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Why is relay latching? :(

Discussion in 'General Electronics Discussion' started by Kookie, Dec 11, 2014.

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  1. Kookie

    Kookie

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    Dec 11, 2014
    Hi, new to the forum, thanks for being here. Could use some help here understanding what’s going on in this simple circuit please. Having problems making it work. Now that I’ve drawn it out, I think it’s wrong but that is how it’s connected.

    [​IMG]

    It’s a door bell circuit. The reason I have a 12 and 5 volt DC supply is the bells work at different voltages. The 5v bell specs are 3-8v ac/dc. The 12v bell is 12vdc. The two switches are somewhat of a mystery. Got them at lowes and they have no specs, but it is a mechanical push button and an LED. They seem to work well at 12v. There is a tiny pcb inside the switch with surface mount resistors I believe and extra diode.

    The relay is from RS. http://www.radioshack.com/12vdc-coil-dpdt-miniature-pc-relay/2750249.html. There is a spec with that that says “Pickup/dropout voltage 9.6/0.6VDC” I’m wondering if this is the problem.

    Now here is the problem: It worked well with one switch hooked up and both bells. When I ran the wires for the second switch and hooked it up, the push buttons latch the relay. I have to reset by power down.

    Can I fix this circuit? Also, after drawing it out here in schematic form I am surprised the LED’s light up. (When I put it together I did it all with components, I was pressed for time.)
     
  2. Gryd3

    Gryd3

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    Jun 25, 2014
    The lights that are built into the buttons could be allowing too much current to pass to allow the relay to dis-engage.
    There are only two conductors on the buttons?
     
  3. Kookie

    Kookie

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    Dec 11, 2014
    Yes, two conductors. Doorbell button.
    I know, not much to go on. I got these on clearance for $6. It doesn't say on the package either, operating voltage, or anything. They light up nice at 12vdc though. Tried them at 5vdc, just dimmer. The momentary sw and LED in the schematic are a packaged deal.
     
    Last edited: Dec 11, 2014
  4. Kookie

    Kookie

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    Dec 11, 2014
    Also, its a 100' run to SW1, 50' to SW2, 50' to the 5v bell, and 200' to the 12v bell. I used #18 2c plenum except for SW2 which wasn't in plenum space. Used PVC jacket for SW2.
     
  5. Kookie

    Kookie

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    Dec 11, 2014
    Relay spec: Nominal coil current - can't tell if it says 60 or 80mA. So adding the second switch w/LED may be bringing that in. Hmmm, maybe a resistor in series?
     
  6. dablakh0l

    dablakh0l

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    Sep 19, 2014
    Without knowing the specs on the switches, you might try wiring the coil to 12v directly, and having the switches pull the coil to ground?
     
  7. Kookie

    Kookie

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    Dec 11, 2014
    So your saying put the relay coil between the supply and switches. The switches pull in ground. right?
     
  8. dablakh0l

    dablakh0l

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    Sep 19, 2014
    yes.
     
  9. Gryd3

    Gryd3

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    Jun 25, 2014
    Well. the LED's in the button will create a voltage drop, and the built-in resistor should be sized to allow for ~20mA to pass through...
    So if both LED's were lit, they could be passing a combined 40mA... I would be surprised if you get different behavior putting it on the ground side instead.
     
  10. Kookie

    Kookie

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    Dec 11, 2014
    Hmmm, then maybe a relay that requires more current to operate would be my solution?
     
  11. Gryd3

    Gryd3

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    Jun 25, 2014
    Or perhaps some additional components. I would have to think on it, as I'm a little scattered at the moment...
    But, from the looks of things, that relay is probably going to have at least 5-6V across it right now...
    The addition of a resistor will cause the LED's to become dimmer, but will decrease the current through the relay possibly preventing the relay from latching.
    Remember though, that those buttons require current to flow to be able to light, so any relay you select will have a current flow through regardless of the button being pushed or not.

    I'm sorry I don't have more details, I'm sure there is an easy fix, but I can't recall anything at the moment.
     
  12. duke37

    duke37

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    Jan 9, 2011
    It may work if you put a resistor (try 100Ω) across the relay coil to dump some current.

    You could probably do away with the 5V supply by using a resistor to drop the 12V to a suitable level.
     
    KrisBlueNZ and Gryd3 like this.
  13. Kookie

    Kookie

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    Dec 11, 2014
    The reason I have the 5v supply is that I have a bunch of them. Got them on ebay real cheap. Dual-output Regulator 5V-3A/12V-5A LCD Universal Regulated Power Supplies Boards. Thought I'd throw one of them at this project. Also, I was concerned about the long wire runs, but with the #18 it seems to be ok.

    Ok, will try the 100Ω resistor. Will be easy to hook up and try.

    BTW, thank you guys very much for your inputs. Its been a looooong time since I played around component level. Its been fun and its nice to be able to talk to others about it.
     
  14. Bluejets

    Bluejets

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    Oct 5, 2014
    Replace the relay with D1 i.e. first relay, with the 12v bell. (drive it directly off the existing switches)
    Put a couple of turns of your 12v circuit wire around a glass reed switch (trial and error for how many turns)
    Use reed switch to energise your 5v bell.
     
  15. Kookie

    Kookie

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    Dec 11, 2014
    PS. When I built it, it worked with one switch I had on hand. I installed it and dynamics changed with the second switch. (Haste makes waste, right, lol.)

    I will measure the current for both switches (draw of the LED's) while I am there.

    Oh, I like the reed switch idea. Hmmm where to get reed switches locally.
     
  16. hevans1944

    hevans1944 Hop - AC8NS

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    Try looking in the home alarm section of Lowes or Home Depot or Sears Hardware or Handyman Ace Hardware for a two-part window switch: the kind that has a permanent magnet you mount to one sash and a reed switch that mounts to the other sash. You will have to disassemble the switch part to get at the reed relay, which is probably soldered and supported between two posts. Check Radio Shack, too, IIRC I saw some reed switches in their components section. RS does offer reed relays too... you could take one of those apart or experiment by wrapping some magnet wire around the case.
     
  17. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Are the LEDs in the switches illuminated all the time? Do they extinguish when the switch is depressed? I like the idea that @duke37 had of adding some extra load across the relay coil. Please let us know if that worked.
     
  18. duke37

    duke37

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    Jan 9, 2011
    To go one step further with the current dump.
    Put a forward facing diode in series with the relay coil, this will drop 0.6V or so. There will be negligible current through the relay until the voltage across the dump resistor exceeds 0.6V.
     
    hevans1944 likes this.
  19. Kookie

    Kookie

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    Dec 11, 2014
    Yes, SW lights on, off momentarily during button press. (IMO, conformation to the user the bell is rung, can't hear it from the long distance.)

    Will do first chance I get. Got the flu, sucks hard. Plus have to work regular job and can't call in sick. My free time is rest for now. I will throw in this hack at some point this week. Thanks yet again for all of your help. I will report back.

    BTW, for some reason the 5v bell stopped working. I don't know why. (The coil that pulls in the clanger needs a physical push to get it to start its ring cycle.) I will fill in more detail when I feel better.
     
  20. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Door bells depend on an interrupter contact in series with their coil. The spacing on these can usually be adjusted by bending the fixed contact with a pair of long-nose pliers. Your 5 V door bell may have the clanger pressing the moving contact too hard against the fixed contact for the magnetic field produced by the coil to pull it back far enough to open the circuit. Sometimes cleaning the contacts with a piece of ordinary bond typing paper works, which cleaning decreases the contact resistance and allows more current in the coil. Don't be aggressive with the cleaning, i.e., no "fine" or "extra fine" sandpaper or emery paper. If it's gonna work, ordinary bond paper is rough enough to remove oxidation on the contacts without damaging them. Sometimes a very slight bending of the fixed contact away from the moving contact works. Do that with power applied to the bell to see how it works. Replacing the door bell almost always works for awhile. I don't think the fifty feet of wire is an important factor since you can "nudge" the bell into operation.

    We don't offer medical advice here (and none was requested), but bed rest and plenty of liquids are about the only thing you can do for a cold or the flu. Don't let them fill you up with useless antibiotics.
     
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