Battery ON/OFF Switch circuit

Discussion in 'General Electronics Chat' started by Humanityrulz, Feb 3, 2011.

  1. Humanityrulz

    Thread Starter Active Member

    Jul 21, 2009
    60
    0
    Hi Everyone,
    I am using 4 X 1.2V AA battery power for my application. To last long my batteries, I am trying to design a low power consumption ON/OFF circuit.
    Battery shouldn't discharge any power while it's switched off,while it's ON better to draw low power.
    I have searched for this type of circuits, I found many interesting ckt s of course. I was doing experiment with attached ckt. I have few doubts about this ckt.
    I am using P-Mosfet Q3:IRF9Z24N and N-mosfetQ1:ZVN3306a.
    I tested ckt with these components, it seems working @12V, 5V, my Worry is when batteries are low consider 0.8V,.so all together 3.2V, will P-Mosfet work? Gate voltage of the P-mosfet is +/_ 20V and Vgs thresold is -2Vto -4V according to data sheet.
    To avoid this, I added LDO between BATTERY source and ON/OFF ckt. This LDO(NCP 551 3.3V output) always 3.3V to On/OFF ckt.

    So My question is, Will 3.3V is enough to drive these Mosfets? If these are Ok then how can I justify in terms of Power dissipation and other factors.
    If not can you advice me any alternative.. by considering the power dissipating and other critical factors...
    HOW can I modify this ckt into reverse polarity protection..to avoid damage to my PIC and other components.. because some times there is chance.. while replacing the batteries..
    I can use 1N4148, but again , there is 0.7V lesser voltage and power dissipation..I guess correct me if I am wrong...and also let me know if any better options...

    Suggest me in this....to understand better and design better...
    Any comments will help....Thanks in advance..
     
  2. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Why not just place the regulator *AFTER* the ON/OFF switching circuit and then your MCU would have regulated 3.3V and the ON/OFF circuit can also benefit from the higher voltage of battery. The 10uA drain of the NCP551 regulator with no load is still a source of power drain to the battery.
     
  3. Humanityrulz

    Thread Starter Active Member

    Jul 21, 2009
    60
    0
    Thanks for quick reply eblc1388, That was my first option.... I don't have good reason why I want to use like this, may be to maintain same voltage across ON/OFF ckt.
    I thought "While ckt ON, Mosfets will receive unregulated voltages... depends on load.. it will receive drastic voltage changes.. which upsets MOSFET performance.." This is my assumption.. not sure.. That's why I used LDO in between...
    I will change the ckt as you suggested..once I satisfy....
    About the drain current you mentioned 10uA drain current.. I could find it in datasheet.. where did you get this.. I found Iq=4uA,..

    Anyway thanks for response..:)
     
  4. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    It should be 4uA typical. Sorry, bad memory. :D
     
  5. Humanityrulz

    Thread Starter Active Member

    Jul 21, 2009
    60
    0
    Any comments on MOSFET driving voltage or other identical MOSFETs works efficiently between 6V max to 3.3V min.....

    And also any suggestions about reverse polarity protection.. will be helpful..
     
  6. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    For reverse polarity protection, you should consider one of the followings:

    1. A series Schottky diode, with max. current rating several times the maximum current of the PIC and loading. This would ensure that its voltage drop is about 0.2~0.3V.

    2. A series resettable fuse + a 1N4001 diode to 0V. The diode will conduct if battery polarity is reversed and activate the fuse.
     
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