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| What is a Capacitor? |
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Experiment E3. Finding out what a capacitor is. Wiring the circuit..... Switch off the 5 volt supply then follow these instructions:- 1. If you are following on from experiment E2 remove all the components except S1. If you are starting with a blank plugboard fit push button switch S1 between H1 & H3 and F1 & F3. 2. Bend the leads of a 100k resistor as shown in the first experiment. Fit R1 between I3 and I7. |
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3. 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 of these faces
in H7 & H9 and the other two leads in F7 & F9. 4. Fit 1k resistor R2 between I9 and I13. 5. Fit the red LED which already has the leads bent into position with the short lead in Y13 and the long lead in J13. |
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6. Examine the 100u capacitor. One lead is shorter than the other. A minus sign is marked on the can near the short lead. Trim the short lead so that it is 0.4 inches long. Trim the long lead so that it is 0.5 inches long. 7. Fit the capacitor with the short lead in J7 and the long lead in Y7. Testing the circuit..... Switch on the 5 volt supply to the programmer module. Electrostatic energy..... Press the top push button S1 and hold it down for about half a minute. Release that button then press the other push button S2 and hold it down. The LED will glow brightly then fade away quickly to nothing. The LED will be on for about a half a second. Repeat this a few times. Notice that the LED is initially as bright as when it is connected to 5 volt supply via the 1k resistor. What happens is this. When push button S1 is pressed the 5 volt supply is connected to the capacitor via the 100k resistor and current flows into the capacitor and gradually charges it up to almost the full 5 volts. When S1 is released the capacitor still has 5 volts across its terminals. At the moment when push button S2 is pressed the charged capacitor behaves as if it is a 5 volt supply but as current flows out of it and into the LED the charge is dissipated and the voltage at the terminals of the capacitor rapidly drops to below 2 volts. What you have learnt by this is that a capacitor is a device which will store electrostatic energy. Time constant..... Now try this. Press S1 and keep it held down for all of this test. Wait for about half a minute then press S2 and hold it down until the LED has just a faint glow. The LED will not completely stop glowing because a tiny current will continue to flow through the 100k resistor. Keep S1 down and press S2 at various time intervals. What you will find is that the capacitor needs to be allowed one hundred times longer to charge than it takes to discharge. If you press S2 more quickly than once every half minute you will find that the LED starts less brightly. This happen when the capacitor is not allowed enough time to charge. Look at the circuit. The resistor charging the capacitor is 100 times higher resistance than the resistor that discharges the capacitor. So the capacitor discharges 100 times faster. You have just discovered the time constant effect of a resistor and capacitor together, another very important effect that we need to be aware of when designing electronic circuits. Decoupling..... Unplug the 100k resistor (R1) and fit a 470 ohm resistor in its place. The charging resistor in our circuit is now roughly half the value of the discharging resistor so S1 only needs to be pressed for half the time that the LED glows. Press S1 for a very short time then press S2. Do this a few times changing the amount of time that S1 is pressed. Now hold S2 down and while that is down press S1 on and off as fast as you can. The LED will glow continuously but its brightness will change. Keep S2 down and Press S1 on and off very slowly. Each time S2 is down the LED will be bright then fade away when S2 is up. Unplug the capacitor. When you have done that hold S2 down and while that is down press S1 on and off as fast as you can. However fast you press S1 on and off the LED will only be on while the push button is down. Fit the capacitor again. Hold S2 down then press S1 on and off. |
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