I want to turn off this circuit using micro controller signal to base of Q4 transistor. Can I use the same transistor model for Q4 as for Q3? I'm attaching the picture of schematic from Proteus Simulations. The SPST button above latches the supply to turn on or off when pushed momentarily. The below given SPST button will be replaced by micro controller which will give signal to base of Q4.

The circuit will be driven by 9V battery or AAA battery cells. The output of this circuit is feeded to voltage regulator and then to micro controller.

I want transistor model for Q4 which will be able to absorb current from above circuitry and the base resistor value also?

The schematic is given below: enter image description here

  • \$\begingroup\$ Why did you make R6 only 200 ohms? (5 V - 0.7 V) / 200 ohms = 21.5 mA. Make R6 also 10 kohm, that will give more than enough base current. \$\endgroup\$ Commented Sep 4, 2018 at 7:21
  • \$\begingroup\$ Welcome to EE.SE! I tried to clear up the language a bit. Please change if I made any mistake in interpreting your question. \$\endgroup\$
    – winny
    Commented Sep 4, 2018 at 8:44
  • \$\begingroup\$ You have the same part number for both Q3 and Q4 so why wouldn't you be able to use the same model for both transistors in your simulation? \$\endgroup\$
    – winny
    Commented Sep 4, 2018 at 8:52
  • \$\begingroup\$ What is your actual goal here? Are you trying to make an MCU control its own power? And trying to avoid using a regulator's own enable/shutdown pin because you think you can do better current wise? Beware that MCU self power control is quite tricky as you have to watch out for paths completed between keeper resistors and I/O protection diodes which can partially re-power the MCU. \$\endgroup\$ Commented Sep 4, 2018 at 15:16
  • \$\begingroup\$ I want to power off the circuit using MCU signal to base of Q4 transitor which will shutdown the power supply at the end of program.And what are u talking about keeper resistor and I/O stuff? \$\endgroup\$ Commented Sep 5, 2018 at 5:53

1 Answer 1


Max current through Q4 would be approximately \$\frac{9V}{10k} = 0.9mA\$, which is perfectly fine.

As pointed above, that 200R base resistor could cause problems, \$\frac{(5v-0.7v)^2}{200}=92mW\$, that's more than a small chip resistor can handle. In any case it's safe to assume that the transistor has current multiplication of at least 10 so it'd be enough to supply 90µA. In practise, use the same 10k resistor as you have in series with the transistor..

  • \$\begingroup\$ I will be using 0.25 W Resistor of package R0805 so it should not have a problem \$\endgroup\$ Commented Sep 5, 2018 at 5:55

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.