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-   -   Switches and currents (http://www.diychatroom.com/f18/switches-currents-151654/)

jlmran 07-26-2012 10:49 PM

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?

stickboy1375 07-26-2012 11:12 PM

Quote:

Originally Posted by jlmran (Post 975228)
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. :)

ddawg16 07-27-2012 12:54 AM

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....

md2lgyk 07-27-2012 05:49 AM

Quote:

Originally Posted by ddawg16 (Post 975265)
....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.

jlmran 07-27-2012 06:16 AM

Quote:

Originally Posted by md2lgyk

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.

J. V. 07-27-2012 10:40 AM

Quote:

Originally Posted by jlmran (Post 975324)
. And, we couldn't afford mechanics.

Neither could we. We walked. :laughing:

ddawg16 07-27-2012 10:44 AM

Quote:

Originally Posted by jlmran (Post 975324)
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....

McSteve 07-27-2012 11:10 AM

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.

Yoyizit 07-27-2012 12:10 PM

Quote:

Originally Posted by jlmran (Post 975228)
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.

jlmran 07-27-2012 01:50 PM

Quote:

Originally Posted by McSteve
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.

jlmran 07-27-2012 01:52 PM

Quote:

Originally Posted by ddawg16

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.

ddawg16 07-27-2012 02:12 PM

Quote:

Originally Posted by jlmran (Post 975565)
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.

Quote:

Originally Posted by jlmran (Post 975566)
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........

zappa 07-27-2012 04:05 PM

Quote:

Originally Posted by jlmran (Post 975566)
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. :mad:

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.

jlmran 07-27-2012 06:11 PM

Quote:

Originally Posted by zappa

Ahhh....memories. :mad:

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!

jbfan 07-27-2012 10:59 PM

Quote:

Originally Posted by J. V. (Post 975454)
Neither could we. We walked. :laughing:

Uphill, both ways!!:whistling2:


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