PN junctions and diodes

When an LED is connected to a power supply to light it, what is the main purpose of a resistor in series with the LED?

  1. ATo limit the current through the LED to a suitable value
  2. BTo change the LED's emission color
  3. CTo raise the LED's reverse breakdown voltage
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Answer: A. To limit the current through the LED to a suitable value

An LED is a diode whose current rises sharply above its forward voltage, so connecting it directly to a supply would let excessive current flow and destroy it. A series resistor limits the current; its value is (supply voltage − LED forward voltage) ÷ desired current.

  • A. Correct. It limits the current to protect the LED.
  • B. The color is set by the semiconductor material, not the resistor.
  • C. A series resistor does not raise the reverse breakdown voltage.
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More PN junctions and diodes questions

  1. What is the main purpose of placing a capacitor in parallel with the output of a rectifier?
  2. How does the forward voltage of a silicon diode carrying a constant current change as the temperature rises?
  3. For an ideal diode (ideality factor n = 1) at room temperature, roughly how much must the forward voltage increase to raise the forward current tenfold?
  4. Which expression gives the depletion (junction) capacitance C of a reverse-biased PN junction in terms of junction area A, depletion width W and semiconductor permittivity ε?
  5. Why does no current flow externally in a PN junction in thermal equilibrium?