How does a diode pass current one way only, and what is it used for?
Diodes: forward and reverse bias, the forward voltage drop, the LED, rectification, and protecting circuits against reverse current and back-EMF.
An Eduqas GCSE Electronics answer on diodes: forward and reverse bias, the forward voltage drop, the light-emitting diode, half-wave rectification of a.c. to d.c., and using a diode to protect a circuit from reverse polarity and from the back-EMF of a relay or motor.
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What this dot point is asking
Eduqas wants you to describe a diode: how it conducts in forward bias but blocks in reverse bias, its forward voltage drop, the light-emitting diode (LED), rectification of a.c. to d.c., and using a diode to protect a circuit against reverse polarity and against the back-EMF of an inductive load such as a relay or motor. The diode is the simplest non-ohmic component and a workhorse of output and power circuits.
The answer
Forward and reverse bias
The forward voltage drop
The light-emitting diode
Rectification and protection
Examples in context
Diodes appear throughout the output and power subsystems of the course. The LED is the most common output indicator and reuses the current-limiting-resistor calculation. Rectifier diodes turn the a.c. mains into the d.c. that electronics needs, and the smoothing capacitor that follows builds on the time-constant topic. The flyback diode is essential whenever a transistor switches a motor or relay, tying the diode directly to the transistor-switching and MOSFET topics that follow.
Try this
Q1. State the approximate forward voltage drop of a conducting silicon diode. [1 mark]
- Cue. About .
Q2. A diode in series with a resistor runs at from a supply. Find the resistor. [2 marks]
- Cue. ; (nearest preferred value ).
Q3. State the purpose of a flyback diode across a motor. [1 mark]
- Cue. It absorbs the back-EMF (induced voltage spike) when the current is switched off, protecting the switching transistor.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20194 marksExplain what is meant by forward bias and reverse bias for a diode, and state the approximate forward voltage drop of a silicon diode when it is conducting.Show worked answer →
Forward bias (up to 2 marks): the diode is connected so the anode is more positive than the cathode; once the forward voltage is exceeded the diode conducts and passes current with only a small voltage across it.
Reverse bias (1 mark): the anode is more negative than the cathode; the diode does not conduct (only a negligible leakage current flows) and blocks the current.
Forward voltage (1 mark): about for a silicon diode.
Markers reward the anode-positive condition for conduction, the blocking action in reverse, and the figure.
Eduqas 20223 marksA relay coil is switched by a transistor. Explain why a diode is connected across the coil, and how it should be oriented.Show worked answer →
Purpose (up to 2 marks): when the transistor turns off, the collapsing magnetic field of the coil induces a large reverse voltage (back-EMF) that would damage the transistor. The diode (a flyback or protection diode) provides a safe path for the induced current to circulate and decay, protecting the transistor.
Orientation (1 mark): the diode is connected in reverse bias across the coil during normal operation (cathode to the positive supply, anode to the collector), so it conducts only the back-EMF and not the normal supply current.
Markers reward the back-EMF on switch-off, the diode giving the induced current a safe path, and reverse orientation so it conducts only the spike.
Related dot points
- Driving LEDs: the need for a current-limiting series resistor, the forward voltage drop of an LED, and calculating the resistor value for a chosen current.
An Eduqas GCSE Electronics answer on driving LEDs: why an LED needs a series current-limiting resistor, the LED forward voltage drop, and calculating the resistor value from the supply voltage, forward voltage and forward current.
- Transistor switching: the bipolar transistor as a switch, cut-off and saturation, current gain, and choosing the base resistor to drive a load.
An Eduqas GCSE Electronics answer on using a bipolar transistor as a switch: the cut-off and saturation states, current gain relating collector and base current, and choosing the base resistor so a small input current turns on a larger load current.
- MOSFET switching: the MOSFET as a voltage-controlled switch, the gate threshold voltage, why it draws no steady gate current, and choosing between a MOSFET and a bipolar transistor.
An Eduqas GCSE Electronics answer on the MOSFET as a switch: voltage control by the gate, the gate threshold voltage, why a MOSFET draws essentially no steady gate current, switching higher-current loads, and choosing between a MOSFET and a bipolar transistor for an output stage.
- Power supplies: rectifying a.c. to d.c., smoothing with a reservoir capacitor, the idea of ripple, and stabilising the output voltage.
An Eduqas GCSE Electronics answer on power supplies: rectifying a.c. to d.c. with diodes, smoothing the output with a reservoir capacitor, the idea of ripple and how a larger capacitor reduces it, and stabilising the output voltage with a regulator or Zener diode.
Sources & how we know this
- WJEC Eduqas GCSE (9-1) Electronics specification (C490) — WJEC Eduqas (2017)