Transistor behaviour

Hi,

I have the circuits presented on the images connected to my Arduino Uno.

Does anyone know why the LED turns off when collector is connected to PIN 13 at LOW state, but turns on when the collector is disconnected?

The transistor used is a TIP122.

Richard

Without the collector connected the base-emitter junction conducts to light the LED, the emitter having about 2 volts on it since the two resistors form a potential divider.

However when you connect the collector and then switch on the base-emitter junction, the collector-emitter becomes "connected" and because the collector is at GND potential, so is the emitter. Hence the LED has GND at both terminals. (In actual fact the emitter will be at about 0.7volts but this won't be enough to illuminate the LED)

jack

Hi Jackrae,

Your explanation helped alot. However, after reading it and lots of googling, i just don't understand the diference between a collector disconnected and a collector connected to something turned off (PIN 13 @ LOW state).

Do you know any good online resource about transistors and this kind of situation -> behaviour of currents through the transistor with one or another pin unplugged ?

Thank you again

Richard

i just don’t understand the diference between a collector disconnected and a collector connected to something turned off (PIN 13 @ LOW state).

But it’s not ‘turned off, pin at low’, it’s connected to a active low, i.e. a ground connection capable of sinking current.

You have to leave the world of digital logic were 1=on=high and 0=off=low behind with yourtransistor circuits and enter the analog world of variable current flow dictated by ohm’s law.

Lefty

I am having difficulty comprehending why you wish to consider "transistor theory" with one of the wires disconnected. Such a situation is irrelevant to the operation of a transistor (unless you are considering IR detection, UV detection or temperature measurement)

The circuit you show, with all wires connected is a classic emitter follower circuit, where the output (emitter voltage) follows the base input voltage and control of the LED is determined by the current flowing into the base. The collector should be connected to a permanent +5 supply.

If you are switching both emitter and collector (separately or together) they are in fact acting as inputs to an AND gate (the transistor) since both need energising to get an output and considering you are using arduino outputs to act as these inputs it's all very much a waste of technology.

jack