If a toggle-switch is rated for 15amp at 250VAC, what is it rated for at 12VDC ??

I know that watts = volts divided by amps, or somesuch equation; does this equation apply when cross-referencing toggle-switches ??

If that be the case, this would be a 300-amp at 12VDC rated switch, right ??

Let's hear from the experts............er, Jim Lane, where are you ??

 

Good question I was wondering this for quite awhile. I have access to quite a few 250vac switch's for free but was afraid that I would end up burning something up.

 

amps are amps. the voltage rating only relates to the amount of voltage the insulation and design can handle. 10 amps at 12v is just like 10 amps at 240v. the conductors and the contacts must be of the same grade. you can use a 240v 10a rated switch on a 12v 10a circuit. you can't use a 240v 5a rated switch on a 12v 10a circuit.

 

True amps are amps, but one is amps AC and one is amps DC.

The real concern however is volts AC and volts DC.

AC is easier to interrupt because voltage will go through zero during the AC cycle making it much easier to extinguish an arc.

DC is constant and is much harder on contacts than AC. Most DC contacts will have a greater distance between them to interrupt the load or will use other methods to help extinguish the arc.

Because of this, if a switch or breaker is rated for AC and DC the DC ratings will usually be much lower than the AC ratings.

For instance Square D QO breakers rated for a straight 240VAC are only rated for 48VDC at the same current rating. Those breakers are rated to interrupt a 10kA AC fault, however when used on a DC system its only listed to interrupt a 5kA fault.

For the switch you're using I would say it would be alright as long as you didn't go over the obvious 15A.

 

DaveB.inVa is on to the answer but it gets a little more complicated than the AC zero crossings stopping the current flow. If the switch is switching a resistive load like a lamp, the switch rated amps for AC will also work for DC. This is because a resistive load cannot store any energy and just makes heat when on. The problem with switch amp ratings is when the load is inductive like a motor. The inductance is the result of the motor windings and iron core forming an electro-magnet. If the switch is opened with the motor running on DC this inductance will discharge its stored energy and burn the switch contacts. Over a many on-off cycles the contacts will fail just like the infamous starter contacts in our Dodge.

One way to help out a switch, making the contacts last longer is to connect a capacitor across the contacts. This capacitor soaks up the energy stored in the inductance and will help swallow the arc. I don’t know how many of you guys remember points in the ignition of old cars and trucks, but a capacitor is used to soak up the arc from the ignition coil being turned off. If you tried to run the old engine without the capacitor the points would quickly fail.

 

Excellent point bent valves. I the thought of an inductor coming into play didn't cross my mind at the time!

If you switch off an inductive load you will get a voltage spike that can be of a much higher value than the applied voltage. Basically an inductor or capacitor doesn't like change. For an inductor it likes to keep current constant. So when the switch is open the collapsing magnetic field creates an extremely high voltage to try to keep that current flowing. This is exactly how an electric fence box works. With a capacitor it stores a charge and likes to keep voltage constant. When voltage drops it dumps the charge and creates a very high current.

Inductive switching surges are not only present on DC but AC as well. They just may not be as prevalent because sometimes you do switch off at less than peak current or voltage.

 

The more I learn, the dumber I realize I am.







And I'm smarter than most of the people I am around all the time; scary ain't it.

 

In the case of passive devices (only carries power, not uses power), those are maximum ratings.

some devices will have a dual rating for ac and dc voltage, some dont.