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Max and min pickup current of the relay

  1. Jul 3, 2012 #1

    Femme_physics

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    I hope I'm translating this correctly from Hebrew.


    http://img716.imageshack.us/img716/7621/mamser.jpg [Broken]


    I have a relay here and I'm told that the min pickup current is 6mA, and the max is 10mA.... does it mean that the relay only works in this range? What happens if there is more than 10mA?
     
    Last edited by a moderator: May 6, 2017
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  3. Jul 3, 2012 #2

    berkeman

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    I've only seen relays rated by input voltage, not by current. The input voltage is specified (like 5V or 12V), and the coil reisistance and tolerance dictate the coil current...
     
  4. Jul 3, 2012 #3

    cepheid

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    Could it be that at least 6 mA must be flowing through the relay coil in order to actuate the switch? If less than 6 mA are flowing, then the switch will not move? That would be my best guess.

    I couldn't say what the maximum current means.

    I agree with berkeman: most relays I've seen just have a specific input voltage that you are supposed to supply to to the input, and when this voltage is supplied, the coil resistance determines how much current is drawn.
     
  5. Jul 4, 2012 #4

    rude man

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    The 10 mA max figure undoubtedly signifies that beyond that current the coil can overheat at the high temperature limit.

    I don't see the big deal about voltage vs. current. The max. spec. can be imposed either way.
     
  6. Jul 5, 2012 #5

    NascentOxygen

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    There is an ambiguity here, FP, in that it's not clear whether you are quoting a spec for the winding of the electromechanical relay, or whether this is a spec for the input to that transistor driver.

    Would I be correct in guessing that you really don't have a relay at all, just a textbook question that states this as a parameter to apply to your calculations?

    My take on it is that 10mA is guaranteed to operate all relays of this part number, while some will be found to operate [satisfactorily] with as little as 6mA. It's quite possible there's no problem in exceeding that 10mA figure by even 50% or so; few electromechanical devices are so fragile as to have a tightly constrained permissible operating range as narrow as 8mA +/- 2mA. Though you might have found an exception. :wink:

    Manufacturer's data sheets are invaluable for discovering the power limits of devices, and establishing precisely what these abbreviated specs really mean in cases of ambiguity.
     
  7. Jul 5, 2012 #6

    Femme_physics

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    Sorry, Nascent, I meant the min-max pickup current of the relay...yes, in that case "rude man's" comment seems accurate

    Thanks
     
  8. Jul 5, 2012 #7

    I like Serena

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    A coil that overheats at 10 mA?
    Not very likely.

    That is way too little current to overheat anything.

    If you want to burn a carton board man, you'll need way more! :wink:
     
  9. Jul 5, 2012 #8

    cepheid

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    So NascentOxygen's remark that 10 mA is the "must turn on" current and 6 mA is the "must turn off" current might be accurate. In between these two values, the state of the relay is uncertain.

    Of course, we're just speculating. These values could very well have something to do with the transistor and nothing to do with the relay.
     
  10. Jul 5, 2012 #9

    I like Serena

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    I agree. This is the most likely explanation.

    A min and max "pickup current" sounds very much like what is needed for the relais.
     
  11. Jul 5, 2012 #10

    NascentOxygen

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    I wasn't disputing that figure, small as it may seem. There may well be microminaturized relays with very fine wire all fully sealed in epoxy from which little heat can escape, and having maximum allowable fixed currents of 10 mA. What I focussed on was the likelihood of an electromechanical device (inherently robust), seeming to have a "destructive" current so close to its rated operating current.

    The OP has not indicated whether this is from a data sheet, or a textbook question. I suspect the latter, and by a well-meaning author who thought he was simplifying the problem for students using his textbook.
     
  12. Jul 5, 2012 #11

    I like Serena

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    I think it's good if Fp gets a sense of what is realistic and what is not.

    Shall we assume the simplest explanation that fits all the facts?

    (Also, with Fp's other thread in mind, I'd like to dance over corpses.
    Can I? Can I? :shy:)
     
  13. Jul 12, 2012 #12

    Femme_physics

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    LOL at the last comment

    And yes, I just wrote that the current needs to be between 6-10 mA for the circuit to work, no need to get too deep into the nitty gritty I reckon..

    thanks everyone!
     
  14. Jul 12, 2012 #13
    Something not quite right here!! ignoring (???) any voltage across R1 the voltage across the emitter resistor is about 11/50 = 0.22A ie 220mA. So for the circuit drawn the current through the relay coil is about 220mA when S is closed.
     
  15. Jul 12, 2012 #14

    I like Serena

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    Erm... shouldn't that depend on R1 which influences the voltage across the transistor and also of the unknown resistance of the coil?
     
  16. Jul 12, 2012 #15
    ignoring (????) was meant to cover any uncertainty. Safe to assume that current into base of transistor is at least 100X smaller than emitter/colector current (could be wrong !!!) then R1 would need to be 100x bigger than Re to be significant... ? ....yes/no...?....5000Ω....but no value is given.... do something with the values and experience of transistor circuits and relays.
    Still safe to assume that Ve is 11V.... 10V .....9V....8V if you want.
    Look at the principles here
     
    Last edited: Jul 12, 2012
  17. Jul 12, 2012 #16

    I like Serena

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    And now that I think of it, isn't 220 mA perfectly alright?
    It's more than 10 mA, so that means the relais will close. :)
     
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