Soft latch power switch questions

Discussion in 'General Electronics Chat' started by Zurn, Sep 20, 2019.

  1. Zurn

    Thread Starter Member

    Mar 4, 2019
    Hi everyone,

    I've been using this soft latch power switch as provided by the EEV blog:


    I'm using two of these switches to control two 9V batteries which are each powering a microcontroller. I'm using a single reed switch to control both of them - each circuit needs they needs separate power redundancy, hence the two switch circuits.

    It works. My problem is that it's VERY sensitive. It sometimes even just turns on when unplug the completely unrelated circuitry from AC power. Touching the MOSFETs with my hands also turns them on.

    So my question is: why is this? Is it the static sensitivity of the fets? Perhaps I made the wrong choice in fets? (I'm using IRF9530PBFs as they're the ones I had on hand).

    And my other question is: how could I modify this circuit to add an on delay of, say, a couple of seconds, so that it doesn't accidentally turn on. I ALMOST figured this out on my own by adding a 220 uF cap to the base of T1 (the one on the left), however this stopped working after a few minutes!

    Any input would be greatly appreciated.
  2. crutschow


    Mar 14, 2008
    You could try reducing the values of the 100k resistors.
    The value of the 1meg would likely have to be reduced, and the 22μF capacitor value increased proportionally.

    Or try adding a small capacitor between the gate and source of the P-MOSFET.
  3. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
    I thought you were using @Alec_t solution from this thread.
  4. Zurn

    Thread Starter Member

    Mar 4, 2019
    Great, I'll try that out (tomorrow).

    That was actually for a different circuit, one that already had the line voltage regulated to 5V, making the use of the IC a cinch. I may end up using that one... but right now I'm hoping I can just swap out a couple of components without having to redo the whole thing (I'm pretty much out of veroboard!).
    Last edited: Sep 22, 2019