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Old 07-26-2012, 10:49 PM   #1
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Switches and currents


Why would a switch be listed/rated for AC only? If one used an AC-only switch on a DC circuit will the universe collapse?

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Old 07-26-2012, 11:12 PM   #2
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Switches and currents


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Why would a switch be listed/rated for AC only? If one used an AC-only switch on a DC circuit will the universe collapse?
No, but I have a feeling the switch will fail much sooner because of the prolonged arc. But if i'm wrong and the universe does collapse, we know who to blame.

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Old 07-27-2012, 12:54 AM   #3
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It has to do with contact material and design.

It's actually a good question with a complicated answer.

Several factors to consider....what type of load? If you switching an inductive load, the resulting dc inductive 'kick' can distroy a contact in no time....just think of how a ignition coil in a car works....and then think of the old points that car dist used to have....and how long they lasted.....and what mechanics had to do to keep them going.......and that was only 12 Vdc.......

We could spend weeks discussing contacts...material....wiping.....surge supression....etc....

So...to answer your original question....yes, an AC switch will work (for a short time) with DC.......the short time will be determined on the type of load (inductive being the worse) and the amps.

BTW....to give you an idea of the complications....look up how electroplating works.....guess what can happen with contacts when switching DC? Yea....you can have an electroplating process take place....
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Old 07-27-2012, 05:49 AM   #4
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....just think of how a ignition coil in a car works....and then think of the old points that car dist used to have....and how long they lasted.....and what mechanics had to do to keep them going.......and that was only 12 Vdc.......
If the OP is younger than maybe 25, you just lost him. He won't know what you're talking about LOL.
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Old 07-27-2012, 06:16 AM   #5
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If the OP is younger than maybe 25, you just lost him. He won't know what you're talking about LOL.
My first vehicle was a 1970 C10 with a 307 and 3 in-the-tree. And, we couldn't afford mechanics.
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Old 07-27-2012, 10:40 AM   #6
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. And, we couldn't afford mechanics.
Neither could we. We walked.
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Old 07-27-2012, 10:44 AM   #7
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My first vehicle was a 1970 C10 with a 307 and 3 in-the-tree. And, we couldn't afford mechanics.
In that case.....you have some idea then.....

"3 in the tree"....haven't heard that one in awhile.....I wonder how many people would even know how to shift one of those....
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Old 07-27-2012, 11:10 AM   #8
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Switches and currents


The major problem as I understand it is that it is more difficult to extinguish a DC arc than an AC arc, at equal voltage and current. This is because household AC crosses through zero volts 60 times per second, whereas DC is simply a sustained voltage. A switch meant to interrupt 20A DC needs to have greater separation between the contacts when opened, compared to a switch meant to interrupt 20A AC.
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Old 07-27-2012, 12:10 PM   #9
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Why would a switch be listed/rated for AC only? If one used an AC-only switch on a DC circuit will the universe collapse?
I've seen relays that were rated at 1A at 120vac or at 28 vdc so DC is almost 5x worse than AC for contact life.
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Old 07-27-2012, 01:50 PM   #10
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The major problem as I understand it is that it is more difficult to extinguish a DC arc than an AC arc, at equal voltage and current. This is because household AC crosses through zero volts 60 times per second, whereas DC is simply a sustained voltage. A switch meant to interrupt 20A DC needs to have greater separation between the contacts when opened, compared to a switch meant to interrupt 20A AC.
I guess I knew this but didn't realize I knew it. Welding with DC is much 'easier' when compared to AC.
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Old 07-27-2012, 01:52 PM   #11
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In that case.....you have some idea then.....

"3 in the tree"....haven't heard that one in awhile.....I wonder how many people would even know how to shift one of those....
The linkages had a bad habit of binding if you didn't manipulate the shifting arm just "so so". That was a time when driving a vehicle had some art form to it.
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Old 07-27-2012, 02:12 PM   #12
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I guess I knew this but didn't realize I knew it. Welding with DC is much 'easier' when compared to AC.
Depends on the load. If purely resistive (no inductance), its not to bad....but add a relay coil....now you have more issues. That is why you see a diode across relay coils that are dc powered.

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The linkages had a bad habit of binding if you didn't manipulate the shifting arm just "so so". That was a time when driving a vehicle had some art form to it.
Yep.....along with double clutching.........you know we are giving away our age........
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Old 07-27-2012, 04:05 PM   #13
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The linkages had a bad habit of binding if you didn't manipulate the shifting arm just "so so". That was a time when driving a vehicle had some art form to it.
Ahhh....memories.

Many a time I had to crawl under my old Ford Econoline and unjam the linkage out of 3rd gear. Or was it reverse? I can't remember now.
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Old 07-27-2012, 06:11 PM   #14
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Ahhh....memories.

Many a time I had to crawl under my old Ford Econoline and unjam the linkage out of 3rd gear. Or was it reverse? I can't remember now.
The C10 was a little more friendly. Pop the hood and stand at the left front fender, reach in with only 2 fingers, then pull up sharply and evenly. What fun!
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Old 07-27-2012, 10:59 PM   #15
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