Delivering power to load using MOSFET

Discussion in 'General Electronics Chat' started by Nano001, Jun 6, 2013.

  1. Nano001

    Thread Starter Active Member

    Jan 12, 2010
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    Hi, I am using the attached circuit using the p-mosfet below to switch power to my circuit, and I am getting strange results.

    http://www.onsemi.com/pub_link/Collateral/MMBF2202PT1-D.PDF

    My Vin is 3.2 V, and the on/off to the gate is 2.5 V / 0V (gnd). I need to periodically source this voltage and about 8-10 mA to my load for 10 seconds every 5 minutes. If I use a power supply, my circuit works just fine, however when I use a 3.0 V coin cell battery (CR2032, about 3.2 V actually) the circuit does not perform correctly. I know it is not a current supplying issue from the battery because when I directly connect the battery to the circuit everything works fine. Also, I am not losing any voltage drop through the mosfet, the voltage delivered to the load is equal to the source voltage when the gate is off.

    If I put two 3V batteries in series to get 6V at the drain, it works. My question is why doesn't it work with a single 3V battery through the FET when I have tested that 3V is fine with a power supply through the FET and directly using a battery?

    Can anyone give me some insight on what could be happening here?
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,042
    3,243
    Have you measured the battery voltage when the circuit is operating?
     
  3. Nano001

    Thread Starter Active Member

    Jan 12, 2010
    88
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    yes, both the battery voltage and output voltage from the mosfet are fine.
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Hi,

    You said the power supply voltage was 3.2V. The 0.2V more than the cell voltage is enough to make it operate.

    How do you control the gate? Show a diagram please with the cell connected.
     
  5. nigelwright7557

    Senior Member

    May 10, 2008
    487
    71
    A mosfet typically takes about 7 volts to switch it on hard.
     
  6. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Not always, logic level MOSFETs operate with voltages down to 3V.
     
  7. Nano001

    Thread Starter Active Member

    Jan 12, 2010
    88
    0

    Actually, the cell voltage is about 3.2 V also, the coin cell says 3 V but a new one is actually around 3.2 V when you measure it out of the package. The circuit with the control logic and load is attached. Any improvements would be welcome.
     
  8. BobTPH

    Active Member

    Jun 5, 2013
    784
    114
    Look at figure 1 in your datasheet. You need about 4V on the gate to turn on.

    Bob
     
  9. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,015
    1,531
    Am I missing something here? It is a Pmos, isn't it shut off when voltage is applied to the gate?
     
  10. Nano001

    Thread Starter Active Member

    Jan 12, 2010
    88
    0
    Yes, the circuit is connected to the source when the microcontroller sends a low signal to the gate.
     
  11. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    First, check that the cell shares the same GND as the microcontroller.

    Second, check the cell voltage stays at 3.2V when the MOSFET is ON. It might drop because the cell cannot supply the demanded current.
     
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