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Microcontroller Controlled High-Side Switch

This is a small circuit that I designed to switch a 12-V power source using an I/O pin of a 3.3V microcontroller. The important thing to note is the high-voltage non-inverted switching. Vin are the high-voltage that need to be switched and for 2N2907, it can be as high as 40V. V2 is the uC output around 3.3V.

Important design notes:

  • Maximum load on 2N2907 is 500mA. This defines the maximum base current using I_c = beta * I_b. The beta for 2N2907 for this load is around 100 which means: I_b = 500mA/100 = 5mA.
  • Now R1 = 12V/5mA = 2.4K, we choose 2K.
  • Pull-up resistor R2 is generally 10 times the biasing resistor. So R2 = 20K.
  • R1 also serves to limit the current to Q1 because the base of 2N2907 is around 12V.
  • R5 limits the current from uC and R3 is pull-down to keep the transistor off when not under active control of the uC.
  • The switching is no more inverted because of two transistors.
  • Maximum current drawn from uC is 270uA.

The LTSpice model of this switch can be downloaded here. A P-Ch MOSFET based version can be downloaded from here.



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