Help with current limiting

Discussion in 'The Projects Forum' started by benha, Mar 9, 2014.

  1. benha

    Thread Starter Active Member

    Jan 4, 2011
    35
    1
    Hi,

    I'm building a 12V circuit which uses a momentary switch to toggle a latching relay. Thus far it's working just fine, but I've noticed that when I close the momentary switch to toggle the relay, the circuit starts to draw a fair bit of current, maxing out at about 1.5A. The relay coil doesn't need anywhere near this much current to trigger, and I can limit the power supply's current output down to about 20ma and still trigger the relay.

    The problem is that the momentary switch's contacts are only rated to 50ma, and I'm worried that I'll burn them out prematurely given the amperage that's flowing.

    I need the full 12V to get the relay to trigger, so I can't solve this with a resistor. Can anyone point me in the direction of a simple circuit that will cap the current flowing through the relay coil at 30 or 40ma?

    Thanks!
    -Ben
     
  2. Dritech

    Well-Known Member

    Sep 21, 2011
    756
    5
    You can use a transistor as a switch. Connect the toggle switch to the transistor base and the relay to the collector. Always connect a snubber diode in parallel with the relay.
     
  3. benha

    Thread Starter Active Member

    Jan 4, 2011
    35
    1
    Thanks! Though you lost me on the flyback diode. That's some jargon I don't follow...
     
  4. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    Posting the circuit that you are having problems with would be a good start at getting your problem solved.
     
  5. Dritech

    Well-Known Member

    Sep 21, 2011
    756
    5
    If your concern is the current passing through the toggle switch and not the over-all current, the circuit in the link below should work.

    http://www.electronics-tutorials.ws/io/io36.gif

    Connect the toggle switch to Vin.

    A Flywheel diode (snubber diode) is used to eliminate voltage spikes caused by inductors (in your case the relay coil) when the supply is turned off. Without the diode, the transistor will get damaged.
     
  6. benha

    Thread Starter Active Member

    Jan 4, 2011
    35
    1
    Okay. Good point.

    I should probably have pointed out that this is not a normal relay. It's a DNI 0127, which is a bistable SPDT latching relay used primarily in vintage Volkswagen headlamp circuits.

    You trigger it to flip flop between states by momentarily connecting pin S to ground. When triggered, it switches pin 56 back and forth between 56a and 56b. Pin 30 provides 12v + to the relay to do its thing when pin S is grounded.

    Image attached.
     
  7. benha

    Thread Starter Active Member

    Jan 4, 2011
    35
    1
    Also, it's my assumption that the current is going through the switch since when I release it the amperage needle on the power supply goes back to nearly zero.
     
  8. benha

    Thread Starter Active Member

    Jan 4, 2011
    35
    1
    Correct, Dritech. I'm not really concerned about the overall current given that this is a momentary button that'll really never be held down for more than a second. I just don't want to burn up the switch.
     
  9. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    Use a series resistor and a diode across the switch or coil.

    Assuming you are correct about not needing full current.
     
  10. benha

    Thread Starter Active Member

    Jan 4, 2011
    35
    1
    Okay, so I'm hearing two distinct options, which I've drawn up in the attached diagram.

    Option A is basically: put a series resistor between the switch and the coil (with a diode for reasons I'm not sure I understand.)

    Option B is: Use a transistor as a switch to isolate the button from the current draw of the coil, with a diode to protect the transistor.

    Is that correct? In option A, doesn't the resistor reduce the forward voltage apparent to the coil? I tried a series resistor this afternoon and with any meaningful resistance value in there it wouldn't toggle the coil, which I ass-u-me'd meant that there was insufficient voltage available to engage the coil. (I made this assumption because pretty meaningful current was still flowing - more than is required to toggle the coil if I dial back the current limiter on the power supply and remove the resistor.

    Option B looks more complex to implement, but presumably without a resistor between the coil and ground it should allow the coil to see the full available voltage?

    Thanks for humoring me in this. I'm trying to learn but presently suffering from "know enough to be dangerous" syndrome.

    -Ben
     
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