Microcontroller Interfacing – Part 8
High Side PNP Transistor Switching
This section covers the use of PNP transistors to perform high side switching with a microcontroller.
High Side Switching
Part 7 covered using transistors to switch loads that require higher currents or voltages than the microcontroller can handle. All the circuits had similar topologies. The load was connected to the power source, and an NPN transistor acted as a switch to ground. Since the switching element (the transistor) was at ground, it is called a “low side switch.”
With low side switching the load is at Vcc potential. Sometimes it is desired to have the load at ground potential, and to switch the power supplied to it. This is called high side switching. Figure 8-1 shows the difference between high and low side switching.
You can’t just connect an NPN transistor’s collector to Vcc and the emitter to the load which is grounded. As explained in the previous section, negative feedback will prevent the NPN transistor from being driven into saturation.