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dredmoody(at)cox.net
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 Posted: Fri May 04, 2007 8:01 am Post subject: Electrical question |
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I think I already know that the answer to the following question is "NO" but there are many of you guys out there who know more about electronics than me so here goes.
I need to install an SPDT switch to select my right or left aux fuel pumps. The EXP BUS power panel will provide the "on / off" switching for whichever pump is selected by the SPDT switch. The problem is this...... the best area for the switch would be centered directly above the fuel selector valve and in that location, I have only 1/2" between the back of the panel and one of the fuel line elbows on the valve. All of the 12VDC rated switches that I have found are too bulky to fit in that restricted space. I have found several SPDT switches rated for as much as 10 amps at 120VAC which would fit there easily but I remember being told long ago that they will heat up too much if I use them for a 12VDC circuit.
Can an AC rated switch be used in this application? Bear in mind that we are talking about boost pumps which are used for short periods of time and draw maybe 2 amps at 12VDC for a minute or two.
Okay all you electronics whizzes out there..... save a harmless lovable dentist from himself.
Thanks,
Dred
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craig(at)craigandjean.com
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| Posted: Fri May 04, 2007 8:54 am Post subject: Electrical question |
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Any switch with an AC rating will have a DC rating - it is just a matter of
finding it. The reason a switch is rated lower for DC than AC AT THE SAME
VOLTAGE is because of the electrical arc formed when the switch is opened.
This is when a lot of the wear occurs. But in an AC circuit the voltage
drops to 0 as the voltage alternates (the "A" in AC) every 1/120 (60 Hz) or
1/100 (50 Hz) of a second. This extinguishes the arc. But you are switching
one tenth of 120 VAC so the arc is reduced.
Take a look at this article from Bob Nuckolls of AeroElectric fame (esp. the
chart at the top of page 2):
www.aeroelectric.com/articles/swtchrat.pdf
Here is the key quote:
"In a nutshell, 125 VAC ratings equate favorably and conservatively to 14
VDC ratings - as long as the switch has a healthy "snap" action . . . all
toggle switches and most rocker switches do. Just because the numbers
stamped on the side of the switch don't mention a DC capability doesn't mean
that the switch doesn't have one. Manufacturers are unable to put ALL of the
information from the chart onto the side of every product, the lettering
would be too small to read!"
-- Craig
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bryanmmartin
Joined: 10 Jan 2006
Posts: 1018
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| Posted: Fri May 04, 2007 8:54 am Post subject: Electrical question |
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It's not the steady current flow that causes problems when using AC rated switches for DC circuits. It's the actual switching process. Interrupting a DC current is harder on a switch contact than interrupting AC current. That's why a switch with a 10A AC rating usually has a much lower DC rating. Those 10A AC switches may work for a 2A DC circuit but it would be nice if you can find out the DC rating of those switches.
On May 4, 2007, at 12:00 PM, <dredmoody(at)cox.net (dredmoody(at)cox.net)> <dredmoody(at)cox.net (dredmoody(at)cox.net)> wrote:
| Quote: | --> Zenith-List message posted by: <dredmoody(at)cox.net (dredmoody(at)cox.net)>
I think I already know that the answer to the following question is "NO" but there are many of you guys out there who know more about electronics than me so here goes.
I need to install an SPDT switch to select my right or left aux fuel pumps. The EXP BUS power panel will provide the "on / off" switching for whichever pump is selected by the SPDT switch. The problem is this...... the best area for the switch would be centered directly above the fuel selector valve and in that location, I have only 1/2" between the back of the panel and one of the fuel line elbows on the valve. All of the 12VDC rated switches that I have found are too bulky to fit in that restricted space. I have found several SPDT switches rated for as much as 10 amps at 120VAC which would fit there easily but I remember being told long ago that they will heat up too much if I use them for a 12VDC circuit.
Can an AC rated switch be used in this application? Bear in mind that we are talking about boost pumps which are used for short periods of time and draw maybe 2 amps at 12VDC for a minute or two.
Okay all you electronics whizzes out there..... save a harmless lovable dentist from himself.
|
--
Bryan Martin
N61BM, CH 601 XL,
RAM Subaru, Stratus redrive.
do not archive.
[quote][b]
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_________________
--
Bryan Martin
N61BM, CH 601 XL, Stratus Subaru.
do not archive. |
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dredmoody(at)cox.net
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| Posted: Fri May 04, 2007 1:05 pm Post subject: Electrical question |
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Thanks for the responses regarding the DC rating of an AC rated switch. Both relpies indicate that the problem point is the arcing that will happen when the switch is opened or closed. That solved my dilemma. In my proposed application the AC rated switch is a SPDT switch which will be used to direct the 12VDC power to either the right or the left boost pump (I have one facet pump mounted in each wing root next to the tank). This "fuel pump selector switch" is fed by the fuel pump switch on the panel, which in my case it is part of the EXP BUS 2V Tray system. So in this application, the selector switch is activated before the "fuel pump switch" on the panel is switched on. Therefore there should be no opening or closing arc at the selector switch. That should make it safe to use a 125VAC 10 amp switch as the selector switch without burning it out or welding it closed at a ripe young age.
Thanks again,
Dred
[quote][b]
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ashontz
Joined: 27 Dec 2006
Posts: 723
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| Posted: Mon May 07, 2007 6:15 am Post subject: Re: Electrical question |
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[quote="dredmoody(at)cox.net"]Thanks for the responses regarding the DC rating of an AC rated switch. Both relpies indicate that the problem point is the arcing that will happen when the switch is opened or closed. That solved my dilemma. In my proposed application the AC rated switch is a SPDT switch which will be used to direct the 12VDC power to either the right or the left boost pump (I have one facet pump mounted in each wing root next to the tank). This "fuel pump selector switch" is fed by the fuel pump switch on the panel, which in my case it is part of the EXP BUS 2V Tray system. So in this application, the selector switch is activated before the "fuel pump switch" on the panel is switched on. Therefore there should be no opening or closing arc at the selector switch. That should make it safe to use a 125VAC 10 amp switch as the selector switch without burning it out or welding it closed at a ripe young age.
Thanks again,
Dred
If you're going to do that, make sure you always use it in that order, selection with the AC rated switch, then ON with the DC rated switch.
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