Guidance needed for an auto power off NOT using a relay circuit

Discussion in 'The Projects Forum' started by enveetee, Dec 6, 2009.

  1. enveetee

    Thread Starter New Member

    Dec 1, 2009

    I have a working circuit for my project which runs off a Lithium Polymer battery. The battery has a nominal voltage of 9.9V and the circuit draws up to 2 amps. (It's a heat regulator circuit for a pair of gloves)

    I'm looking for guidance on how to make a circuit which will disconnect the heater circuit when the LiPo battery voltage reaches about 9V.

    I'm quite happy using a dedicated voltage monitoring chip (MAX8211 or MAX 8212) and a relay to disconnect both the monitoring circuit and heater regulator circuit, however I was wondering if it was possible to use a MOSFET and if so, what components should I use.

    Cheers for any advice, tips or even actual circuits!

  2. bluebrakes

    Senior Member

    Oct 17, 2009
  3. BMorse

    AAC Fanatic!

    Sep 26, 2009
    or if you are going solely on voltage levels, use an LM339 or another opamp, so you can set your upper and lower thresholds.... and use a Mosfet to cut the power off/on, since the relay would drain the battery more....
  4. KMoffett

    AAC Fanatic!

    Dec 19, 2007

    I think one of those (with it's associated resistors), and an N-channel a IFRZ34N with a 1K pullup resistor...will work.

  5. enveetee

    Thread Starter New Member

    Dec 1, 2009
    Have I missed something

    Did you read my post? No mention of time or temperature.

  6. enveetee

    Thread Starter New Member

    Dec 1, 2009
    Ken, how much current does the IFRZ34N draw in it's non-conducting mode?

    With a relay, once it is de-energized, I can be confident that the current drawn will be zero. I need to be sure of this because once the IFRZ34N is not conducting, my user could leave the circuit connected and eventually damage the LiPo

  7. SgtWookie


    Jul 17, 2007
    You can determine the leakage current of the IFRZ34N via it's datasheet.

    It will likely be substantially less than the LiPo's own internal leakage current.

    Your relay idea will work, but the battery drain will be substantial, and the relay will be making/breaking a connection with a fair amount of current. You'll need to use a relay rated for 3x the current to get at least a reasonable life span from it.

    Here's an idea using an N-ch MOSFET with the trip voltage set by a micropower Zener diode. C1/R5 should really be replaced by a N.O. momentary switch to provide a reset; otherwise if the consumer doesn't manage to connect the battery without "bouncing" the connections, it will not turn on by itself.

    Leakage current will be under 1uA, except for the selected MOSFETs' leakage current.