| Electrical Question GMC?? [message #114942] |
Mon, 14 February 2011 10:31  |
Jim Wagner
Messages: 339
Registered: February 2004
Location: Brook Park, Oh
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If a toggle switch is rated at 20a@125vac what would it be rated for if I use it with 12vdc.
Thanks in advance.
Jim Wagner
Brook Park, oh
76 Glen Brook
71 Vega
69 Corvette
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| Re: Electrical Question GMC?? [message #114943 is a reply to message #114942] |
Mon, 14 February 2011 10:40  |
idrob
Messages: 645
Registered: January 2005
Location: Central Idaho
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| Jim Wagner wrote on Mon, 14 February 2011 08:31 |
If a toggle switch is rated at 20a@125vac what would it be rated for if I use it with 12vdc.
Thanks in advance.
Jim Wagner
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Same current rating, 20 amps... But... since breaking a DC circuit causes a larger "spark" (think of welding), the switch technically should be DC rated. However, in practice, at relatively low currents, there is seldom a problem other than possibly a shorter switch life.
At least that is what I think I know.
Rob Allen
former owner of '76 x-PB
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| Re: [GMCnet] Electrical Question GMC?? [message #114945 is a reply to message #114942] |
Mon, 14 February 2011 10:43 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
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Jim,
There is no good answer to that question. If the manufacturer doesn't
provide a DC rating, it's probably because the material, switching speed,
heating characteristics, etc., are incompatible with the DC current's
tendency to arc across opening contacts.
If you really need to use an ac-only rated switch for DC, it should probably
be only in a non-critical application, and with a de-rating factor of about
5; in other words, no more than 4A through your 20a ac switch. For more
load or reliability than that, use the switch to control a relay.
Ken H.
On Mon, Feb 14, 2011 at 11:31 AM, Jim Wagner <slwjmw@sbcglobal.net> wrote:
>
>
> If a toggle switch is rated at mailto:20a@125vac what would it be rated
> for if I use it with 12vdc.
>
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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| Re: [GMCnet] Electrical Question GMC?? [message #114946 is a reply to message #114944] |
Mon, 14 February 2011 10:56 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
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Jim,
I think that's much too optimistic a de-rating. The major reason for ac
current ratings being higher than those for DC is because a contact-damaging
arc is sustained across the contacts when they open, especially if feeding
an inductive (coil) load. With DC voltage as a driver, the tendency is for
current to continue to flow through the arc for an extended period while the
load "dies". With ac, the voltage passes zero every 1/120th of a second so
that the arc across the contacts MUST die that quickly. A DC rating of
about 20% of the ac rating is probably more appropriate. And even that
should be for non-critical applications.
Ken H.
On Mon, Feb 14, 2011 at 11:40 AM, James Hupy <jamesh1296@gmail.com> wrote:
> I believe that 20 Amps is the maximum current flow, regardless of the
> voltage. There is some differences between AC and DC. To be on the safe
> side, I would limit the current flow to about 75 percent of the maximum
> rating, in this case that would be about 15 amps.
>
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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| Re: Electrical Question GMC?? [message #115022 is a reply to message #114970] |
Mon, 14 February 2011 21:59 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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I have a 3.0 amp fuse in series with my HEI and have never blown it. The HEI on a GMC draws around 1.6 amps. Just use a switch. Adding a relay for low current devices just introduces another possible point of failure with no useful gain.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: Electrical Question GMC?? [message #115036 is a reply to message #115022] |
Mon, 14 February 2011 23:15 |
idrob
Messages: 645
Registered: January 2005
Location: Central Idaho
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| Ken Burton wrote on Mon, 14 February 2011 19:59 |
I have a 3.0 amp fuse in series with my HEI and have never blown it. The HEI on a GMC draws around 1.6 amps. Just use a switch. Adding a relay for low current devices just introduces another possible point of failure with no useful gain.
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I second that, relays are also possible failure points. I learned that with my headlight relay system. I lost the low beam relay, and until I could substitute it for the high beam relay, I had to drive blinding everyone who came at me. Then when changed out, I had no high beams. That was a long evening.
Rob Allen
former owner of '76 x-PB
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| Re: Electrical Question GMC?? [message #115038 is a reply to message #115036] |
Mon, 14 February 2011 23:33 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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I'm not arguing against ALL relays. Relays preform a necessary function is areas requiring higher current with minimal voltage drop. As an example can you imagine the size of the switch and cables you would need to run the starter. Early cars and some airplanes used this approach. Over time they improved the circuit and starter solenoid came into use to eliminate these problems.
In low current applications it usually makes no sense to add a relay.
There are other low current applications where a relay could / should be used like triggering multiple circuits with one switch, or triggering a circuit with a ground connection when positive is the voltage you need to turn off and on. There are also several other applications where low current relays should be used.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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