I am really new to electronic components, and I am working with a PIC to use a port pin to drive an LED with 12v and approx 115 ma. This needs to be a high side switch, so I am asking for a recommended transistor and some advice on whether I need a current limiting resistor on the base. It looks like a 2N3906 will work (will it switch a 200ma load?), but I am too new to this to be sure.
Any suggestions would be appreciated.
Thanks
Bipolar transistors (the common PNP and NPN types) are controlled by current, not voltage. If you want to turn one on or off with a microcontroller or other logic chip, you must put a resistor in series with the base. The actual value depends on the characteristics of the transistor and the load current. Usually a few kiloohms.
Usually, with 5V logic, it is easier to use a FET like BS170, 2N7000 etc. These are voltage controlled and can be directly connected to the output of the uC.
Dave
It doesn’t matter if you use a BJT or a MOSFET, you must use two transistors because of the level shift required by the difference between the MCU output voltage and the voltage being switched. A quick Google search on “high side switch circuit” would turn up plenty of examples.