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How do you read the ratings of a relay?

  1. Jan 10, 2016 #1
    Hey guys, hopefully this is the right subforum..

    Anyways, I am wondering how to read the ratings of a relay. Here is what I am looking at:


    The rating for this piece is 10A @ 240VAC/125VAC, or 10A @ 30VDC. Would it be safe to use these relays with mains power(110v US, 20A)?

    Say I want to switch on/off a lamp. The lamp has a 100watt light bulb. I know that 110v and 20 amps comes out of the socket, and I know that 110v AC is not a problem for the relay. But what about the 20 amps? At 110v, wouldn't the light bulb technically need around about 0.9 amps to run? Would this be safe to assume that technically only ~0.9 amps is being fed through the wires at any time?

    Another example, say I have my PC. My power supply is rated for 600 watts with 80% efficiency. So this gives me a max wattage at any time to be 720(am I right here?). With Ohms law you could calculate that at 720 watts max, and 110v, the maximum amperage would be around 6.5? Once again, this would be safe for the relay, right?


    Also quick question. If I wanted to protect the relay from ever receiving over 10 amps, I would just use a fuse, right?
  2. jcsd
  3. Jan 10, 2016 #2


    Staff: Mentor

    Your thinking is correct. Adequacy of the relay depends on the current it must carry, not the rating of the supply.

    You are also right that a 10A fuse in series is a good way to be sure the relay will not be overloaded.
  4. Jan 10, 2016 #3
    Okay thanks! Just wanted to make sure before I play around with it to prevent burning down anything :)

    Now I've heard that some fuses take a little bit of time to "break" once they go past 10 amps. Granted this time is very short, but should this be worrisome at all to the relay? What were to happen if by any chance the relay was hit with a quick shock of 11 amps? Or is this even possible?
  5. Jan 10, 2016 #4


    Staff: Mentor

    Ht he limiting thing on a relay or switch is how much current can it interrupt. The risk us thst the relay is commanded to open when carrying more than rated current.

    But I'm not clear myself when they use "slow blow" fuses instead of regular ones.
  6. Jan 11, 2016 #5

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    Fuses have a time overcurrent graph that shows how long they take to open for a given current.

    you might also peruse this site for introduction to subject of overcurrent

    slow down your thinking like a frame-by-frame football instant replay
    it takes time for current to heat something to melting point
    the objective is to pick a fuse that melts before anything else does

    learn what is I2T it's a handy concept

    old jim
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