Unwanted voltage on open relay contacts.

Discussion in 'The Projects Forum' started by NM2008, May 7, 2009.

  1. NM2008

    Thread Starter Senior Member

    Feb 9, 2008
    135
    0
    Hi,
    I have a 5v Omron G6H-2100 relay being turned on and off from this radio control interface, http://www.robotmarketplace.com/products/0-TD-RCE200B.html

    http://www.robotmarketplace.com/products/images/rce200.pdf

    When connected, this interface powers the relay on and off fine, but I have a problem with the contacts on the relay. It is a DPDT relay, which I am switching 5 volts with, from the same supply used to power the interface and relay itself.

    At the moment I am using one pole on this relay.
    With 5v to its common contact.
    When the relay powers up it latches and puts 5v on one of its contacts, as normal.
    But at this the point the other contact should be open and at 0v until the relay is powered off.

    For some reason I am getting a voltage, 3.8v on the contact that should be open.
    So on the common there's 5v(input), on the closed contact there's 5v and on the contact that should be open there's 3.8.V

    This also occurs when the relay is in its off state and I still get 3.8v on the contact that should be open.

    What I noticed was that an l.e.d will pull this 3.8v down to within .1 of a volt.
    The l.e.d does not light at all when I do this.
    When the l.e.d is taken off it will rise back to 3.8v and remain steady there.
    So at the moment I have 47k resistor (which brings it down to .1v) on the each contact to pull down the 3.8v without greatly effecting the 5v that should be there with the contacts are closed.

    There is no problem with the relay, I have tried a few replacements, to the same effect.
    This problem does not exist when the relay is pulled from the circuit and powered in similiar manner, all open contacts are at 0v, they way they should.
    There is no major resistance between any of the contacts on the relay when in circuit.
    The circuit itself is fine, simple straight forward, relay on/relay off. No shorts.
    And I have the diode across the relay coil.

    Could this be some form of feedback from the interface? the device will only switch negative not positive, so the relay turns on/off through negative.

    Another possibility I was thinking of was, could this be some form magnetism from the relay coil inducing 3.8v onto the open contact, why is there a drop to 3.8v with with little or no current behind it?

    I don't think this is normal relay behaviour, but having to use resistors to hold relay contacts to 0v does not sound right.

    I don't know, this is a head scratcher for me?:confused:

    Hopefully someone can shed some light on this, and whats happening.

    I look forward to your comments.

    Thanks for your time.
    Regards NM
     
    Last edited: May 7, 2009
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Can you post your circuit?

    What are you controlling with the relay?

    Maybe the contacts of the relay have a large resistance between them and when you measure the voltage with the voltmeter the voltage drops to 3.8 from 5V due to the current drawn by the multimeter. Try to measure the voltage with an oscilloscope if you have one.
     
  3. NM2008

    Thread Starter Senior Member

    Feb 9, 2008
    135
    0
    Mik3,
    I am controlling nothing at the moment, I haven't put it to use yet.
    I do not think this problem will effect me if I drive other relays from it, the coils will pull it well down.
    But there is a possibility that I may use it to to send a signal to the gates of F.E.T's.
    This is what is concerning me, will that 3.8v or 5v cause intermittent triggering when it should be at 0v, or open circuit?

    I don't understand this, the relay contacts work fine when the contacts are tested for continuity in circuit and out.

    Its almost like a static 5v on the open contacts?

    Regards NM
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Measure the resistance between the NC and NO contacts of the relay.

    If you connect a 10K resistor between the gate and source of the FET it will not turn on if the 3.8 voltage drops with this resistor.

    Do you have the datasheet of the relay?
     
  5. tkng211

    Well-Known Member

    Jan 4, 2008
    65
    2
    If you findd out there isn't any significant leakage between the relay contacts, you better add a branch ciruit in parallel with the coil to see if there is any change. The branch consists of an electrolytic capacitor 100uFand a series resistor 10 ohm.
    Maybe putting a capacitor of 0.iuF(100nF) across the contacts could help either.
     
  6. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    I would be looking for a mis-wire but without seeing the print.... ?
     
  7. DonQ

    Active Member

    May 6, 2009
    320
    11
    Sounds like there is a parallel (parasitic) resistance across the contacts. If you're measuring with a digital multimeter, it probably has a 10MΩ resistance. If there is a parallel resistance of 3MΩ across the contact, your Ohm-meter would read it as "open" except for maybe on the highest range. The voltage divider of 3MΩ leakage and 10MΩ meter would show about 3.8 volts. This would be the same for the other contact in the "off" state.

    The LED pulling it low is also consistent with this.

    Measuring with a 'scope would have a 1MΩ resistance (in X1 mode). This would probably show about 1.25V. Still a problem, but a good way to get another symptom.

    A simple test would be to put a 10K resistor between the voltmeter leads, at the point where the leads enter the meter. This would change the input impedance of the meter to approx 10KΩ. Now measure the voltage. I would suspect you would get about .017V (17mV).

    If this is the case, just put a similar resistor (or maybe even less resistance) from the contact of the relay to ground, like another poster said. This will be overpowered by the contact closure (zero ohms), but make the voltage divider of 3MΩ and 10KΩ really lopsided go give you a very small "open" voltage despite the leakage.


    tkng211. Putting a snubber across the coil will do nothing to help this.
     
  8. tkng211

    Well-Known Member

    Jan 4, 2008
    65
    2
    DongQ, thanks for your advice. I just presumed that there might be something in the driving circuit that caused the problem, if not the leakage between the contacts of the relay.
     
  9. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Leakage between the contacts of a relay? Maybe if it were submerged in water. :rolleyes:
     
  10. DonQ

    Active Member

    May 6, 2009
    320
    11
    Just seemed to me that if the same voltage is on the normally closed and the normally opened, (one with the coil energized, one with it de-energized), it seems unlikely that leakage from the coil would cause this symptom. Leakage from the +5 that is on the common of the NO and NC contacts seems more likely.

    The conductors pass through a number of holders within the relay, then the case of the relay, into the circuit board or breadboard, and through the traces. Any junk at any of these places could easily cause 3MΩ of leakage. Solder resin on the board, or the coffee someone spilled into the breadboard, a cheap relay construction, or even sweaty fingerprints on a circuit board...

    Actually, something unlikely to cause it is immersion in clean water. Water doesn't conduct well at all unless it has something in it to ionize (salt, etc). Besides, you would probably notice it being submerged in water:D, whereas you may not notice the solder flux between connections (or whatever). I would consider it not unusual for there to be MΩ resistance between non-connected points in an experimenters breadboard/prototype/kludge circuit.

    Measuring with a high-impedance volt-meter can show you things you might not expect. If you had an even higher impedance meter, you would probably see the full 5V there. Higher impedance meters are not readily available, and it's easy to lower the impedance like I suggested. I made a couple of assumptions because I only had second-hand knowledge of only one symptom, but I'd still bet money (been there, seen that!)

    It will be interesting to see if NM2008 gets back on this...
     
  11. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Don, all your points are valid. I must admit that I didn't consider the possibility of this being a PCB mount relay. Over the past year I discovered the liquid flux I was using is more than mildly conductive and must be thoroughly cleaned. This liquid flux will easily pass nearly a mA. I still have some old paste flux (that's over 30 years old) which has never measured any leakage worth mentioning. Environmental controls have dramatically changed things that worked well for decades. Now we also have solder that grows wiskers!!!

    FYI: Yes, distilled water is nearly a none conductor but there isn't to much of that laying around. :D
     
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