Circuits / Getting started
RC circuit with a square wave
C1 charges and discharges with τ = R·C; compare it with the period of G1
Tap a part to see its voltages and currents. It is a model, not a measurement.
Predict, measure, explain
- Predict A 0 to 5 V square wave at 100 Hz charges the capacitor through 1 kΩ. Does it reach 5 V?
- Measure Watch the capacitor's voltage. Then raise the generator to 1 kHz.
- Explain
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Almost: at 100 Hz each half lasts 5 ms, five times RC, so it swings from 0.1 to 4.9 V. At 1 kHz a half lasts 0.5 ms and it only swings from 1.9 to 3.1 V.
What do you need?
- Resistor 1 kΩSearch on Amazon
- Capacitor 1 µF 25 VSearch on Amazon
- Jumper wires × 1Search on Amazon
- BreadboardSearch on Amazon
How does an RC circuit charge and discharge?
A capacitor charged through a resistor does not jump to the new voltage: it approaches it along a curve set by the time constant, τ = R · C. With R1 at 1 kΩ and C1 at 1 µF, τ is 1 ms. After one τ the capacitor has covered about 63 % of the way, and after five it is practically there. Here the generator G1 drives a square wave between 0 and 5 V at 100 Hz, so each half of the wave lasts 5 ms, five time constants. The capacitor has time to fill and empty almost completely: it swings from 0.07 V to 4.93 V. Raise the frequency to 1 kHz and each half lasts only 0.5 ms. Now C1 cannot keep up and only moves between 1.93 V and 3.07 V around the middle. That is a low-pass filter at work.
Values from the Protoboard ISO simulation: a model, not a bench measurement.
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