Getting to know transistors

Some students had a bit of a problem getting good behaviour from the transistor familiarisation exercise.
When I got home, I tried it using a single 9V battery as the power supply. My circuit is as follows: (CLICK TO ENLARGE)

transistor intro 1

The red power rail was powered by the 9V battery.

The positive rail went into the long leg of the LED which was attached to a 2.2 k ohm (red red red, gold = 22×10^2 = 2200 ohms) resistor then into the collector pin of the transistor (the right most pin on the diagram). The middle pin, the base, went through a 100 k ohm resistor (brown black yellow, gold) resistor and the yellow arrow was just  a jumper wire that could connect the other end of the 100 k ohm resistor to the 9V rail, hence pumping current through the base. The green wire was just a jumper from the emitter to the ground rail. When I connected the yellow jumper to 9V the collector -> emitter path opened and the LED light up quite well. I was using the ‘fatter’ LED.

The LED still had some, but only a little, bit of light shining when then the base pin wasn’t connected to anything but it wasn’t much. Hence I think the power supply packs you were using had some ‘issues’ and are forcing through some current through the LED into the emitter at ground. Try the 9V battery and you’ll get a better result than you did in the meet-session.

Actually I then inverted the transistor so the “collector” was at ground and the emitter had the 9V, LED and 2.2 k ohm resistor on it, and it worked even better than before with almost zero light coming from the LED at all.

So try the circuit. Also try it with a 5V supply (you may need to reduce the resistor values accordingly – i.e. reduce their values by approx 5/9 of its previous value.

Maybe we need a power regulator on the power pack.

Anyway… Keep trying….. and learning.


One thought on “Getting to know transistors

  1. In the nest session I’d like to start playing with the incredibly useful 555 timers. So please complete the transistor familiarisation exercises in the manual and pre-read the 555 timer info. You could also pre-assemble a 555 timer circuit. I would advise using the variable resistor for the control resistor, so we can manually adjust the signals the 555 timer spits out.


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