12v DC switch

[sail4evr]

Hi, can I run 12V led lighting through a typical residential 120v AC lighting switch. Perhaps (10) 12v 10watt LED lights.? 100 watts total. thanks for your help

 

[pcgeek11]

Yes.

 

[Thebobo]

Easy peasy

 

[LTC8K6]

100 watts at 12 volts will be 8.3 amps, for reference.

 

[PowerEngineer]

The difference between switching AC and DC is that by its very nature 60 Hz AC will have points in time (every 8.3 milliseconds) the current goes to zero (whereas DC current stays constant over time). When you flip a switch and the contacts begin to move apart, you'll start drawing a bit of an arc. This arc will essentially extinguish itself when the AC current passes through zero, and this gives the ionized air between the contacts a short time to de-ionize before the AC voltage is high enough to have it restrike across the contacts. With DC, the arc does not extinguish itself but has to be drawn out far enough quickly enough to break the arc. So in general, DC current is harder to switch than AC current. This video shows you what I mean:

So the strictly correct answer is that you should only switch DC using switches with appropriate DC ratings. That said, you wouldn't be the first person to use an AC switch in a DC application. I recollect that he rule of thumb is that an AC switch can probably clear up to its AC rated current provided that the DC voltage is one third (or less) of its AC rated voltage. So if your AC light switch is rated for 120v/15amps, then it might be okay to use in your 12 volt and 8.3 amp application. I'm not sure how long the AC switch will hold up as switching may burn up the contacts over time. Personally, I would use a DC rated switch (or use the AC switch to turn the power supply for your 12 volt lighting on and off).

 

[William Gaatjes]

The difference between switching AC and DC is that by its very nature 60 Hz AC will have points in time (every 8.3 milliseconds) the current goes to zero (whereas DC current stays constant over time). When you flip a switch and the contacts begin to move apart, you'll start drawing a bit of an arc. This arc will essentially extinguish itself when the AC current passes through zero, and this gives the ionized air between the contacts a short time to de-ionize before the AC voltage is high enough to have it restrike across the contacts. With DC, the arc does not extinguish itself but has to be drawn out far enough quickly enough to break the arc. So in general, DC current is harder to switch than AC current. This video shows you what I mean:

So the strictly correct answer is that you should only switch DC using switches with appropriate DC ratings. That said, you wouldn't be the first person to use an AC switch in a DC application. I recollect that he rule of thumb is that an AC switch can probably clear up to its AC rated current provided that the DC voltage is one third (or less) of its AC rated voltage. So if your AC light switch is rated for 120v/15amps, then it might be okay to use in your 12 volt and 8.3 amp application. I'm not sure how long the AC switch will hold up as switching may burn up the contacts over time. Personally, I would use a DC rated switch (or use the AC switch to turn the power supply for your 12 volt lighting on and off).

Although i agree, there is one thing to keep in mind and that is the level of the voltage.
If he is using 12V DC only, there is going to be considerably less arcing when compared to 320 DC (220 AC rectified), because of lot less electric field strength. Less field strength means less ionization and less contact material being burned away because of the arcing.

 

[PowerEngineer]

Although i agree, there is one thing to keep in mind and that is the level of the voltage.
If he is using 12V DC only, there is going to be considerably less arcing when compared to 320 DC (220 AC rectified), because of lot less electric field strength. Less field strength means less ionization and less contact material being burned away because of the arcing.

Yes, I agree that lower DC voltage makes it likely that the AC switch will work. I want to point out, however, that typical AC light switches are intended for switching loads with reasonably high power factors where the phase angle difference between the voltage and current wave forms are pretty small. This means that the voltage that is building up across the open contacts immediately after the AC current has extinguished itself should be starting from a small fraction of its peak value because it too will be close to a voltage zero. And given that we're only dealing with 125v AC, I am thinking that the difference in field strength on opening for the AC and DC cases might be less than you think. The initial ionization has more to do with the current which at 8 amps is pretty high (the resistance across the arc is low which also makes the voltage difference across the gap low until the arc extinguished), and I think that pitting will be a function of the current and the time to clearing. I am not really sure, but would not be surprised to see more pitting for the DC case. As you point out, the build up of field strength across the open contacts after the arc has cleared is much lower for the 12v DC and therefore there would seem to be no danger of restrike across the contacts once the arc has cleared.