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Page 10: THE TRANSISTOR

Transistor Pinouts

Click to see a table of pinouts

The transistor is a 3-leaded electronic device. The three leads are given the names: COLLECTOR, BASE and EMITTER.

In simple terms the input lead is the BASE and the output is the COLLECTOR. The EMITTER is connected to the negative rail (for an NPN type) and is common to the input and output.

A typical transistor is shown in Fig. 2 with the equivalent circuit symbol.

The transistor is an amplifying device. It has a very close analogy to the water transistor show in Fig. 3.

Each drop of water that enters the base “turns” the transistor on and causes a larger amount of water to flow through te speaker to produce a loud “thud” or “click.”

This is why the transistor is called an amplifying device.

The only unusual feature with a transistor is the base must be fed with a small amount of voltage (.6v) before the transistor will allow current to flow.

This can be equated to putting a few drops of water into the funnel and then further drops will create a thud.

In general, we can consider the transistor is capable of amplifying 100 times. In other words, if 1mA is fed into the base, 100 milliamps will flow in the collector-emitter circuit.

Transistors are also capable of amplifying very small currents. If 1/1,000th of a milliamp is fed into the base, the current flow in the collector will be 1/10th of a milliamp. PNP AND NPN TRANSISTORS There are two types of small-signal transistors. One is called NPN and the other is PNP. These names are derived from the type of material used in the manufacture of the junctions.

The PNP type is a mirror image of the NPN type and you will see that the NPN type is generally connected with its emitter lead to the negative rail while the PNP has its emitter connected on or near the positive rail as shown in fig. 5.

It is very important not to mix up PNP types with NPN types as NPN types will not work in place of a PNP type and vice versa. IDENTIFYING THE TRANSISTORS IN THE KIT Because this e-book is available world-wide on the web, the type of transistors you can purchase locally may be different from those we have described. If you buy a kit locally, the transistors supplied may be substitutes. They will work exactly as discussed but the pinout may not be included in the kit. Click HERE to see a table of transistor pinouts. Contact us if the pinout is not included and we will add it to the table. We have concentrated on the fact that one of the transistors is a PNP type and the other is NPN. The circuits we have described are not critical in operation and will accept almost any type of small signal transistor - providing they are PNP or NPN.

For a beginner it is not nice to be given an unbranded component or one with a coloured dot on it but in our case the circuits are so flexible that the most common types in each country will work perfectly ok.

In the parts list and on the circuit symbols page we have given a list of transistors for each type and the first thing you should do is mark the top of the PNP transistors with red nail polish and the NPN transistors with white-out. This will keep them separated as it is very easy to make a mistake and fit the wrong type of transistor.

If we have supplied a transistor not included on the list, we will have already marked it with a white dot for NPN and red dot for PNP so that a mistake cannot be made.

WHY WILL ANY TRANSISTOR WORK?
Almost any transistor will work in our circuits because we are using them in a non-critical way, on a low voltage and not expecting an impressive performance. When transistors are manufactured, they are made in very large batches. They are then tested for collector-emitter breakdown voltage, current gain as well as a number of other parameters. Every device is then given a type-number and even those that are left over from the “batching” process are ok for the circuits in this book. Sometimes you can get unbranded transistors in junk packs and these will also be suitable.

If you are using parts from your parts-box, the only thing to remember is to find 4 NPN transistors and 2 PNP transistors. You will also need to know the pin-out of the leads.

Click HERE to see a table of transistor pinouts.

TRANSISTOR PIN-OUTS

Fortunately transistors have only three leads however there are 6 different ways of naming these three leads and that’s exactly what different manufacturers have done.

Most of the time the pin-out is a result of the way the transistor has been fabricated however it is important to know the pin out of the devices you are using as it will take a lot of soldering and desoldering to try all the different combinations.

We have provided diagrams for the most common devices and a pin out will be included with any devices used in our kits if they are not on the list. BIASING A TRANSISTOR There are two resistors that must be connected to a transistor so that it will work. These are the base-bias resistor and load resistor. For an NPN transistor, fig 6 shows the placement of the two resistors:

In Project 1, the base-bias resistor for the NPN transistor is the touch plate (plus the 47k) and the load resistor is 1k.

The base bias resistor is a very high value so that only a very small current flows into the base. This is all the transistor needs as it amplifies the base current at least 100 times and allows the higher current to flow through the LOAD resistor. FITTING A TRANSISTOR The overlay on the PC Board has a “D” shape showing where each transistor is placed. The transistors we will be supplying in the kits will fit exactly over this shape and the leads will fit down the three holes. The diagrams below show how the transistors fit down the holes. If you have transistors other than BC 547 or BC 557, the shape of the transistor will be slightly different and the leads will be in a different position. Refer to the notes contained in your kit.

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