Switch maximum rating Watt/Ampere/Volt?!

Using Arduino General Electronics
1

Hi there!

Just a quick question if I understood this right:

I have a switch which is rated max. 3A at 125V (375W).

Is it correct that the max. current at 12V would be 31.25A?
Is this correct?

Thanks!

2

No. The switch has a maximum voltage it will safely break.
It also has a maximum current and this will be lower at DC since you have a phenomena called arc-over that happens at DC.

So a switch rated at 3A at 125V AC would probably not be any more than 1-2A at 12V DC.

// Per.

3

Also consider that the heating effect due to the contact resistance is proportional to the current squared, not to the power those contacts are carrying.

4

Okay!
Thank you very much for clarification!

5

Zapro:
So a switch rated at 3A at 125V AC would probably not be any more than 1-2A at 12V DC.

Maybe the current rating is based on the size of the contacts instead for this particular switch.

Most relays have separate current/voltage ratings for AC and DC printed on them, where indeed the AC ratings are usually much higher in both voltage and current. You sometimes see snubber circuits in combination with a relay to help switching off inductive loads.

6

asuryan:
Hi there!

Just a quick question if I understood this right:

I have a switch which is rated max. 3A at 125V (375W).

Is it correct that the max. current at 12V would be 31.25A?
Is this correct?

Thanks!

Breaking a DC circuit is far more difficult than AC as there is no zero-crossing to quench the arcing as
the contacts open. The more power available to sustain the arc, the harder it is to break it, and the
more rapidly the contacts will erode in use.

Typical ordinary switches are only rated upto 24 or 48Vdc, as higher DC voltages are much more
troublesome. Both the DC voltage and peak current matter, as they contribute to arc size and
thus whether it will self-quench over a particular distance. Higher voltage and power DC switches have to
be physically larger in size, and have powerful springs to separate contacts as fast as possible.

High power DC is much easier to switch in silicon than in air, upto a few 100V.