Control circuit for mechanically latched relay

Discussion in 'The Projects Forum' started by 4wsilver, Jun 25, 2015.

  1. 4wsilver

    Thread Starter New Member

    Apr 14, 2015
    I have a mechanical latch (contactor) relay (KIB 9806) which will latch closed with a positive pulse on the coil contacts and unlatch with a negative pulse on the coil contact. I have a Mini-BMS HousePower board which will monitor a 300 AH 12 volt bank of Lithium cells. I am trying to marry these two devices. The BMS will output a solid 12 volt signal to a coil when energized and I need to convert that to a pulsed signal. I thought of just using a capacitor between ground and the other coil connection but am worried about some leakage current once the capacitor charges.

    If a problem occurs, the circuit board has alarm contact which I could use to trip the device and if the capacitor were charged, then the positive terminal could be shorted to ground and at the same time the 12 volt supply could be opened to the contact with a NC contact, providing a negative pulse on the coil contacts. Once the latched relay trips all power is removed from the circuit and everything turns to rest position. I "think" it might work but sounds like a good candidate for a "relay race."

    I would like to hear any and all ideas and don't be afraid to criticize my thoughts.
  2. #12


    Nov 30, 2010
    I think your concerns about capacitor leakage are without merit. Besides, if the capacitor did leak, it surely wouldn't leak more than the original current required to latch the relay! That kind of leakage is called, "complete failure".

    I would be more concerned that when the 12 volt signal stops, it merely stops and does not provide a good ground path for the charge in the capacitor to fire it's stored current backwards and unlatch the relay.

    If my comments seem awfully strange, it is a misunderstanding. I feel a need for a drawing to clarify your question. Can you help with that?
  3. Reloadron

    Well-Known Member

    Jan 15, 2015
    You have one of these Battery Disconnect Latching Relay and you want it to work with one of these HousePower BMS units.

    Every mention of the battery disconnect relays makes mention of "It only requires a short "pulse" of voltage to activate or deactivate the relay". So where are you getting "negative pulse" from? Looks like a latching relay pulse on and pulse off. During normal operation the BMS monitoring unit has a constant 12 volt output to the solenoid, but the low side is switched, like open collector in a mosfet world, here are the BMS specifications:

    The problem here is you are trying to use the wrong type of Solenoid / Contactor. My read for the relay circuits is they are all open drain mosfet types. You are switching the solenoid / contactor negative side. You would need to take that signal and use it to trigger a one shot to pulse your contactor, additionally the pulse would need to deliver the needed current to get the contactor to latch. This would likely work much better using a contactor designed for use with the BMS unit that you have. Finally, while I am not 100% on the latching relay you have if it is pulse On and pulse Off any glitches could possibly latch the thing On and Off?