arc will last, maybe long enough to destroy the switch.

Enough with circuit theory, I agree with others, just try the darn thing as long as you stay within the switches current ratings.

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Old 13-01-2011, 19:16   #8
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I agree with Foggy,

The interrupting current in DC is what will kill your switches. But here a regular automotive DPDT, as specified, will more than do the job for the current draw.

My Bebi lights should be here next week!
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Old 13-01-2011, 19:28   #9
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Foggy,
He's switching an LED Anchor light (Bebi). No big inductive load.
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Old 13-01-2011, 19:32   #10
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Don--

Too many folks don't understand how switches should be used.

But from my earlier post "Enough with circuit theory, I agree with others, just try the darn thing as long as you stay within the switches current ratings. "
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Old 13-01-2011, 19:48   #11
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if you have a grainger around Switch, Toggle, Dpdt - Toggle Switches - Switches - Electrical : Grainger Industrial Supply

DPDT center off 125volt 15amp switch
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Old 13-01-2011, 20:23   #12
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the switch should have an amp rating as well as the voltage rating, and under normal loads your switch you found will work, and has way more capacity for more voltage and or AC use if you choose... but, in the event of short circuit, you may fail your switch before your fuse, so in essence you will fix replace your fuse and it still may not work and then have to check the switch... but this is the case anyways, as the short may be faster then the fuse...
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Old 13-01-2011, 20:35   #13
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The 240 VAC rating is a testing / rating standard, perhaps national code or testing service rating. It simply means it won't arc or short through to the surroundings at the 240 volts. The important rating is the power. Electric gear will amost always have the allowable amps listed. Volts times amps is power. Any switch you find will easily handle the miliamps of the BIBI fixtures. The more important point will be selecting reliable parts. You have a great light; use good switch gear recommended for marine use.

By the way, Mike at BIBI built a couple of Windex lights for me two years ago. I have prototype 1 & 2. (main & mizzen instruments) They wire into the base of the nav light fixture power and shine on the wind pointer. I haven't seen them on their site yet but he said he was planning to add them into his mix. I really like these. The orange tabs appear as though they each have their own brilliant light.
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Old 16-01-2011, 07:14   #14
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Quote:
Originally Posted by hellosailor View Post
Strider, a 240v rated switch (ac/dc doesn't really matter here) is simply rated 20x beyond what you need. No worry.
no...no...no... it matters a great deal!
I am an electrical engineer and licensed electrician and electrical supervisor so I will clear this up so everyone understands.

A 240V 10A AC mains rated switch must be derated 80% if used to switch DC at 12V DC. Therefore a 240VAC 10A switch is rated the same as if you used the same switch at 12V 2A DC maximum.

The reason is exactly as other posters have said due to the fact that AC is switching 50 (or 60) times though the 0V point per second (ie,50-60Hz) due to the AC waveform. In stark contrast 12VDC is constant current which tends to maintain an arc if you try to switch it off so this is much more difficult to switch and extinguish the arc and the reason for the much lower rating of a switch when used for DC instead of AC.

I hope this clears things up for everyone.
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Old 16-01-2011, 08:16   #15
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Originally Posted by hellosailor View Post
Strider, a 240v rated switch (ac/dc doesn't really matter here) is simply rated 20x beyond what you need. No worry ...
Switch contacts suffer greater arcing & heating duress switching DC than AC, particularly if inductive loads are present. This is primarily due to the self-extinguishing (or quenching) nature of an AC arc. Very fast-acting “snap” switches tend to mitigate this deficiency. Accordingly switches and relays generally have much lower ratings for DC than for AC. I know of no generally reliable de-rating factor for applying DC to AC only rated devices.

A resistor-capacitor network called a "snubber" can be connected in parallel with a switch contact to reduce contact arcing.

Wetting current is the minimum amount of electric current necessary for a switch contact to carry in order for it to be self-cleaning. Normally this value is far below the switch's maximum current rating. Accordingly, switches with vastly overrated (for the application) contacts should be avoided.
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