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# 12 volt DC switch vs. 110 volt AC switch

coolclay
I dwindle a bit in electrical theory, building electrical equipment and what not. One thing that has always stumped me is what physically is the difference between a 12 volt DC switch and a 110 volt AC switch. They label them very clearly so there has to be a reason one can't be used for the other. Even though I have not had any problems substituting 110VAC switches in 12VDC circuits. The only problem I can theorize is that maybe the components inside can only handle so much voltage and/or amperage. If this was the case I would think the one built with higher tolerances could be substituted for the other but not vice versa. Any one have any ideas, I've looked online but never really found any closure.
coreymanshack
AC wave looks like this ~~~
DC wave looks like this -----

The hardware used with these waves are very different. DC and AC both have their uses.
ocalhoun
For just a basic switch, the only thing that should really matter is the amperage and voltage used. (More so the amperage.)
Too much current through too small a switch could cause overheating, melting, failure, and destruction. Too much voltage through a switch that wasn't designed for it might cause too much arcing, making the switch possibly break, and certainly not last as long.

The difference between an AC or DC switch for the same amperage and voltage? At a basic level, none.

There's no reason you shouldn't be able to use a 110 volt, 1 amp switch for a 12 volt, .5 amp circuit, for example, but the reverse would be dangerous and likely to fail.

This is not necessarily the same for other electrical components though! Don't get creative with it unless you know what you're doing. Take a basic electronics course, or at least read a book or website about it.
coolclay
Thanks coreymanshack, I already knew that though, but thanks.

Ocalhoun-
That was my take exactly. I just thought I would double check to back up my theory. And yes I do already know quite a bit about electricity, I am a household electrician, well sort of. I install appliances during the winter, and run lines, outlets, install breakers and what not.

I just wasn't positive because they label them very well, and don't say anything about being backward (less current) compatible.
coreymanshack
I was thinking you said power supply for some reason. But yeah, ocalhun is right.
Indi
 ocalhoun wrote: The difference between an AC or DC switch for the same amperage and voltage? At a basic level, none.

(Careful! ^_^; That may be true, but it also may not. An AC signal can be a complicated beast, and the voltage and current do not necessarily need to be in phase with each other, resulting in a lower power overall. For a DC signal, the power is just P = I × V, but for an AC signal, the power is P = I(rms) × V(rms) × cos(φ), where φ is the phase difference between the voltage and current signals. i don't know a lot about how electrical components are commonly rated, but i suspect that the required current rating for a 12 VAC 100 W switch is very different from the required current rating for a 12 VDC 100 W switch.

But more relevant here is the fact that if you assume the same power draw, the current through a 110 VAC switch will be much, much lower than through a 12 VDC switch. So you could use a 100 W 12 VDC switch in place of a 100 W 110 VAC switch, but not vice versa. In other words, coolclay was talking about - using 110 VAC switches in a 12 VDC circuit... is probably not wise.

i think you're right though - the current is the key here. Make sure you're current-safe, and everything should be fine.)
SonLight
How well the switch is insulated is an important safety consideration. For a 12 volt switch, only a little insulation is needed to protect the user and prevent short circuits. For a 110 volt switch, the handle should be well isolated from the wires, and the insulation needs to be much stronger.

There are enough differences that generally it would be wise to use any switch only in the type of circuit it was designed for, although it should be safe enough to use a 110-volt rated switch in a 12-volt circuit provided the power carrying capacity is sufficient.
jwellsy
If you're really interested and extreamly bored, try going through a couple of IEEE arc-flash calculations. The results are how many Kcalories of energy can potentialy be released and dictates what kind of Personnel Protective Equipment is required before the panel containing the switches could be opened for maintenance.
chatrack
Ac swich is designed to handle high voltage than high current... & visaversa
davidcyr2000
AC and DC switch contacts are supposed to be manufactured with different materials in order to withstand the arcing that occurs, and the resulting degradation of the switch contacts. AC circuits typically break cooler than DC circuits because the current drops to zero 120 times per second, which means arcing stops at least by the time the current goes to zero. Current in a DC circuit does not drop to zero immediately when the contacts open when there is an inductive component in the circuit. In a DC circuit, the voltage will spike when the contacts open and the arcing will be significantly more than in the comparable AC circuit. This results in more rapid contact degradation through pitting from the high temperature arc. The voltage rating only talks about insulation factor. Current rating only talks about current capacity, neither of which is related to the other. So an AC switch is NOT equal to a DC switch.
chatrack
 davidcyr2000 wrote: AC and DC switch contacts are supposed to be manufactured with different materials in order to withstand the arcing that occurs, and the resulting degradation of the switch contacts. AC circuits typically break cooler than DC circuits because the current drops to zero 120 times per second, which means arcing stops at least by the time the current goes to zero. Current in a DC circuit does not drop to zero immediately when the contacts open when there is an inductive component in the circuit. In a DC circuit, the voltage will spike when the contacts open and the arcing will be significantly more than in the comparable AC circuit. This results in more rapid contact degradation through pitting from the high temperature arc. The voltage rating only talks about insulation factor. Current rating only talks about current capacity, neither of which is related to the other. So an AC switch is NOT equal to a DC switch.

Hi davidcyr2000,

You give more in depth in this case... thanks
jetgirltaxi
 coolclay wrote: I dwindle a bit in electrical theory, building electrical equipment and what not. One thing that has always stumped me is what physically is the difference between a 12 volt DC switch and a 110 volt AC switch. They label them very clearly so there has to be a reason one can't be used for the other. Even though I have not had any problems substituting 110VAC switches in 12VDC circuits. The only problem I can theorize is that maybe the components inside can only handle so much voltage and/or amperage. If this was the case I would think the one built with higher tolerances could be substituted for the other but not vice versa. Any one have any ideas, I've looked online but never really found any closure.

I don't imagine there is any physical difference. The only thing that matters is the AMPERAGE rating of the switch. Current, not voltage, is what generates heat. 12 volt circuits certainly can pull some very heavy amps, think about the starter relay in a car engine. As for alternating versus direct current, I can't see how it matters to the switch.
menino
Yes, I agree with you'll, .... brings me back to my school and college days that AC stands for Alternating current and DC for direct current.

If you want to convert dc to ac, it is possible using an inverter, source (http://en.wikipedia.org/wiki/Inverter_(electrical)).
chatrack
I think in the question, it is not mentioned about the amount of current that has to be handeled.
pyro53
Read and learn, note source, this is real info, tried and proven over the decades. Whatever you people are smoking when you write this stuff - you need to stop - you are dangerous, to yourselves and innocent people around you.

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[PDF] SWITCHESAC VS DC
... in load carrying capability are dramatically non-linear and are best appreciated
by carefully inspecting ahigh-quality switch carrying both AC and DC ratings. ...
www.eaa.org/sportaviationmag/2005/0502_switches.pdf - 2010-01-27

SWITCHES AC VS DC
by Art Bianconi (EAA 92330)
Technical Counselsor 1216
99 Dover Green
Staten Island, NY 10312
Some years ago I w».s fortunate to be able to work alongside engineers from
Underwriters Laboratories (UL) during destructive testing of electrical devices.
It was part of my apprenticeship as a designer for a major electrical manufacturer and it was during this period that I acquired an appreciation for the vital
differences between AC and DC ratings for switches.
I share this with you because I am growing increasingly concerned at the
widespread lack of appropriateness most aircraft builders demonstrate
when selecting switches for the cockpit environment. Each time a builder asks
me to perform a pre-FAA inspection of the aircraft, I carefully inspect the
switches and to date over three-fourths of the projects inspected have turned
up AC rated or non-rated switches in DC circuits.
There is a large scale misconception that any switch can be used so long as
its current rating exceeds the maximum load in the circuit. "Current is current;
what difference does it make whether it's DC or AC? Besides, I'm using a 125
volt AC switch in a circuit with only 12 volts!" The differences in load carrying
capability are dramatically non-linear and are best appreciated by carefully
inspecting a high-quality switch carrying both AC and DC ratings. Typical of this
is the roller and bar micro switch made by MICRO Corporation (Part No. DT-
2RV23-A7). Rated at 10 amps at 125 or 250 volts AC, the same switch can
only carry .3 (that's three-tenths!) of an amp at 125 volts DC. If DC voltage is
increased to 250 volts, the current rating drops even further to .15 amps! In
real terms, this represents less than 1/60 of the original load carrying ability
and all we did was go from 250 volts AC to 250 volts DC!
Those of you who can still remember the old Kettering coil ignition systems
will recall that when the condenser in the distributor went bad, the points generally turned blue and melted down in just a few minutes. Cockpit switches
don't have the benefit of condensers to absorb the electrical inertia present in a
DC circuit and, as a result, the gap temperatures get hot enough to weld contacts,
even those made with exotic high temperature alloys.
The reason for this is simple enough to appreciate: because AC current
changes directions 120 times a second in a 60 cycle circuit; there are 120 times
when there is no current flowing at all. The current actually helps turn itself off
the moment it sees a gap and switch desrgners use this phenomenon to help
reduce the cost of manufacturing AC switches. In DC circuits, however, the
"push" is constant even when the points begin to open and the resulting arc is
DC current's way of demonstrating its resistance to termination.
"But won't my circuit breakers protect me?" No, they won't. Fuses and CB's
provide overload protection and a welded set of contacts will not, by themselves,
cause an increase in circuit load. Furthermore, what often happened during UL testing
was that the points welded shut making it impossibleto open the circuit. Cycling the switch
to the open position was often misleading — yes, the lever moved but inside
the switch, the cam had separated from the welded points and while it appeared
to have interrupted the circuit, the circuit was, in fact, still hot. If the circuit involved was your fuel transfer pump or fuel boost pump and you thought it
turned off when in fact it was still running, what would the consequences
be? If it were a flap or elevator trim motor or a gear retraction device, how
would a tripped circuit breaker save you if the activating switch was welded
closed and in a mode other than what is required for a safe landing?
A DC rated switch will cost you about 3 times more than an AC rated switch
of identical current capacity. If your panel sports 10 switches (which is not
likely) the difference will be less than \$35 (in 1986 dollars). You've gotten this
far. Is it worth jeopardizing your investment or your safety by cutting corners
with cheap or improperly rated switches.
abhinavm24
pyro nice info.
good to see someone trying out his best.
Well answering the query switches are just a method to make devices turn ON/OFF on demand.
Their sole purpose is to complete the circuit! when needed,
what it takes to complete the circuit is to just give metal connections in the circuit which is btw same for both AC/DC hence the purpose.
cleo2010
I used a AC 110 volt light switch in a 12 volt DC car circuit earlier today. I was rigging a switch to turn on my cars cooling fan. The cars sensor was bad and it was overheating because the cooling fan would not come on. I had to drive the the car while the parts store ordered it. I hooked it up to the battery. The AC switch burned out within seconds so it would not turn off. The switch must have either started arching across or it welded something together inside. I threw that switch away. I re-rigged it with 10 gage stranded wire and a 30 amp AC breaker for the switch that I purchased for
\$4.00 from a surplus store. So DC is nothing to mess with. If there was a short it would heat up untill it started a fire. The car's cooling fan has heavy gage wire and a 30 amp breaker for a reason. My part comes tomorrow but I have to drive several places before I can get it and put it in. I'm taking the fire extinguisher with in the car just in case.