A student is designing a model railway crossing signal. The circuit uses several electronic components including a capacitor and a light emitting diode (LED...
Question 1 Report
A student is designing a model railway crossing signal. The circuit uses several electronic components including a capacitor and a light emitting diode (LED). The LED is connected in series with a resistor to indicate when a train is approaching.
(a) State the function of a capacitor in an electronic circuit. [1]
(b) Explain why a resistor must be connected in series with an LED. [1]
Answer Details
(a) A capacitor stores electrical charge (energy). [1]
A capacitor consists of two conductive plates separated by an insulating layer (dielectric). When a voltage is applied, electrons accumulate on one plate and are depleted from the other, creating an electric field that stores energy. This stored charge can later be released back into the circuit. In electronic timing circuits such as a 555 timer, the charge and discharge rate of a capacitor determines the output timing. In power supply circuits, capacitors smooth out voltage fluctuations.
(b) A resistor must be connected in series with an LED to limit the current flowing through it, preventing damage. [1]
An LED is a semiconductor device with a very low forward resistance once it begins conducting. Without a series resistor, the current drawn from the power supply would be far higher than the LED's maximum rated current (typically 20 mA for a standard indicator LED). This excessive current would overheat the junction inside the LED, destroying it almost instantly. The series resistor creates a controlled voltage drop that restricts the current to a safe level. The required resistance is calculated using Ohm's law: \( R = \frac{V_{\text{supply}} - V_{\text{LED}}}{I_{\text{LED}}} \).
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