TTEC4847 - Electronic components

Bipolar Transistors

Background info:

The transistor originated from the “Thermionic Triode”, invented in 1907. This was however inadequate in terms of today’s use, as they were fragile and had a high power consumption. It wasn’t until 1954 in which the first commercial transistor was produced.

When it comes to the basics, transistors operate to amplify current, and can be used as a switch. There are two main types of bipolar transistors; NPN (negative positive negative) and PNP (positive negative positive). These letters standing for the different legs on the transistor. The leg of the collector is the positive lead, the base is the lead responsible for activating the transistor, and the emitter is the negative lead.

Technical explanation:

The current is amplified through the collector to the emitter, in which the base is supplied a small current or voltage allowing the large current/voltage to go between collector and emitter. As seen in this diagram bellow, when the switch is changed to close the circuit, current goes through the 1k resistor and to the base. The bulb can now turn on as the 9v is directed to ground.

The reason for using a resistor before the base in a transistor is because of the high volt/current input, which can easily damage the transistor with excess heat etc.

Test procedure:

In order to test a transistor, the best way is using a multimeter set on "diode-test mode". If using an NPN transistor, by putting the negative lead on either the collector or emitter legs, and the positive lead on the base leg, the reading should be similar to that of a diode (e.g around 0.7v depending on what type of transistor's being used). Vbe will be higher than Vce, and the reading between collector and emitter there should be zero continuity.

Problems:

There are a number of problems commonly experienced with transistors i.e manufacturing faults etc. However the most common problem would have to be the fact that theycan blow relatively easy if the current in the circuit is greater than the transistors maximum Ib. The transistor will either short circuit (in which its resistance becomes very little or zero) or it open circuits (resistance becomes extremely high).

Reflection:

Overall I found transistors to be quite puzzling at first, but after using them in the practicals for a while I finally got my head around them and now understand the ways in which they can be used as a switch or to amplify current.

Light Emitting Diodes (LEDs)

Background info:

Light emitting diodes, abbreviated to "LEDs" are really just small lightbulbs that are more common than people think. They are used in TV remote controls, traffic lights, LED tv's, general house hold appliances and many more common day items.

LEDs aren't like normal light bulbs, however, but are instead lit up by the movement of electrons through a semi conductor material. They also have a very long lifetime, surpassing that of a normal light bulb by thousands of hours due to the low heat immittance.

The first practical LED didn't come about until around 1962, designed by Nicki Holonyak Jr, although at this stage there was only one colour of LED (red). LEDs weren't commercially successful until around 1970, as the price was far too high.

Technical explanation:

LEDs are made of semiconducting materials (materials with varying ability to conduct electrical current), most commonly aluminium-gallium-arsenide. The difference in colour is determined by the semiconductive material, and also mixing different LEDs together.

They share a similarity with zener diodes, in which current can only flow one way - from anode to cathode. The cathode side can is notable as to having a shorter leg than the anode, and the cathode also has a flat spot on the lens exterior.

The semiconductive materials are doped with impurities, to make an N-type material the semiconductor must have extra electrons, in which electrons move from a negatively charged are to a postively charged area. P-type materials have 'holes', in which electrons can move from hole to hole, from negative charge to positive charge. When the electrons pass through the P and N types this radiates light, this light being in the form of photons, which are released as a result of the electrons moving in such abundance.

The amount of light an LED produces is a result of the amount of current passing through it. This current is always limited by a resistor, and also stops the LED from blowing/burning out.

Testing:

There are a few ways to test an LED. The easiest ways are either by a visual defect, i.e clouded lens near the LED chip (blown), or with a multimeter. Using a multimeter in "diode-test mode", by connecting the negative lead to the cathode, and the positive lead to the anode, the multimeter should show a voltage drop reading of between 1.5v and 3.5v (depending on what type of LED is being tested).

Problems:

The main problems with LEDs come down to a few things; They must be grouped together to suit certain applications, as they are too small on there own (normally less than 0.25 inches). They are relatively expensive to produce, compared to a normal incandescent light bulb. White LEDs are known to fade over time. And single LEDs are limited to colours, as the semiconductors produce the different colours and there isn't a very big variety.

Reflection:

I found LED's very easy to understand, as they really are quite basic when it comes down to it. In terms of personal use, it was frustrating at times finding out the reason some of my circuits weren't working in practical class was due to blown LEDs, which affected the whole circuit as current can't pass through them when blown.

Some pictures/video from practicals:

Injector circuit calculations: