Wildman Rocketry Giveaway!

# Thread: 12v vs. 125v switches

1. ## 12v vs. 125v switches

I have tried to research this a bit, as I thought I recalled seeing it addressed in an earlier thread, but apparently I am either mistaken or inept.

I have a DPST toggle rated for 20 amps at 240 volts. What is the capacity of this switch at 12 volts? I tried to apply Ohm's Law, as I had a very good estimate of resistance, but I didn't understand the answer. How do they rate a switch if they don't know what the resistance of the circuit will be?

2. ## Re: 12v vs. 125v switches

The switch you describe is good for 20 amps at any voltage 240 or below. Amps are the rate of flow of electrons, as far as a switch is concerned, just need to keep the current from getting so high that it fuses the contact points.

Ohms law only applies to a resistive load, which is not what a switch is in a circuit.

Sometimes you will see a switch rated for moderately higher current if the voltage is a lot lower, because voltage contributes to arcing, which can also result in fusing the contact points.

3. ## Re: 12v vs. 125v switches

Switch contact current ratings are often severely derated for DC use. For use with resistive loads (like it will see in a launch controller), your switch should handle the same current at low voltage DC as it will at 240VAC without a problem.

The problems come when an AC rated switch is used to control an inductive load, such as a DC motor. Inductive loads on DC cause severe arcing when the contacts open. And because the current through the circuit is constant (not passing through zero 120 times per second, like it does with AC), an arc may not extinguish once started. The same problem MAY occur in resistive circuits if operated at high DC voltages (like trying to use a 240VAC switch to control 240VDC). Drawing a continuous arc between the contacts will destroy the switch almost immediately.

4. ## Re: 12v vs. 125v switches

20A is 20A. If you push (load) 21A through it at any pressure (voltage), you'll release the "magic smoke".

From Ohm's Law perspective, since R approaches zero, I is undefined. Therefore, E is irrelevant.

Of course, AC voltage/current can be greater, but that's a little more math.

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