View Full Version : Amp rating of switches.
Apologies for the rudimentary nature of this post, electrics are a mystery to me - the meagre knowledge I occasionally acquire I seem to promptly forget... :dispirited:
I'm putting together a small stirring contraption that uses one of those 25rpm microwave motors. The motor runs off the mains - 240VAC - and I'm going to use a mini SPST toggle switch as an on/off. The switch I bought is this one:
...it's rated at 3A at 125V or 1A at 240V. The motor has a power rating of 4W so therefore draws .016A (it'll barely get any load so it's unlikely to draw much more than this) - I'll wire the live through the switch and the neutral direct to the motor.
Whilst looking for switches I came across the 10A rocker switches that you can buy from Halfords for use in 12VDC car applications - the connectors look pretty chunky and that got me thinking about whether this hardware would do the job - okay, it's three times the price and I've heard about relatively low DC currents blowing AC hardware rated for much higher amperages, but it's just added a layer of confusion... An amp's an amp, right? Seemingly not - anyone have a layman's explanation that I might have a chance of getting all of my three brain cells around..?
12-10-2016, 10:56 PM
Not sure what you are asking here Wal. the Maplin one will do the job but its a bit small physically .
The 12V DC one would probably do the job but if its rated at 12V the insulation might not be good enough at 230V.
The trouble with DC is that you get a big arc when its switches that is why they are rated lower. AC does not get the same arc as the voltage will go down to zero every half cycle and will extinguish the arc. I know its a bad explanation. This is a bit more robust http://www.ebay.co.uk/itm/SPST-Standard-Mains-Toggle-Switch-On-Off-1-5A-250Vac-Inc-Legend-Plate-STS-SPST/361483778745?_trksid=p2141725.c100338.m3726&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%2 6asc%3D20150313114020%26meid%3D903c745abf1c461ca3a e2d993927da0b%26pid%3D100338%26rk%3D1%26rkt%3D20%2 6sd%3D231916583861
12-10-2016, 10:57 PM
Personally, I would go with the Maplin toggle switch or equivalent. At least it's rated for 240V AC use, and you will be using it well within its max ratings. The 12V switches aren't necessarily going to be safe for mains use. And an amp isn't an amp when you're comparing AC and DC! With DC, as the switch contacts open, you start to get an arc forming that will continue until the contacts have separated enough stop the arcing. This leads to damage to the contacts fairly quickly, and you need a switch rated for DC operation as it will have some kind of mechanism to stop the arcing. However, with AC, even though it might start to arc as the contacts open (if you do this at the peak of the cycle), as the current drops towards zero any arcing will stop anyway, and by the time the voltage is swinging the other way, the contacts will probably be far enough apart that no new arc forms. That's a pretty simplified explanation, but I hope it helps give a bit of insight in a hand-waving kind of way :smile:
(overlapped with Clive but at least we agree!)
12-10-2016, 11:04 PM
Yes Neale but you said it better.
>Not sure what you are asking here Wal.
Hehe, I'm probably less sure than you are Clive. I guess I was just getting the flavour of amps confused along with how the voltage affects how much current can be handled. Clearly it's always the best idea to use the right gear - in my case stuff that's rated for AC, but out of interest (and I'm unlikely to be trying it at home...) is there a way of ascertaining whether it's safe to use DC rated gear in an AC circuit..? Actually, I might have found the answer - does this get me close?
I used the calculator here (https://www.batterystuff.com/kb/tools/ac-to-dc-amperage-conversion-run-through-an-inverter.html). So looking at those results from the calculator, am I right in thinking that the current I'm drawing in my 240VAC application would be equivalent to 3.52A in a 12VDC world, theoretically making the 12VDC switches safe to use..?
13-10-2016, 08:19 AM
In this case, that calculator is a red herring. The fact that it mentions AC and DC is not really relevant and is rather misleading. For a switch, you have to look at voltage rating, current rating, and whether it is AC or DC rated. Switches will often have both an AC and a DC rating and you have to look at the appropriate one. If a switch is designed for 12V DC, then pushing it to mains voltage means that you are trusting levels of insulation and safety for which it is not designed. It might be OK, but personally I wouldn't trust it. The contacts might handle the voltage and current (there is no magic threshold that says 12V is ok but 13V is not) but the casing and materials are not rated for mains voltage. Get a low-voltage circuit wrong and you might blow a fuse. Get mains wrong and it can kill. The toggle switch is clearly designed for mains voltage so is the safe choice.
Understood, Neale! Thanks for your explanations. No danger of me trying out DC switches in an AC application - firmly in agreement that you use the equipment that's been designed for the job. Heh, lack of knowledge puts me in the category of a child that asks 'but why...?' Repeatedly!
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You may well find that if you were to go to Halfords and look at the ratings on the switches, they may have an AC rating written on them. It's just they won't list it against the details of the switch, as they're only wanting them used in vehicles.
Switch ratings depend on AC/DC, voltage, and load type.
Personally, I wouldn't bother with that toggle switch, but if I did, I'd want it mounted to a metal control panel that is properly grounded.
I'd much rather use a rocker switch (round ones are available for those who don't like making square holes!), and preferably an illuminated one as it lets you know when things are switched on, and have power.
>Personally, I wouldn't bother with that toggle switch
Yeah, I was looking at it again - it's a bit of a feeble thing - it had the shortest contacts which was what I was after. Just bought a couple of 10A 240VAC rockers from RS - half the price, double the quality - I have a feeling that the contacts will be a bit too long (the case is very shallow) but I'll give it a go.
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