<^< C-R Discharging | Course Index | Passive Wave-shaping Circuits >^>
| Objective:
| To investigate the action of a C-R charging circuit when connected to a d.c. supply.
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| Underpinning knowledge:
| Capacitance, charge and voltage. Time constant.
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| Apparatus:
| TINA circuit file, or apparatus to make the following circuit.
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| TINA Circuit File:
| Click here to download circuit file
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| Initial Settings:
| C = 100µF, R = 1MΩ, 10V d.c. Supply.
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| Requires:
| Paper, Linear graph paper, pen or pencil.
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| Instructions:
(a) Change the switch so the capacitor is connected to the discharge resistor. Then start the simulation in transient mode.
(b) Take readings of capacitor voltage every 20s from 0 to 200s and then every 50s from 200s to 400s. At each point, measure and record the capacitor voltage in a table.
(c) Plot a graph showing capacitor voltage, VC, plotted against time, t.
(d) Determine the time constant of the circuit (C x R).
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Observations:
Comment on the shape of the graph. Determine the time taken for the capacitor to fall to 37% of its initial voltage (i.e. 37% of 10V or 3.7V). Confirm that this time is the same as the time constant (C x R) for the circuit.
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| Further work:
Construct a tangent to your curve of VC plotted against time at t = 0. The line of this tangent should pass through 0V on the voltage scale at a time equal to the time constant (C x R). What can you infer from this?
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<^< C-R Discharging | Course index | Passive Wave-shaping Circuits >^>
