NO switch with MOSFET/JFET, Vs=3V, no neg V

Discussion in 'General Electronics Chat' started by hants91, Jul 20, 2011.

  1. hants91

    Thread Starter New Member

    Jul 20, 2011
    Hi, I am new to the forum and looking for some help. I have a circuit which I want to power up when a switch (operated by mechanically) is in Open state. I do not want to used relay.

    State 1: Switch Open - Circuit Power ON
    State 2: Switch Closed - Circuit Power Off

    I considered P-channel MOSFET IRML5103, when the Gate is grounded State 1 activated. But for State 2, I need neg. supply at Gate which I do not have.

    P-N-P transistor can do the job. But it have 0.7 voltage drop which is not acceptable as the unit run by 3V battery and the circuit can run upto 2.2 V.

    I appreciate your suggestions.

  2. bertus


    Apr 5, 2008

    I have moved your post to the general chat area.

    Can you post a schematic of the intended circuit?

  3. tom66

    Senior Member

    May 9, 2009
    You do not necessarily need a negative supply for the PMOS. If the PMOS is on the high side of the load, then pulling the gate low will turn it on; letting it float to the drain voltage will turn it off. Ensure Vgs never exceeds +/-20V though; you might use a zener and series resistor for this.
  4. SgtWookie


    Jul 17, 2007
    You didn't state what your loads' current requirement is.

    Here's one way you might do it with a couple of PNP transistors and some resistors:


    The resistors were calculated for a 3mA load, and Q2 as having a gain of 70.