Help with DPDT relay.

Discussion in 'General Electronics Chat' started by doby, Mar 21, 2016.

  1. doby

    Thread Starter Active Member

    Aug 17, 2011
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    Last edited: Mar 21, 2016
  2. MrSoftware

    Active Member

    Oct 29, 2013
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    Can you post a schematic of your circuit?
     
  3. Marcus2012

    Member

    Feb 22, 2015
    349
    26
    Hi, Can I ask what voltage and current you are switching with the relay please?
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,014
    3,234
    So when does it block, if not at 0V? :confused:
    If the contacts stay permanently closed, then the contacts are likely welded together by overcurrent or inductive spikes from the load.
    What is the load?
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I'm confused now, you just posted you found the problem?
    Max.
     
  6. doby

    Thread Starter Active Member

    Aug 17, 2011
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    I don't have a proper schematic because I'm using prebuilt circuits, but I have a drawing (attached).

    I'm using the relay to isolate li-ion cells from charging and protection circuits. When 5V is supplied by USB the relay switches the cells to connect to the charging circuits. Then once the usb is removed they switch back to the protection circuit and the cells are connected in series to deliver the load. The resting position for the relay should be so the protection circuit is connected, but after charging the cells and removing the usb they don't seem to switch back.

    All the switches shown in the drawing are actually the relay contacts (I know this isn't how it should be drawn). I'm using two relays, both DPDT, basically one for each cell.

    5V <2A

    1A per charge circuit (allegedly) although I've read that its actually more around 850mA, so approx 1.7A combined. Could be welding I suppose although they switch back instantly when I reverse polarity (reset).



    I know, I thought I had but I was mistaken (once again!)
     
  7. Marcus2012

    Member

    Feb 22, 2015
    349
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    OK so that voltage and current fits ok with the ratings. I see the release voltage is 0.5V but if you say the voltage did go below this it should have released. I know this might be a silly question but have you definitely cut all the traces that might inadvertently short the output pins of the relay? I neglected this last week while half asleep, took me a full ten minutes before the penny dropped. :oops:
     
  8. doby

    Thread Starter Active Member

    Aug 17, 2011
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    No shorted pins. I've found that tapping the relays makes them release or at least makes them get good contact in their resting position. What could be making them stick? I guess I have damaged them somehow, but theres no way I could have been pumping over 2A through each.

    Edit: Reading into it I think arcing contacts could be the issue. What's the best method to snub an arc in this case?

    Edit 2: Just read about someone that had relays that were sticking after being exposed to neodymium magnets and guess what, mine have possibly been in close proximity to some too. This makes much more sense, going to order replacements.

    Edit 3: Replacements work as expected.

    Note to self (and others): Keep relays away from magnets.
     
    Last edited: Mar 23, 2016
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