I've just started dipping my toes into electronics and got hold of a few bits and pieces. I thought I would mess around with the transistors I have to see them in action. I setup a simple circuit with the collector attached to the positive rail, the emitter connected to a LED then onto the negative rail. I just wanted to see how the LED will only light when I run a current through the base of the transistor. The problem is the LED lights WITHOUT a current running into the base. What have I done wrong?

 

Hello,

Can you post a schematic of the circuit you made?
That way we can see if you did something wrong.

Bertus

 

Thanks, will do.

 

8-bit, you are using a positive AND a negative supply voltage, correct?
In this case, the negative voltage at the emitter and zero voltage at the base (I assume) cause together a positive B-E voltage which opens the transistor.
In this case, of course there is a base current.

 

 

To make that circuit schematic work you need two modifications:

first, turn the LED over so that the cathode (not the anode) is connected to 0V.

second, the bottom lead of your transistor must be the emitter (usually drawn with an arrow to indicate current direction.) If you have the resistor connected to the collector, you will have to turn the transistor over by interchanging the collector and emitter.

 

Thanks Mike. Luckily the LED was the right way around but the transistor may not be. I'll post an image of my circuit so you can see maybe what the problem is.

 

Here's the image. Hope you can make it out okay?

 

I realize now there is no need for the red connecting wire at the bottom of the image!

 

Or the black one? If the LED is lit all the time, try shorting the base to the emitter. If the LED is still lit, the transistor is shorted. (Based on the assumption that it the pin order is EBC)

 

The black connects from the battery's input that just out of picture.

 

Hello,

Your transistor is shorted by the strips inside the breadboard.

Bertus

 

The black one goes to the other terminal of the battery.

 

Hello,

Your transistor is shorted by the strips inside the breadboard.

Bertus

Ah! So thats the problem? But surely it wouldn't short without a current to its base? I assumed it would act as an open switch as it stands in the image.

 

Hello,

The metal strip inside the breadboard is shorting the C-E of the transistor, so it can not switch.
Putting the emittor and the resistor in an other row will solve the problem.

Bertus

 

Thanks Bertus, will alter it and see what happens.

 

I did not think that it would be able to short as no current can flow between the collector and emitter until the transistor is 'turned on' by a base current.(?)

 

Well, altered the mistake you pointed out and it works fine! Thanks

 

Hello,

Did you understand what I said abou moving the wires?

Bertus

 

Yes thank you.

But it seems something odd is happening. When I apply a current to the base of the transistor the led lights up but I can't see how it does as the collector isn't connected to the positive rail(??)

I assumed the base current allowed a current to flow from collector to emitter but I've noticed the collector isn't actually connected to a the positive rail at the moment. :s