Monday, Jan 24, 2011 at 21:06
Hello Bonz
As you probably guess it depends a bit on both current and voltage - the interesting effect is that the reversal of polarity with AC quenches the spark that would normally occur and as its a pretty uniform change is does a good job of putting the fire (spark out) hence much more power can be handled before destructive arcing can occur , in the absense of any other
information a handy rule is to derate to about 1/5th i.e. 10amps for a switch becomes 2 amps DC.
Of course this is a rough and ready rule and if a switch is properly designed to handle sparking then almost equal current can be switched.
The voltage caused by opening the switch with a resistive load might be 12v, but as soon as an inductor (which holds/stores energy) is the load then as you open the switch then a thousand volts could be generated for an instant.
This 1000v spike could go anywhere depending on the design of the switch
if done right it doesn't do much damage but if not it could arc in any direction - hence you can't have a hard and fast rule.
Two other things to consider are
1/ So what if the switch fails ? - if its a non-critical application you can use the switch at higher currents.
2/ Intersting thing is that a slight arc can actually be benefical to a DC switch as it can clean the contacts from oxidizing (called a wetting current) - but thats only for those have have driven all the tracks and have spare time on their hands, so I'm never really going to understand that one properly.
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