I am using an IRFZ44N MOSFET (n-type) transistor to regulate the current of a Lithium-ion battery, and I'm trying to use the battery to supply power to both the drain and gate of the transistor. My goal is to allow current to flow from drain to source when I apply voltage to the gate (when I attach the (+) wire to the gate), and stop the current flow when I connect the transistor gate to ground (when I attach the (-) wire to the gate). In my schematic I left the circuit open, but depending on what I want my gate voltage to be I connect either the (+) or the (-) wire momentarily.

My problem is that I don't understand why I'm reading certain values on my ammeter. When I connect the (+) wire to supply voltage, I should see a current of 3.7V/10ohms=370mA, but I'm only reading a value of around 30mA. However, if I use a separate power supply for the gate of the MOSFET with everything else unchanged, I get an ammeter reading of 370mA as expected.

I know the gate-source voltage is the same in both scenarios because I measured it with a voltmeter (measured about 3.5V), and when I use a separate power supply I'm also using a 3.7V battery.

Why does using the same power source to supply the gate voltage only let 30mA through?


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ It's a MOSFET. There is no base. Or emitter. Or collector. \$\endgroup\$ May 4 '18 at 23:27
  • \$\begingroup\$ And as drawn your body diode is forward biased so you won't be able to decrease the current below about .3A. Not to mention that you need Vgs to be >Vt which you can't get by using a voltage equal to or less than the source potential. \$\endgroup\$
    – John D
    May 4 '18 at 23:36
  • \$\begingroup\$ Nchannel fets connect to the Negative battery side. The little arrow thing in the symbol actually goes straight across S->D and it conducts that way (S+>D-) always. Only the other way is switched (D+>S-) \$\endgroup\$
    – Henry Crun
    May 5 '18 at 0:03
  • \$\begingroup\$ Why don´t you use a transistor to switch between positive and negative in mosfet gate? Also you need a greater votlage source as JOHN D said. \$\endgroup\$ May 5 '18 at 0:25
  • \$\begingroup\$ when you "short" the battery the voltage must drop below VgsTh, thus increasing the FET's resistance and limiting current in that regard. In short, you'll float source-vgsth, which might not be above vgsth unless the battery is double the threshold. \$\endgroup\$
    – dandavis
    May 5 '18 at 13:37

Assuming R1 is a load you want to switch, you need to fix the circuit and choose a mosfet that will switch at 3.7V (or less). The IRF44Z is not even characterized for Vgs of less than 4.5V and will have very significant Rds at Vgs of 3.7V.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Thanks for the design, but I still can't pull more than 200mA through my load (even at no resistance), and I need 1-2 amps. I'm confused as to why I can't pull more current because when I use an external power supply (also 3.7V) for the gate, I have no problem pulling 1-2 amps. Any ideas why? \$\endgroup\$
    – J. Doe
    May 5 '18 at 3:48
  • \$\begingroup\$ Did you change the NFET or are you still using an IRF44Z? The IRF44Z needs 9V between the gate and source to fully turn on. Measure both the battery voltage and the gate-source voltage when using this circuit and using an external power supply for Vgs. \$\endgroup\$ May 5 '18 at 4:23
  • \$\begingroup\$ I'm still using the IRF44Z, and I'm measuring the gate-source voltage to be about 3V. This shouldn't be a problem because I've used this same transistor to pull 2 amps through drain-source when I've had a gate-source voltage of only 3. I think I'm missing something in my understanding of transistors because I don't understand why I can get different source-drain resistances with the same gate-source voltage \$\endgroup\$
    – J. Doe
    May 5 '18 at 18:44
  • 2
    \$\begingroup\$ Tiny differences in parameters could make a big difference in your results when operating in this region. You are, frankly, wasting your time trying to operate an IRF44Z in this region as a switch, get a mosfet with a very low Vth or use a charge pump based gate driver to generate a reasonable gate voltage. \$\endgroup\$ May 5 '18 at 21:19

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