Why does this circuit work?

[Wotan]

I have made a simple circuit that works but I don;'t know why it works. I have looked up how transistors work but this circuit doesn't make sense to me. The collector is connected to the positive when the transistor is an NPN BC548 transistor. In theory this makes me think the circuit should work when the collector is connected to the negative since the collector is negatively polarised.

Anyway here is the circuit i made (2 shots of the same circuit)






I copied it off this video -

 

[jackorocko]

NPN transistors work (as in, turn on, conduct) when the base voltage is more positive then the emitter voltage by approximately 0.7 volts.

 

[Wotan]

More positive? Do you mean a higher positive voltage?

 

[jackorocko]

if you are asking if -5 is more positive then -10, yes

 

[Wotan]

When you say more positive do you just mean higher voltage?

 

[jackorocko]

-10V is greater(higher) then -5V, but as I said above it is certainly not more positive. I don't know how to be much clearer.

 

[(*steve*)]

I don't know how to be much clearer.

If you put a voltmeter across the emitter and base, you would find that the voltmeter measured a voltage (of about 0.7v) and that the positive end was at the base (for NPN transistors).

Incidentally, you would rarely read a much higher voltage than this (maybe up to a volt) because the base emitter junction is a forward biased diode. For this reason, there is always either a resistor (or a resistance, or the source of the base current has high impedance -- it all means the same thing) in series with the base or emitter so you don't kill the transistor.

When we have said 10 volts on the base, 5 on the emitter, what we don't say is that this would include any voltage drop across base and/or emitter resistors.

 

[Wotan]

Ok, thanks everyone for your replies.