Current calculator?

[crashwg]

Wondering if there's a simple way of figuring this out.

I've got a 5VDC Relay with contacts rated at 1A @ 120VAC/24VDC.
The power I'm attempting to switch is a wall-wart:

Input is 100-240VAC @ 0.6A
Output is 12VDC @ 2.0A

According to the the rating of the relay, the input is within spec but I'd feel safer switching the output.  Problem is I don't know how to do the math to figure the rating of the relay @ 12VDC or even if it works that way...

[DaOld Man]

I would not use this relay on the secondary (low voltage side) of the transformer.
However, it should work ok on the primary (high side).

[MonMotha]

Most relays have a "safe operating region" diagram.  This will indicate boundaries as a function of both voltage and current.  There will usually be separate plots for AC and DC as DC tends to arc whereas AC arcs will self-extinguish upon zero crossing.

Generally, there is a "maximum voltage" and a "maximum current" that are reached and some sort of derating (usually just given as linear) between extremes.  The points where you get into the derating vs. voltage/current "hard limits" are device dependent.  Without such a plot, assume that even at 12VDC, you can't go above 1A on the contacts.

If you have a real datasheet (not just a little glossy info sheet), it should have this plot in it as well as a ton of other data which may or may not be important to you.

Switching the primary (input) side is no problem here, though, as previously mentioned.

[crashwg]

What is the danger of using the relay anyway knowing it's rating and the rating of the wall-wart?

[MonMotha]

Generally just premature failure due to contact wear. At 2x the rated current, you'll probably see ~1/10th the rated lifespan in terms of number of cycles, but this can vary quite widely depending on the construction of the relay.  Worst case is that it will start having poor closure due to pitting on the contacts causing increased heating and a possible fire, though that's pretty unlikely.  Might damage your downstream device, though.

Will you actually be drawing all 2A from the wall wart?  Just because it can source it doesn't mean you'll draw it.  The device hooked up to the brick will only draw what it needs to, which can vary a lot depending on what kind of device it is.

Really, best to find a relay with the proper ratings if you do want to switch the secondary side.

[bkenobi]

Will you actually be drawing all 2A from the wall wart?

That's what I was going to say.  The transformer is spec'ed to be capable of supplying up to 2A, but it will only do that if the device it's powering is drawing that much.  If this is the transformer that came with the device you are powering, then it might draw that much.  However, if you are using a generic transformer, then it's probably not going to pull that much current.

I suppose the only way to know for sure is to either find some documentation for your device or to measure it yourself.  If you have a multimeter, you might be able to figure it out if you can access the power and ground from your device.