Besides capacitance, which rating must be checked when choosing an electrolytic capacitor?

The working voltage rating must be higher than the supply voltage. A capacitor run above its voltage rating can fail or burst.

What is the time constant of an RC circuit?

The time constant is C × R, in seconds when C is in farads and R is in ohms. A charging capacitor reaches about 63% of the supply voltage after one time constant.

Calculate the time constant of a 100 μF capacitor charging through a 47 kΩ resistor.

The time constant is 4.7 s. T = C × R = (100 × 10⁻⁶) × (47 × 10³) = 4.7 s.

CCEA A-Level Technology and Design 8900

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Electronic Control SystemsOption A

Topic overview Control systems, feedback and electronics safety Resistors, Ohm's law and power ratings Capacitors and RC timing Potential dividers, sensors and comparators Switches, pull-up and pull-down resistors Output devices and LEDs Transistors, Darlington pairs and thyristors Logic gates, truth tables and flip-flops 555 monostable and astable timers

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Electronic Control Systems

Capacitors and RC timing

3 things to know
AS 1 Paper 2

Overview

Electrolytic and non-polarised capacitors, voltage ratings, the RC time constant with worked delays, charging and discharging graphs, and RC timers with transistors.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What does a capacitor do in a circuit?

Key term

How does an electrolytic capacitor differ from a non-polarised capacitor?

Key term

Describe a non-polarised capacitor and where it is used.

Questions from this subtopic

Each one with the model answer underneath.

1
What does a capacitor do in a circuit?
Model answer
A capacitor stores electrical charge and releases it later. Capacitance is measured in farads, usually as microfarads (μF), nanofarads or picofarads.
2
How does an electrolytic capacitor differ from a non-polarised capacitor?
Model answer
An electrolytic capacitor is polarised and must be connected the right way round. Electrolytic capacitors give large values, from about 1 μF to thousands of microfarads.
3
Describe a non-polarised capacitor and where it is used.
Model answer
A non-polarised capacitor, such as a ceramic or polyester type, can be fitted either way round. Non-polarised capacitors have small values and suit noise suppression and short timing.
4
Besides capacitance, which rating must be checked when choosing an electrolytic capacitor?
Model answer
The working voltage rating must be higher than the supply voltage. A capacitor run above its voltage rating can fail or burst.
5
What is the time constant of an RC circuit?
Model answer
The time constant is C × R, in seconds when C is in farads and R is in ohms. A charging capacitor reaches about 63% of the supply voltage after one time constant.
6
Calculate the time constant of a 100 μF capacitor charging through a 47 kΩ resistor.
Model answer
The time constant is 4.7 s. T = C × R = (100 × 10⁻⁶) × (47 × 10³) = 4.7 s.

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AThe total resistance is 2 kΩ.

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CThe series resistor adds another 1 kΩ.

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