0
\$\begingroup\$

I have this 5V voltage regulator circuit that is supposed to power these 2 circuits:

  1. NodeMCU (will automatically turn on if plugged in)
  2. Turn on/turn off the LCD and sensor when the pushbutton (BTN1) is pressed. (The circuit on the right blue box.)

The problem is, when I read Q3's collector using a multimeter, it shows 5V, but when the LCD and sensors are plugged in, the voltage drops. Am I missing something?

enter image description here

\$\endgroup\$
2
  • \$\begingroup\$ A power transistor like TIP32 typically hasn't got much gain. With 100K in the base there is too little base current to make it conduct significantly. \$\endgroup\$
    – Unimportant
    Dec 27, 2020 at 13:30
  • \$\begingroup\$ Wow thanks! Didn't notice that. \$\endgroup\$
    – steven
    Dec 27, 2020 at 15:48

1 Answer 1

Reset to default
1
\$\begingroup\$

TIP32 has very low hFe, 10-50 according to datasheet, so it needs high base current.

You need to lower the value of R10 accordingly, but honestly the best solution would be to replace TIP32 with a P channel MOSFET which will have a lower voltage drop when turned on and will not require drive current. Make sure to pick a MOSFET that has a RdsON value suitable for your application according to desired maximum voltage drop at the current you'll use, and the datasheet should specify this RdsON at a Vgs of 4.5V. If it is only specified for a Vgs of 10V then it will probably not turn on with your 5V gate drive.

Note you could alos remove the 555 and do the switching with the micro.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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