blinkin LED

[zachtheterrible]

Hello. I know that this circuit that I've drawn up on the bottom here doesn't work, but I am trying to figure out why. I'm going to explain how I think that it should work, and if someone could tell me where and why my reasoning is flawed, I'd be a happy man. I also put the circuit that works on here. Here goes:

The way I see it, C1 charges up through R1 and R2, and when enough voltage is attained, Q1 opens up and C1 discharges through Q1, and while Q1 is open, the led lights up. Then the cycle starts all over again. WHERE AM I WRONG!?!?


Thankee so very much

 

[Nigel Goodwin]

Hello. I know that this circuit that I've drawn up on the bottom here doesn't work, but I am trying to figure out why. I'm going to explain how I think that it should work, and if someone could tell me where and why my reasoning is flawed, I'd be a happy man. I also put the circuit that works on here. Here goes:

The way I see it, C1 charges up through R1 and R2, and when enough voltage is attained, Q1 opens up and C1 discharges through Q1, and while Q1 is open, the led lights up. Then the cycle starts all over again. WHERE AM I WRONG!?!?


Thankee so very much

We've already been through this exact same thread before - you can't use one transistor, you need two to give the positive feedback (where each one inverts).

 

[zachtheterrible]

Yes . . . I know we've been through this before, and I thought that I understood it, but then i realized i didn't.But the thing is, I dont really get WHY each one needs to invert. I know the part that each one gives 180 degree phase shift, and combined 360 degree. Maybe if you could tell me where my theory is wrong, I'd have an understanding. Thanx

 

[Nigel Goodwin]

Yes . . . I know we've been through this before, and I thought that I understood it, but then i realized i didn't.But the thing is, I dont really get WHY each one needs to invert. I know the part that each one gives 180 degree phase shift, and combined 360 degree. Maybe if you could tell me where my theory is wrong, I'd have an understanding. Thanx

An oscillator needs positive feedback, it also needs a gain of greater than one - while the base and emitter of a transistor are in phase, the voltage gain isn't greater than one - so there's no way it can oscillate.

A transistor gives voltage gain (out of phase) between base and collector, so by using two transistors you have both voltage gain, and the required 'in phase' input and output.

 

[zachtheterrible]

Why won't the single transistor in my circuit provide any gain?

 

[Nigel Goodwin]

Why won't the single transistor in my circuit provide any gain?

Read my previous answer! - from base to emitter there's no voltage gain!.

 

[zachtheterrible]

But WHY is there no voltage gain from base to emitter???

 

[Nigel Goodwin]

But WHY is there no voltage gain from base to emitter???

Because that's how transistors work! :lol:

The voltage between base and emitter is always 0.7V, for an NPN transistor the emitter is 0.7V lower than the base - as the base voltage increases, the emitter voltage increases by the same amount. As there's a fixed 0.7V between base and emitter it's pretty obvious there's no voltage gain there.

Time for one of my famous analogies! - think of a transistor as a lever sat on a fulcrum!. The base is one end of the lever, the collector the other end of the lever, and the emitter is the fulcrum.

OK, so you press on the base, the collector goes up, as the fulcrum is closer to the base the collector goes up further than the base goes down - this is gain!.

Now, press on the base again, but this time have your load on the fulcrum, where does it go - nowhere. For a more correct analogy the emitter would actually be away from the fulcrum, but not quite as far as the base is - so you don't get any gain, and in fact make a slight loss.

Does that help you to understand it at all?.

 

[zachtheterrible]

AT LAST!! I think ive finaly got it!! So, basically, the emitter and base r @ the same voltage, and therefore . . no gain. Thanx Nigel!