Transistor Circuits

Safety: HOT COMPONENTS

Please be aware that during this experiment the resistors and LED could become warm Handle them carefully and do only as instructed. 1. References
http://wild-bohemian.com/electronics/flasher.html http://www.creative-science.org.uk/transistor.html

2. Equipment
Small piece of solderless breadboard (Maplin 1.73 each) 9V, PP3, Battery and connector Wire for connections 2 NPN transistors (2N3904) 1 x 560 resistor 2 x 470 resistors and 2 x 470 variable resistors (or potentiometers) 2 x 100k resistors and 2 x 100k variable resistors (or potentiometers) 2 LEDs 2 Capacitors, 10F AVO meter for troubleshooting. Stopwatch

3. Introduction
In 1956 the Nobel prize for physics was awarded to Shockley, Bardeen and Brattain for the invention of the transistor. The transistor has enabled the modern telecommunications revolution.

4. The Transistor
The transistor is an electronic device that transforms small electrical currents (and voltages) into larger copies of the original - it is what is called an amplifier and is said to have 'gain' (magnification). The transistor has three wire connections called; the emitter (E), the base (B) and the collector (C), see Fig. 1. By wiring the device up with other simple components an amplifier can easily be constructed. A typical transistor has a gain of about 100 times. The physical theory describing the transistor involves understanding the movement of electrons (and the absence of electrons - holes) in P and / or N doped semiconductor materials. What follows here is not a detailed account of the theory but a simple set of experiments that demonstrates the transistor working.

5. How it works
A diode is a two wire electronic component that only conducts electricity when connected the 'correct way i.e. with the potentials applied correctly. It is composed of N semiconductor junction. The transistor is a three wire component composed of a sandwich of either PNP or NPN junctions. Electrically it is as if the transistor is composed of two diodes wired back to back. The common middle region (the base - B) of the transistor is much thinner than the other two regions. Because the diodes are opposed to each other no current would normally flow when a voltage is applied between the emitter and the collector - EC (although there may be a tiny leakage current). If a voltage is applied across BE (B positive and E negative for an NPN transistor) this junction will be forward biased so a current will flow in this circuit. However, because the base region is very thin (and also because when wired up correctly the collector is at a high potential and so attracts electrons) as much as 99% of this current will actually flow right across the base region to reach the collector (C). So we have actually made the EC circuit of the transistor conduct by applying a current into B (set up by a small voltage across BE). Now the current flowing from the emitter must be equal to the sum of i) the 99% arriving at the collector and ii) the 1% that is left flowing through the base. So the base current is small, only 1% or so. But as we have seen the collector current can not exist without the little base current and so it is effectively controlling the collector current. This collector current is a larger copy of the base signal and so we find the transistor produces a current gain! Current gains of 100-200 are typical for a transistor. Usually the EC part of the circuit is used as the output and the base is used as the input of the amplifier. The EB circuit is low voltage and low current while the EC is at a much higher potential and higher current. As power = voltage x current we must therefore have a higher power in EC and so a power-gain is possible with such a simple circuit. Of course the transistor does not amplify this small base signal by 'magic', the extra power is derived from the supply driving the transistor circuit. The transistor needs a battery, or other supply, to work its 'magic'. round' a P and

6. Experiment 1
Connect in series a 9V battery, an LED and a 560 ohm resistor as shown in Fig. 2. The LED should light if connected correctly (reverse LED connections if nothing happens). The LED needs about 3V at 10mA to light and this can be achieved by putting an appropriate sized resistor in the circuit: R = V / I = (9 - 3)V / 0.01A = 600 ohms (Note: actually we use a 560 ohm resistor in these experiments as it is an easily obtained 'preferred value' that is close enough to work).

7. Experiment 2: A simple circuit that wont work.

As the human body is used in these experiments electrical shock hazards need to be considered. With a 9V PP3 battery these experiments are completely safe. These experiments must only be made with batteries. A mains powered supply or 'battery eliminator' should never be used.

Disconnect the lead to the positive battery terminal. Place a finger of one hand onto the positive 9V battery terminal and a finger of the other hand to make connection with the free lead. Now we have a series circuit as before but with the additional resistance of the body included. The resistance of the body is complex and will depend on the voltage applied and most importantly the contact resistance between the skin and wire connections. The body can present a wide range of resistance anywhere between 10,000 - 1,000,000 ohms. For the sake of argument let us say it is 50,000 ohms = 50k ohm. We now get: I = V / R = (9 - 3) / 50,000 = 0.0001A = about 0.1mA which is about 1/100 the current needed to light the LED (about 10mA) and so not surprisingly nothing happens!

8. Experiment 3: The transistor amplifier

We have heard that a transistor can amplify about 100 times so we can make use of this property to boost the signal from the small current flowing through the body so that it can light an LED. Carefully wire up the simple transistor circuit shown in Fig 3. When one finger on one hand is placed on the positive battery terminal and the other finger on the other hand is connected to the base connection of the transistor a tiny current (of a magnitude that we have just calculated - about 0.1mA) flows into the BE circuit of the transistor. Because of the gain of the transistor this sets up a CE current (where the LED is connected) of roughly 100 times this: I = V / R = 0.0001 x 100 = 0.01A = 10mA and so the LED lights

Experiment 4: The Darlington Pair

So what will happen if we have two transistors in cascade (one feeding another)? This is indeed possible and is called the Darlington Pair, see Fig. 4. Quiz students as to what they might think the total gain of such a system will be: Will it be 2 x 100 = 200 or 100 x 100 = 10,000 times 1 Try getting the whole class to form a human series chain, holding hands, with one hand from the first, and one hand of the last person making the connection between the positive 9V battery connection and the base of the Darlington pair.
(* see footnote 1)

9. Experiment 5: Flashing LED Circuit

Now you have some experience with transistors have a go at wiring this flashing LED circuit. There are many possible applications for the circuits below, especially for kids, who love flashing lights. Here's some possible uses. Railroad crossing signal for model railroads. Safety blinkers for bicycles, etc.

The demonstrator should be aware that if the positive supply directly touches the base, and there is a short to the positive supply, the current that passes will damage the transistor(s). In the Darlington pair circuit for example a 100k ohm resistor should be placed in series with the base circuit to limit current (the resistor will hardly effect the function of the circuit because the gain is so very high around 100 x 100 = 10,000 !).

 Fun stuff for Halloween, like making those plastic Jack-O-lanterns blink (try using ultraviolet LEDs here). Christmas decorations. Blinkers to locate items in the dark. Flasher circuit
9V

Breadboard wiring diagram

100k

470

100k

10F

10F

Troubleshooting your circuit easy mistakes to make: Putting the LED in the circuit the wrong way around Mixing up the common, base and emitter legs of the transistor (they should both be the same way around on each side of the circuit i.e. translated not mirrored). Resistors swapped lowest value resistor should be paired with the LED. Once this circuit is working there are many things that you can investigate 1. Time the flashes of 1 of the LEDs. How does this compare with the value RC? (Where R is the 100k resistor and C is the value of the capacitor). 2. 3. 4. Replace the 100k and 470 resistors with a 100k and 470 variable resistor, respectively. Begin by one at a time varying the resistance of the 470 resistors. What is the effect? Return the LEDs to maximum brightness and now vary the 100k resistors one at a time. What effect does this have on the circuit? You may like to time the flashes of each LED in turn.

470