Electronic – PNP or NPN Transistor for this schematic

npnpnpschematicstransistorsvoltage-regulator

I am quite new to electronics and hoping to build a voltage regulator using the schematic below. I will be using a 7806 regulator and a 7.4 Lipo battery (yes, the schematic is probably a little overkill for 6V regulation but never mind that). The transistor required is a TIP2955 which is a PNP transistor.

A website I came across says this about PNP transistors:

Only when there is no current at the base will it turn on. And to ensure that no current enters the base, the base must be grounded (connected to ground). If any current goes into the base of a PNP transistor at all, the transistor will not conduct across from emitter to collector.

The schematic shows the PNP's base connected to positive, not ground. Will this schematic work and how? I'm rather confused as the preceding description seems to imply that it won't.

schematic
(source: zen.co.uk)

Best Answer

That quote is just wrong.

That circuit is correct, but not for your application as it will require about 8V minimum input to give 6V at the output.

It works by supplementing the output current of the 78xx (7806 for 6V) regulator if the regulator takes significant current (about 600mA) with additional current through the PNP transistor.

Your Lipo batteries will not have a constant output voltage, they will probably vary between 8.4V when fully charged down to about 6V or even less when discharged.

What do you need the 6V for? How much current do you need? What is the acceptable voltage range for the load?