Light Switch

I agree that switching DC is harder but as I said a 120 volt AC switch will not have a problem with 12VDC... At least not at any current you are likely to encounter save on the power leads to an inverter and likely not even then.

It has to do with the 10 to one (Really 14 to one) voltage difference.
Every switch is rated both AC and DC volts and current. Just not all publish the DC ratings. . I used a standard AC switch for the porch light on my Scamp. the actual light was an automotive headlamp cartridge type (Modern) one element was bad (Likely the low beam) so I wired it up and put it in a Jelly Jar Porch light that I hung on the side of the Trailer and plugged into a 12 volt accessory outlet. that is about a 30-50 watt lamp. Most porch lights are 25 or less.

wa8yxm wrote:
So you know a switch rated for 120 or more volts AC (Standard house type switch) will have no problems with 12 volt DC. In fact it's kind of over kill. Cheaper but still over kill.

Switching DC is harder than switching AC, since AC has a zero voltage (and hence zero current) crossing 120 times every second. Any arcing that starts as the switch opens will generally dissipate during these times. With DC, on the other hand, the contacts need to move far enough apart to break the arc. If you look at the ratings for switches that have both AC and DC ratings, you'll see that the DC ratings are commonly lower than the AC ratings.

That being said, for non-safety-critical applications, I'd have no qualms about using an AC-only switch for DC applications if the voltage and current ratings are suitable. The failure mode generally is that the contacts eventually get welded together and the switch fails to open (i.e. it's stuck on permanently), and if that merely causes inconvenience rather than likely harm, it's not a huge deal to replace the broken switch.

So you know a switch rated for 120 or more volts AC (Standard house type switch) will have no problems with 12 volt DC. In fact it's kind of over kill. Cheaper but still over kill.

If you are willing to use an AC switch, Del City is the source I used to replace the single throw switch in my porch light with a double throw so I could make it a 3 way switch.

I just installed this one to replace a DF-IS001 on a porch light. If you look at the data sheet, you'll see it's DC rated. Perfect fit, it snapped right in. So far so good.

If you can wait a couple of weeks to get them (from China), I just got 5 for $1.98 and ship free on ebay. Search for "5PCS White ON/OFF 2 Position SPST Boat Rocker Switch 6A/250V 10A/125V AC". Presently the link is "http://www.ebay.com/itm/232274699192".

These fit perfectly and you can't beat the price!

Sorry, got confused with another thread I am following. :o

Better to get a DC rated switch.

tenbear wrote:
You are right Bobbo, but we are talking about switching a meter, only a few milliamps at most. I don't think there will be much arcing.

He is not switching a meter with a few milliamps. It is an overhead light that could be 2 to 4 amps. According to www.bulbs.com an 1156 bulb is 26.9 watts at 12.8v. That divides out to right at 2 amps per bulb.

I used words like "can", "may" and "could" for a reason. In some applications, it really won't matter, but in others it really could. If the person making the substitution doesn't understand the possiblities, he may make a substitution in a place where it will matter.

I never said "don't do it", I merely educated as to the issues so he can make an informed decision. I did tell him that "I" would pass on the AC rated switch, because DC rated switches are available.

Also, a properly rated DC switch is little more expensive, if any at all. It just takes a little more looking. I know, I bought a spare ShurFlo water pump and wired it to pump water from 5 gallon totes into my freshwater tank. I put a 14v DC rated switch on the wire I use to power it. It was no more expensive than the AC switches I found everywhere, but it did take a little more looking to find it.

Bobbo wrote:

tenbear wrote:
I have used the AC switches on DC and they have worked just fine. I don't know what they do differently for a 12v DC switch but I haven't found it to matter.

It MAY work fine. It may not, at least for long.

DC arcs a lot longer and a lot hotter than AC when you break the connection. If the switch is not made for the longer/hotter arc, it can burn the contacts. You won't see the problem immediately, but it MAY lead to premature failure.

AC is 60 hertz, which means that the power stops completely 120 times a second as its direction reverses. That is why its arc is shorter, and therefore cooler. DC never stops until the switch completely breaks the contact, and that is not when the contacts separate. It is when the arc that occurs as the contacts are separating stops.

An example in my personal experience: This is not from AC vs DC but from using a switch on a circuit with higher voltage than it was rated for.

I had an electric fence around a garden pond to keep my dogs out of it. DW liked to work in her pond so I put a switch by the pond that broke the connection on the fence wire. It did not turn off the fence pulser. I originally had a switch rated for that voltage. Over time (outside in the weather) the switch finally broke. I went to a big box store and bought a cheap 120v/15amp light switch and wired that one in. Imagine DW's surprise the next time she went out, turned off that new switch, and touched the fence. The voltage was high enough to arc across the open contacts continuously. The voltage to the fence was NOT interrupted. She had words for me.

The same type of thing CAN happen with an AC rated switch on a DC circuit.

You are right Bobbo, but we are talking about switching a meter, only a few milliamps at most. I don't think there will be much arcing.

tenbear wrote:
I have used the AC switches on DC and they have worked just fine. I don't know what they do differently for a 12v DC switch but I haven't found it to matter.

It MAY work fine. It may not, at least for long.

DC arcs a lot longer and a lot hotter than AC when you break the connection. If the switch is not made for the longer/hotter arc, it can burn the contacts. You won't see the problem immediately, but it MAY lead to premature failure.

AC is 60 hertz, which means that the power stops completely 120 times a second as its direction reverses. That is why its arc is shorter, and therefore cooler. DC never stops until the switch completely breaks the contact, and that is not when the contacts separate. It is when the arc that occurs as the contacts are separating stops.

An example in my personal experience: This is not from AC vs DC but from using a switch on a circuit with higher voltage than it was rated for.

I had an electric fence around a garden pond to keep my dogs out of it. DW liked to work in her pond so I put a switch by the pond that broke the connection on the fence wire. It did not turn off the fence pulser. I originally had a switch rated for that voltage. Over time (outside in the weather) the switch finally broke. I went to a big box store and bought a cheap 120v/15amp light switch and wired that one in. Imagine DW's surprise the next time she went out, turned off that new switch, and touched the fence. The voltage was high enough to arc across the open contacts continuously. The voltage to the fence was NOT interrupted. She had words for me.

The same type of thing CAN happen with an AC rated switch on a DC circuit.