Maximum Wattage for 220V 5A relay?

Discussion in 'General Electronics Chat' started by McPit, Aug 17, 2014.

  1. McPit

    Thread Starter New Member

    Aug 17, 2014
    11
    0
    Hi there,

    I bought and installed the following relay on a small 750W heater... and it ended up burning...
    http://www.ebay.com.au/itm/AC-220V-Delay-Timer-Time-Relay-0-60-Minute-H3Y-2-Base-/370526092370


    My understanding is that the relay should tolerate up to 220x5=1100W, and that I should have ample margin with my 750W.
    But the seller is now saying differently:

    For any device, the , the Instant start current will far more than rating current.
    if you used for long time, it will burnt the relay.
    usually, you need leave half more allowance for the device.
    for the relay, the max current is 5A, and you'd better use a device rating current at about 2A.
    or you just used a bigger current relay.

    it is the common sense



    Your thoughts?
    Thanks a lot
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    I agree with the seller.

    Look at the ratio of cold resistance to hot resistance in this wiki table
     
  3. McPit

    Thread Starter New Member

    Aug 17, 2014
    11
    0
    Not sure I understand.
    (Maybe I should specify that the relay is placed away from the heater, and therefore not exposed to its warmth at all?)
    I'm not sure I understand why a 60 minutes timer relay with specifications for 220V and up to 5A ends up burning when it is connected to a 220V, 750W (=3.4A) consumer.
    What am I missing?
    Thank you!
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    The cold resistance of a Nichrome heater is about one third of what it is when it heats up. Your 750W heater is a 2000W heater when the relay first pulls in...

    This is called inrush
     
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  5. crutschow

    Expert

    Mar 14, 2008
    12,974
    3,220
    A contact will bounce for several tens of milliseconds when the relay closes and that, along with the low cold resistance of the nichrome heater which creates an inrush current of 10A or so, causes significant arcing when the contacts close, burning the contacts. So you should use at least a 10A rated relay, 15A being preferable. You will still get the arcing but the higher rated contacts are better able to tolerate that.
     
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