Transistor Tester
- 1. Fredrick Kendrick Heller Cook Book 3 ET1410 / ET1210 1 Transistor Tester Parts List 1 – LM555CM 1 - 10ΩF Capacitor 1 – 33kΩ Resistor 4 – 220 Ω Resistor 2 – 1N4148 Diode 1 – Red LED (3mm) 1 – Green LED (3mm) 9V Power Source
- 2. Fredrick Kendrick Heller Cook Book 3 ET1410 / ET1210 2 Theory: With all the circuits I make, I end up with a lot of circuits hanging about. Not many of my projects are permanently used, their parts being recycled into new projects. Even fewer achieve the final accolade, "box status", so it is a measure of how pleased I was with the design that this project was instantly built into a box with its own interfacing circuit. So let's see what we need to do to build an intelligent transistor tester. First off, remember that we think of electricity flowing from the positive terminal to the negative one, whereas it is actually a flow of electrons the other way round. Before the true nature of electricity was known scientists made a guess as to the flow direction, a 50 / 50 chance that they got wrong. So now we are lumbered with the twin concepts of conventional current or electron flow. Transistors nowadays, are mainly made using silicon, initially germanium was used but is seldom found now. By adding certain impurities in minute
- 3. Fredrick Kendrick Heller Cook Book 3 ET1410 / ET1210 3 amounts you can create N type or P type silicon, the difference is how the electricity moves through it. If you connect a voltage up to a piece of silicon you will get electricity flowing through it. With N type silicon electricity conducts by electrons, negative particles, travelling from the negative to the positive terminals. Whereas in P type silicon positive particles carry the current going from the positive to the negative terminals. Now these positive particles are called holes and don't really exist. Yes read that again it's right. A hole is the absence of an electron, a vacancy where it can sit. Any electron filling the vacancy will leave a hole behind it that can be filled by a different electron. So while the electrons shuffle towards the negative terminal the hole makes its way to the positive one. It's much easier to think of the hole as a particle in its own right, rather like considering a bubble rising to the surface rather than all the surrounding liquid slipping down a bit. Well having got that concept I can describe a transistor and how it works. Simply take these two types of silicon and make a sandwich as shown in figure 1. Looking at this it come as no surprise to learn that we call this an NPN type transistor. It is possible to make transistors out of the other type of sandwich, the PNP as we will see later. Now in this figure I have shown an applied voltage across the sandwich, no current can flow because the electrons can't get over the P layer in the middle, it is all to do with electric fields stopping them moving through the P type region. Now if we take a wire connected to the P type middle region or base of the transistor and apply a voltage to it you will cause a current to flow from the emitter to the base. All the free electrons in the bottom part of the sandwich see all the extra holes in the base and would rather like to combine with them. Like some other aspects of life, each hole manages to attract several, even hundreds of electrons to it. One electron combines with the hole but all the others are running so fast that they end up with enough energy to burst through into the P layer, flow right across it and end up in the collector region. So a small current flowing in the base collector region will cause a large current to flow from emitter to the collector. You can see where the terms emitter and collector come from now can't you? So why is the middle one called the base? Well that's historic and comes from how this sandwich is actually made. They start with a base of P type material and deposit two spots of N type material either side of it. This type of topology is used because you can only connect conducting wires to the top or bottom of the silicon, not in the middle. So in essence a small current flowing cause a larger one to flow and the ratio of the difference between these two currents is called the "gain" of the transistor. FredrickKendrick© ®
- 4. Fredrick Kendrick Heller Cook Book 3 ET1410 / ET1210 4 December2014toMarch2015