I have circuit where I am using transisitor 2N3904 and Mosfet IRF9540. Following is the schematic:

enter image description here

In the above circuit, chargePin is the digital pin of Atmega328p(arduino uno). Solar+ is the pin where solar voltage is coming. Battery+ is the battery voltage pin. So depending upon the few conditions, MCU will enable the chargePin which will enable the transistor and then Mosfet will connect the solar voltage to battery voltage to start charging. But what's happening is, chargePin is disabled but still the solar+ is connected to battery+.

Is there any error in the above circuit.


1 Answer 1


You should horizontally flip the MOSFET, so source will be on solar output. MOSFET is controlled by VGS, which should not depend on output. ALso depending on voltage you may need another resistor between gate and T1 collector, so VDS would remain in certain range.

  • 1
    \$\begingroup\$ The gate is closest to the source in any FET symbol. Flipped YES but so source will be on battery NO. The source is the Solar+ input. \$\endgroup\$ Jan 13, 2017 at 9:37
  • \$\begingroup\$ Also it may depend on output if fully charged, by control function but I know you meant by Vgs control voltage \$\endgroup\$ Jan 13, 2017 at 9:39
  • \$\begingroup\$ May. But i am pretty sure that this is not what he means. Judging by the question \$\endgroup\$
    – user76844
    Jan 13, 2017 at 9:40
  • \$\begingroup\$ perhaps you can correct your answer \$\endgroup\$ Jan 13, 2017 at 9:44
  • \$\begingroup\$ "another resistor between gate and T1 collector" that, and/or zener diode to limit the Vgs of Q2. \$\endgroup\$
    – winny
    Jan 13, 2017 at 11:24

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