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What is a Resistor?


From the bottom up we have
470, 1K, 2.2K, 4.7K, 10k, 22k, 100k, 1M.
Understanding resistance and the use of resistors is fundamental to all aspects of electronics. In the picture on the left are some of the most common resistors. The value of the resistor is marked on the body using coloured rings but don't worry too much about learning to read the values. You will soon become familiar with the common values.

That is how you must approach this whole course. You must not expect to immediately understand everything that is told to you. From time to time things will be introduced that are too complex for you to understand with your current knowledge. Let the ideas flow into your mind and trust that later it will all fall into place.

Experiment E1.

We are going build a simple circuit to make a light emitting diode glow. Before we can do this we need to understand about voltage, current and resistance. Our plugboard is run from a 5 volt supply. If we were to connect the light emitting diode (LED) directly across a high power 5 volt supply it would take so much current that it would glow very brightly for a few minutes and then fail. LEDs have a strange characteristic. They need a little less than 2 volts to show any light but if the voltage goes much over 2 volts they are destroyed!

If we subtract 2 volts from 5 volts we are left with 3 volts. Somehow we need to connect the 5 volt supply to the LED in such a way as to lose 3 volts somewhere. That is where the resistor is needed. If we connect a resistor in series with the LED and connect the pair to the 5 volt supply we can achieve exactly what we need, 3 volts across the resistor and 2 volts across the LED.

We need to choose a value for the resistor in this circuit which gives the correct current into the LED. The current can be as high as 20 milliamps but if we allowed the LED to take that much current our battery would run down too quickly. A good compromise is 3 milliamps. If we go much below that the LED may look rather dim.

A simple rule with resistor values is that a 1000 ohm (1k) resistor allows one milliamp to flow for each volt that is dropped across it. Our resistor will have about 3 volts across it so if the value is 1k ohms about 3 milliamps will flow into the LED.
Wiring the circuit.....

1. Make sure the programmer module is turned off with both slide switches to the left.

2. Fit a link of red wire between Z4 and J1.

3. Bend the leads of a 1k resistor as shown in the picture. Fit the 1k resistor R1 between I3 and I7.

4. The push button switch is manufactured with bent leads so that it holds itself into a printed circuit board. Use a pair of small pliers to squash the leads flat.
 5. The four leads of the push button switch are arranged in pairs, each pair coming out of opposite faces of the body. Fit the push button switch with two leads from one face in H1 + H3 and the other two leads in F1 + F3.

6. The LED is flattened in one place. The lead nearest to the flat area is shorter than the other lead. Trim the short lead to 0.4 inches. Bend the long lead 0.1 inches from the body directly away from the short lead. Bend the long lead again 0.4 inches from the short lead. Fit the LED with its short lead in Y7 and long lead in J7.

Testing the circuit.....

Connect the batteries or the DC supply and switch on only the 5 volt supply by sliding the switch to the right. Do not connect the programmer module to your computer.

When the normal start up sequence has finished the control PIC will switch on the 5 volts supply to the plugboard.

Press the push button S1 on your test circuit and the LED will glow.

Leave the 5 volt supply to the plugboard turned on. Remove the 1000 ohm (1k) resistor and fit a 470 ohm resistor in its place (I3 & I7). When you have made that change press the push button and the LED will glow much brighter. The resistor is now roughly half the value so the current will be about twice as much and the LED glows brighter.

Remove the 470 ohm resistor and fit a 2.2k ohm resistor in its place (I3 & I7). Press the push button and the LED will glow quite dimly. The resistor is now roughly twice the value of the 1k resistor so the current will be about half of the original current so the LED glows dimly.

Think about it.....

You have just discovered how to use ohms law, the most important theory in any circuit design work. Yet as with many of the fundamental electronic theories its practical use is easily understood. It is impossible to design even the simplest electronic circuit without knowing how to apply ohms law.

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