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04-02-2014, 09:22 PM   #1
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## limiting factor of switch

What is the limiting factor of an electric switch? Amps, or watts? Situation: I can get a 250V, 20A toggle switch, STDP, that I'd like to use for a 12 or 24 V system. Am I limited to 20A even at 24V, or could the switch conceivably handle more amperage (ie, 200) at the reduced voltage? Thanks.

04-02-2014, 09:30 PM   #2
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THAT is actually a very good question.

A lot of factors go into a switch....AC vs DC, voltage and current.

For the most part....wattage is NOT a factor.

If you look at a switch, it should have specs for both AC and DC. In most cases, the DC current rating will be higher...most cases....but depends heavily on the contact material.

With DC, if your going to control a relay....you really need to make sure you put a diode across the relay coil to suppress the inductive kick. Otherwise, your switch has to absorb it...and that will shorten it's life quickly.

Additionally, voltage is also a big factor. A switch rated for 480 Vac is quite a bit different than one for 120 or 240Vac.

At the end of the day, it's pretty simple. Look at the spec's for the switch.

Edit....re-read your post......don't even think of trying to put 200a DC across that switch. Once again, it depends on what your load is. Inductive is worse than resistive. Case in point...the starter solenoid for your car starter (resistive and inductive).

May I suggest some reading.....Read up on Edison and Tesla.....history tends to put Edison on a pedestal....but in reality...it's a good thing Tesla came along.....

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Last edited by ddawg16; 04-02-2014 at 09:36 PM.

 04-02-2014, 09:56 PM #3 Member   Join Date: Dec 2008 Location: Fairbanks, AK Posts: 1,893 Rewards Points: 1,092 thanks. i've been reading a bit, and the mechanics of the switch is also important. 12V switches tend to (or do) have wipers to make contact instead of a knife and ham setup (my analogy). i will definitely get a 12V switch of the proper amperage and bag the idea of using a 120V one.

04-02-2014, 10:06 PM   #4
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Quote:
 Originally Posted by jklingel thanks. i've been reading a bit, and the mechanics of the switch is also important. 12V switches tend to (or do) have wipers to make contact instead of a knife and ham setup (my analogy). i will definitely get a 12V switch of the proper amperage and bag the idea of using a 120V one.
You should see a 2000A draw out contactor.......
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04-03-2014, 01:14 AM   #5
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Quote:
 Originally Posted by ddawg16 You should see a 2000A draw out contactor.......
I googled it; pretty impressive.

04-03-2014, 05:43 AM   #6
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Quote:
 Originally Posted by jklingel What is the limiting factor of an electric switch? Amps, or watts? Situation: I can get a 250V, 20A toggle switch, STDP, that I'd like to use for a 12 or 24 V system. Am I limited to 20A even at 24V, or could the switch conceivably handle more amperage (ie, 200) at the reduced voltage? Thanks.

The DC rating of an AC switch is only a small portion of its AC rating !
Do not use an AC switch on DC, especially if its a large load.
The internal arcing at switch off, will very quickly destroy the switch !
Best to use a switch designed for DC operation !

A switch designed for 20A AC will handle only 1 or 2A at DC!

FACTOID !

 04-03-2014, 01:00 PM #7 Member   Join Date: Dec 2008 Location: Fairbanks, AK Posts: 1,893 Rewards Points: 1,092 Thanks. I did not know that. FYI, however, I see a common mis-use of "factoid"; I learned this the hard way, from some students. Embarrassing.... "A factoid is a questionable or spurious (unverified, false, or fabricated) statement presented as a fact, but without supporting evidence."
 04-03-2014, 01:22 PM #8 Member     Join Date: Jun 2012 Location: Northern Calif. Posts: 4,908 Rewards Points: 1,670 Most of the light switches you will find in a U.S. hardware store or big box, will have zero DC rating. If there is a lot of DC in your area they might carry one or two snap switches with a DC rating. Or, you can order something like http://www.amazon.com/Leviton-12021-2W-Single-Pole-Industrial-Self-Grounding/dp/B003AUDC4I
 04-03-2014, 05:38 PM #9 Member     Join Date: Oct 2010 Location: Brisbane, Australia. Posts: 4,326 Rewards Points: 5,610 Thanls ! Did not know that !
 04-03-2014, 05:56 PM #10 Member   Join Date: Dec 2008 Location: Fairbanks, AK Posts: 1,893 Rewards Points: 1,092 you're welcome. i learned a lot from my high school students over the decades. man, there are some incredibly fast and well educated kids out there. one little twerp had to tell me that he missed ONE question out of 1600 on the SAT test; he wrote them to inform them that he thought they were wrong, and they probably were. cheers.
 04-03-2014, 11:16 PM #11 Member     Join Date: Oct 2010 Location: Brisbane, Australia. Posts: 4,326 Rewards Points: 5,610 The reason for the difference in ratings is ! At switch off, a dc circuit will arc across the switch contacts, and because it is DC the arc will be maintained longer, when compaired to an AC circuit, which will last only for the remainder of the AC cycle, where it will die. But no so with DC, it can remain for some time, this tends to burn out the switch contacts real quick ! Special DC switchs are available which have a greater contact gap with a quicker more snappy action. Which is less likely to draw an arc ! Here's a picture of old older style switch that was designed to be used on AC or DC. A large contact gap and a very snappy action. Last edited by dmxtothemax; 04-03-2014 at 11:20 PM.
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 04-04-2014, 12:22 AM #12 Member   Join Date: Dec 2008 Location: Fairbanks, AK Posts: 1,893 Rewards Points: 1,092 ok. good to know.
04-04-2014, 01:00 AM   #13
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Quote:
 Originally Posted by dmxtothemax The reason for the difference in ratings is ! At switch off, a dc circuit will arc across the switch contacts, and because it is DC the arc will be maintained longer, when compaired to an AC circuit, which will last only for the remainder of the AC cycle, where it will die. But no so with DC, it can remain for some time, this tends to burn out the switch contacts real quick ! Special DC switchs are available which have a greater contact gap with a quicker more snappy action. Which is less likely to draw an arc ! Attachment 83445 Here's a picture of old older style switch that was designed to be used on AC or DC. A large contact gap and a very snappy action.
To add to that.....one of the big factors is the type of load. What dmx is describing is typical of inductive loads, such as a relay. That is why one needs to put a diode (snubber) across the relay coil. Additionally, the contact material is quite a bit different for DC vs AC.

A classic example of a DC switch....the dist contacts on the old style dist.
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04-04-2014, 08:10 AM   #14
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Quote:
 Originally Posted by Oso954 Most of the light switches you will find in a U.S. hardware store or big box, will have zero DC rating. If there is a lot of DC in your area they might carry one or two snap switches with a DC rating. Or, you can order something like http://www.amazon.com/Leviton-12021-.../dp/B003AUDC4I
I've wired three way switches for added interior lights in work van for three different trucks over the years....
never a problem.....

mmmmm... what's that I smell.. :- )
of course dc rated would be better though

04-04-2014, 11:35 AM   #15
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You can get 3way DC toggles and rockers for vehicles 12vdc at auto parts stores, marine stores, and you might even find them in a big box.
But the toggle looks like this.
Attached Images

Last edited by Oso954; 04-04-2014 at 11:52 AM.

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