Electrical Faux Pas

[wattsup]

@z_p_e

Sorry to bother you again about transitors but is it possible to explain how the transistor works in more laymen terms. Voltage enters at base but where does it go? I mean in terms of input and output or is there a mixed input/output?

[z_p_e]

wattsup.

The BJT transistor is a current-controlled current-source. That means one terminal (the base) receives a current, and the current flowing between the other two terminals (collector to emitter) is controlled by this current.

Do you understand how a diode works? You forward bias it (make it conduct) by applying around 0.6V across it. It is the same for the base of a transistor. The voltage is used only to cause this "diode" to conduct. The more the base conducts, the more current you will have flowing from collector to emitter. But for simplicity, the base is isolated from the emitter and collector. (I am trying to use layman's terms).

So the input voltage to the base does not go anywhere. It is very similar to applying a voltage to a resistor with one end grounded.You are creating a current in that resistor. We do the same in a base, we create a current in it. The return path to ground for the base is through the emitter though, even though you can think of the base as being separate from both the collector and emitter.

Think of the base as a control for the emitter and collector current, that's all. How much current is required on the base to control the collector-emitter current is a function of the gain of the transistor. Don't think of the base an input, it is a control.

An example: if you want 100mA collector current, and your transistor has a gain (HFE) of 100, you will only require 1mA of current in the base.

I hope this hasn't caused more confusion. Let me know if you understand it.

Cheers,
Darren

[Evil Roy Slade]

Nicely done ZPE,

I just want to clarify something in case wattsup gets all excited about 1ma turning into 100ma.

There needs to be a power source connected to the collector to provide current. The base current is merely CONTROLLING the collector current as ZPE describes.

An analogy is how you control the water flow out of a tap by turning the knob. The water pressure is created by the reservoir not by the act of turning the tap. If the reservoir level is low then you will only get a trickle of water no matter how much the tap is turned on (so to speak).

So if the power supply connected to the collector is incapable of supplying 100ma then you will only get what it can provide. But it's not 'free' current.

ERS

[z_p_e]

Thanks ERS.

It's nice to get some help once in a while.

Yes, wattsup, the source (battery etc) flows only from collector to emitter.

If you wanted to, you could use ERS's analogy and say that sufficient current on the base is equivalent to opening the water tap. The pressure on one end of the water valve is the amount of supply voltage, and the rate of water flow through the water valve is equivalent to the current level from collector to emitter.

[wattsup]

@z_p_e and @ERS

Thanks, it's starting to sink in. Finally!!

So let's say that I had a transitor with a gain of 100 HFE;

I put positive of a 12 vdc supply to the collector.
   (I know I would need a good size transistor for this).
I put the positive of a 12 vdc bulb to the emitter.
I ground the bulb to the negative of the power supply.
I send 0.012 volts into the base.
Stop. This is where I am getting concerned.

1) So, will the emitter only give 1.2 volts to the bulb?
2) Now if I put .12 volts on the base, will the emitter put 12 volts to the bulb.

3) Now you said the base goes to the emitter also.
Is the base using the emitter as a ground?

4) What happens if I put the transistor base on the negative side of a circuit, but still send the 12 vdc positive into the collector that is given to the emitter and bulb? Would the base negative cause a short to the emitter positive? I might have done that a few times? Would this blow a transistor? I've already baked a few of them. lol

5) So let's say I do not use the collector at all. I only put 12 volts to the base. Will this 12 volts automatically be sent to the emitter and still light up the bulb? I have noticed this once and thought the transistor was shot. Is this normal?