Using relays - Soldering Iron timer

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pdavis68

Joined Nov 27, 2013
42
Over the last year, I've gotten really really bad about leaving my soldering iron on. Sometimes for days. I've been going through tips like it's going out of style and I need to put an end to it. So I had this idea for a simple little timer circuit.

I've got a little arduino pro mini I'm programming. There will be a button to start a roughly 30 minute count-down timer. This will turn on a relay that will activate my soldering iron. Additionally there's a 2 digit 7-segment display to display the countdown as it proceeds. Pressing the button resets the timer...

So that's the basic idea. My question is about the relay. I have zero experience with relays and I just want to verify the specs on this. I pulled this little blue plastic radio shack PC mount relay from a box of stuff. Says it's rated for 10A@120VAC. So just to verify: Way more than enough to handle my 60W iron. Correct?

I kind of have a hard time imagining that little relay pushing 1200W.
 
I pulled this little blue plastic radio shack PC mount relay from a box of stuff. Says it's rated for 10A@120VAC. So just to verify: Way more than enough to handle my 60W iron. Correct?

I kind of have a hard time imagining that little relay pushing 1200W.
It's probably rated for 1200 watts resistive. That is to say; it is not designed to switch highly inductive loads.

The biggest determining factors of a relay's size are the amount of current it must pass when closed and the amount of energy that will be dissipated into and between it's contacts in the form of heat (arcing) when interrupting either full load or locked-rotor current as applicable.

Any relay identified for switching inductive (i.e. motor or ballast) loads is going to be noticeably larger than those small blue PC-mount relays Radio Shack sells. Usually most 8- or 11-pin "ice cube" relays tend to be good for around 2 horsepower at 240V single phase. If you need any more power than that, then you're generally stepping up to a NEMA/IEC contactor with heavy double-break contacts:


An example of the sort of arcing that you'll see with inductive loads, and why motor-rated relays are much larger for a given amperage:
 
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