September 11, 2010
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.