I'm looking for a high current DC switch (able to switch something like 5V@30A) and I've encountered several DPDT that might do what I need, however they are only rated for AC applications (3A@250VAC or 6A@125VAC). Are these rating convertible to DC? or is that something that only the facturarer would know?
AC ratings are typically much more lenient and DC much more strict because of arcing problems:
- AC arcs tend to not last very long, and their polarity is effectively random so that the material transfer from one contact to the other pretty much cancels out.
- DC arcs are much more difficult to stop, and the material transfer is always the same direction so as to erode one of the contacts.
So the DC specs are typically much lower than AC for the same switch. Relays are the same way.
Switches are weird. As AaronD states, it is much easier to interrupt an AC arc than an DC arc, and therefore the contacts in an AC switch do not need to separate as far or as rapidly, plus the contact material does not need to be as robust.
But, as someone else said, the lower the voltage, the less arc there tends to be, so one can get away with using a 240VAC switch to switch DC (near the switch's rated current) at a lower voltage.
And you also can have difficulties as low currents, with both AC and DC. In general one should not use a "power" switch to switch currents below maybe 100ma or so, since the higher current switches rely on the spark from the current to keep the contacts clean, and at low currents the contacts can glaze over. (This is a bigger problem with relays than with switches, I suspect.)