Relay contact can't open sometimes.

Discussion in 'General Electronics Chat' started by ahgan84, May 18, 2014.

  1. ahgan84

    Thread Starter Member

    Dec 19, 2011
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    Hi guys,
    I have attached my relay application schematic below. I have also attached the relay picture that I used.
    Now I face the problem that the relay can't be turn off (open contact) after turning it on. My load is 240VAC TV or monitor.
    I have tested a few times and it can't be turn off 10 times out of 100 times. How could that be? If my relay is spoilt, it straightaway can't be on or off right?
    And my relay got a diode for protection.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Try a 2n7000 Fet instead of a bi-polar tr.
    Max.
     
  3. ahgan84

    Thread Starter Member

    Dec 19, 2011
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    Thanks for the suggestion, but why?
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    FET is a voltage operated device, not current operated.
    Max.
     
  5. ahgan84

    Thread Starter Member

    Dec 19, 2011
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    Care to explain more? How is it related to my application?
     
  6. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    The circuit looks fine. You can find the trouble with a voltmeter:

    Turn relay on and off until it sticks on. Probe collector with meter. If a low voltage is there, the transistor is bad. If it is essentially the 5V supply, the relay is bad.

    Relays can go bad for a number of reasons, including an overload on the contacts so they tend to weld together.

    If the transistor is bad check the diode is in properly.
     
  7. ahgan84

    Thread Starter Member

    Dec 19, 2011
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    Thanks for the suggestion.
    A weird thing I notice is that:
    The normal workable relay wil gv a clear click sound every time on and off right? Mine too. If it is bad, u would expect that it won't click when it can't off bcos the contact are weld together. But for my case, u can still hear a very faint click sound when you try to off it. Jz that the relay can't off. How come like that? Now is it the transistor bad or the relay is bad?
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    A relay still makes a sound even when the contacts are welded, they do not fully retain the relay armature.
    Max.
     
  9. ahgan84

    Thread Starter Member

    Dec 19, 2011
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    So what do you think went wrong? Is it that the contact had been welded together already? Or the relay had not fully discharged the charge in it? Anyway to prevent this happen?
     
  10. ErnieHorning

    Member

    Apr 17, 2014
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    The datasheet for that relay is hard to find but I found one that says that the minimum turn off voltage is .5 volts.So with a bi-polar transistor, you may not be getting low enough to for the relay to always release.

    I agree with trying a FET… or a different relay.

    http://relayhome.cn/pic/other/2013-02%2F2013-02-12-15-04-21-97.pdf
     
    Last edited: May 19, 2014
  11. ahgan84

    Thread Starter Member

    Dec 19, 2011
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    But I'm not getting this problem always. I have this problem only 10 times out of 100 times. So, does this mean the turn off voltage is sometimes below 0.5V and sometimes above 0.5V?
    And I can still hear a faint click sound after I try to off the relay. Just that it won't release the contact and off. So, I don't think the BJT is involve in this...
     
    Last edited: May 19, 2014
  12. Lestraveled

    Well-Known Member

    May 19, 2014
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    Often an output pin does not pull "low" enough to fully turn off a transistor with just a series resistor to the base. I suggest you add a resistor between the base and the emitter of the transistor. A resistor equal to the value of your series base resistor should do the trick.
    Look at buffer/inverter/interface transistor circuits. They all have a base-emitters resistors. This resistor fixes the "not low enough low" of I/O pins and collector-base leakage current.
     
    Last edited: May 19, 2014
  13. ahgan84

    Thread Starter Member

    Dec 19, 2011
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    But if the transistor base voltage is not zero, how come I can still hear a faint click sound after I try to off the relay? Just that it won't release the contact and off.
     
  14. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You have to make some definitive voltage tests to establish what exactly is going on at the time it does not turn off properly, tests at the relay coil and at the contacts.
    Max.
     
  15. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Ugh... 0.5V is the hold in voltage, or the maximum voltage you can apply and still expect the relay to open. If this is actually the problem again a voltmeter will detect it.

    Driving a transistor base off from an output pin is never an issue. Never. There is a negligable ammount of current that needs to be sinked there. Plus there alread IS a resistor from base to ground, it is the same R in series with the base, and it is quite capable of sinking the C-B reverse leakage all by itself.

    ahgan84: your circuit as drawn is fine. I have used it many many times too good use. Do you have a voltmeter to check the collector voltage when off?
     
  16. Lestraveled

    Well-Known Member

    May 19, 2014
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    One good experiment is worth more than 100 opinions. Get a meter and determine what is going on. Either the relay is sticking or something is causing the transistor to conduct enough current to keep the relay from releasing. Based on the spec sheet, about 8 mA. is enough to hold the relay on.

    ErnieM: Output pins NEVER drive completely to ground (0 volts). They typically sink to between .2 to .4 volts. The transistor only needs 100uA of base current to keep the relay on. (Assuming Hfe=100). A silicon transistor is in full saturation at .65 volts (base-emitter voltage). It is never, never a good idea to omit the base-emitter resistor.
     
  17. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    This particular relay may be at issue with a very tiny hold in voltage. Any leakage thru the transistor may be just enough to sometimes hold it on.

    Aside from that this circuit HAS a base-emitter resistor when the driving source (a PIC microcontroller output) is actually a driving sink FET. When the output is low there simply is no driving source of current here, and that output FET only has to sink a fraction of a microamp, so it is just about zero volts.

    Even if there was .2 to .4 volts on that output pin (and there is not) it would solely due to current going into that pin, which implies the voltage on the base is even less as the current thru the R is going away from the transistor. You're not going to be pulling out reverse base current as the grounded emitter can't source current to ground, and the collector is reverse biased so all that supplies is Icbo, which is the current we want to pull to ground to keep the transistor off.
     
  18. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    That does sound like the relay is releasing. The pun intended but the point is also true: things are moving inside to make the noise.

    That does not mean the contacts have not welded together: I believe I've had some open frame relays that still "clicked" but I could see the contact lever stay in one connection.
     
  19. MaxHeadRoom

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    Jul 18, 2013
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    +1 (#8).;)
    Max.
     
  20. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    I have seen sticky relays where residual magnitism holds the relay closed, usually prevented by a small air gap or thin non mag. shim to prevent iron to iron contact. Is the relay rated for the load ?
     
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