circuit analysispnptransistors
I am new to electronics and this question might seem silly.
I do not understand why in the transistor below current seems to flow from collector to emitter, because based on what I have read, I'd say this is a PNP transistor — isn't current meant to flow from emitter to collector in a PNP transistor?
Also, isn't a small current meant to go from the emitter into the base? It does not seem like it's happening here.
So as you mentioned it says that the transistor is essentially two diodes.
You should, but may not, know that the typical voltage drop required over a diode to make it conduct is ~0.7V but can vary depending on the diode of course. So if you just 'stick' a voltage across the terminals as when you increase the voltage over the diode current flows:
As the resistance across a diode is very low when this voltage is applied we can work out that the current would be extremely high: I = V/R, simple to see that the lower R is the higher the current, and this can be very damaging to the base terminal, I believe a datasheet of the particular transistor will give you more information on what size current it can take.
What this is saying is you need to have a current limiting resistor in front of the base terminal on the transistor which does exactly what its name describes, limits the current. As the voltage drop across the transistor will remain at 0.6-0.8V we can work out the size resistor we would need quite easily. R = (Vin - Vdrop)/I, 'I' being the base current that it can take, Vdrop being the voltage drop from the base to the emitter and Vin being the supply that is going into base, you also need to look at the hfe of the transistor so see if it will be able to give you the amount of current you need, which coincidentally can be limited, or 'tailored' with a resistor at the emitter pin so the transistor is less reliant on the hfe, but I am sure you will get on to that in the future!
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?
Best Answer
Yes, it's supposed to, and yes, it does.
That is, the emitter is at -14 volts, and the collector is at-15.5 volts, so the emitter is 1.5 volts positive with respect to the collector. So current flows into the emitter and thence to the collector.