Bad Relay?

Discussion in 'General Electronics Chat' started by tryingtolearn, Feb 5, 2011.

  1. tryingtolearn

    Thread Starter New Member

    Apr 4, 2010
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    Is it possible to test if a relay is bad or not? I am trying to drive a 120v relay (12v to coils, switches ON 120v, relay rated for 120v) from a pic mcu. The coils ohm out. It gets power. But it never throws. It has a NO and NC pole. It stays on the NC and never moves from what I can tell ohming it. Any other possibilities that I can use to see if it is bad? Thanks.
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
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    It is certainly possible to test a relay, but first you need to know what type it is. Can you find out its part number, and perhaps get a data-sheet for it? This information will also help you tell whether your device can drive the relay.

    For instance, can you find out the nominal coil voltage? What is the coil current? I note that you mention coils - does this mean more than one per relay? Could it be a latching relay? Could it be a polarised type?
     
  3. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    I assume the MCU is driving a resistor/transistor that is driving the relay, and not driving the relay directly. Can you measure the MCU's output to see if that's changing? Can you measure the transistor's collector (or drain) voltage to see if that is changing?

    Ken
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    You should post a schematic of how your relay driver is wired.

    A PIC cannot drive a 12v relay coil directly; it needs to have a device (such as a Darlington transistor, a high-gain bjt, or logic-level MOSFET) to sink the current from the relay's coil.

    The relay coil also needs to have a reverse-EMF protection diode across it, or the driver will likely get "zapped" when the coil current is turned off.
     
  5. tryingtolearn

    Thread Starter New Member

    Apr 4, 2010
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    782XAXCL-120A - The only data sheet I've been able to find is http://www.mouser.com/catalog/specsheets/850-0210.pdf

    I've attached a quick sketch of the circuit from the mcu.

    Not sure which pin is the Drain. From pin 3, voltage changes when conditions are met and the mcu sends signal.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    See that "-120A" on the end of your part nuimber?
    That means the coil is rated for 120VAC.
    Look on the bottom left of the 3rd page for the part number breakdown.
    The relay coil won't engage until it gets about 102VAC across its' terminals.
    Measure the resistance across the coil, it should read about 4.43k Ohms.
     
  7. tryingtolearn

    Thread Starter New Member

    Apr 4, 2010
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    Thank you for pointing that out. Sorry for being slow on that one.

    One more question.
    If I go ahead and still use it and I put 120v on the coil, is there a possibility of messing up any of my parts with how I have the circuit built?

    Thank you for taking the time to help.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    You cannot safely energize the relay coil with your existing driver circuit.
    You need to isolate the 120VAC mains from the logic side of things.

    You might use a 5v relay that has contacts rated for 120VAC, but that's getting a bit silly. Better to pick up a relay that actually has a coil rated for 5vdc or 12vdc, and contacts rated for your load voltage/current.
     
  9. tryingtolearn

    Thread Starter New Member

    Apr 4, 2010
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    0
    Thank you for your input. I will get another relay.
     
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