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School-college Physics Notes: Electricity 4.4 Uses of a simple diode device

Electricity section 4: 4.4 The diode electrical device - how does it work & what is it used for?

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[KEY POINTS and learning objectives for this page, after initial notes]

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4.4 The diode electrical device - how does it work and what is it used for?

See also Part 3.4 An experimental investigation of I-V characteristics of a diode

graph of current versus p.d. volts for a diode

The current through a diode flows in one direction only

This is because a diode has a very high resistance in the reverse direction.

A diode is a special device made from a semiconducting material based on silicon (classed as a semi-conductor).

Since the current only flows one way through a diode, it can be used to convert an ac current into a dc current (rectifier, rectification).

With alternating current (ac), the current changes direction in a cycle, but with direct current (dc) there is no reversal in current direction, it flows one way with a constant voltage.

Oscilloscope traces comparing ac and dc current signals - showing the alternating + <=> - oscillation of the alternating current p.d. and the constant p.d. of a direct current.

Diagram 2. shows what happens if pass an ac current through a diode - 'before and after' trace after the dc output has been smoothed.

In some devices in the home the output from e.g. the transformer in your computer power supply, is rectified to convert it from ac to a dc supply.

 

Uses of a diode

Diodes are used as rectifiers (converting an ac signal/current to dc), signal limiters, voltage regulators, switches, signal modulators, signal mixers, signal demodulators, and oscillators etc. etc. in other words - rather useful in electronic circuits.

Diodes are used radio transmitters and receivers.

See also Part 3.4 Investigating the current - voltage characteristics of a diode - graph explained

 

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Key points about electric circuits in physics courses - function and use of a diode

Information sources for Doc Brown's key points: IGCSE-GCSE physics are based on textbooks & syllabus-specifications for students taking the UK AQA, Edexcel, OCR 21st Century Science, OCR Gateway science suite, WJEC, CCEA and CIE GCSE physics 9-1 level science examinations.

A structured, exam-board-aligned summary on diodes, tailored for GCSE/IGCSE Physics students:

Diodes – Summary Revision Notes

What is a Diode?

A diode is an electronic component that allows current to flow in one direction only. It has:

  • Low resistance in the forward direction (forward bias)
  • Very high resistance in the reverse direction (reverse bias)

This makes it a non-ohmic component, meaning it does not obey Ohm’s Law.

Circuit Symbol of a diode

The triangle points in the direction of conventional current flow (positive to negative)

The vertical bar blocks current in the reverse direction

How a diode Works

Condition Behaviour Explanation
Forward Bias Current flows after threshold (~0.6–0.7V) Resistance drops sharply; diode conducts
Reverse Bias No current flows Diode acts as an insulator; very high resistance

Uses of Diodes

Diodes are used in circuits where directional control of current is needed:

  • Rectifiers: Convert AC to DC (e.g. in power supplies)
  • Protective circuits: Prevent damage from reverse polarity
  • Logic gates: In digital electronics
  • LEDs: Light Emitting Diodes are special diodes that emit light (see Part 4.5)

I–V Graph Characteristics of a diode

  • Non-linear graph
  • Forward bias: Little current until threshold voltage, then sharp rise
  • Reverse bias: Flat line near zero (no current)

This behaviour confirms that diodes are non-ohmic.

Required Practical (All Boards)

Aim: Investigate the current–voltage (I–V) characteristics of a diode

Method Overview:

  • Connect diode in series with ammeter and variable power supply
  • Use voltmeter in parallel across diode
  • Measure current and voltage in both forward and reverse directions
  • Plot I–V graph

Expected Result: Sharp rise in forward direction; no current in reverse

Typical Exam Board Specification concerning use of a diode

Key Focus
Required practical on I–V characteristics; diode as non-ohmic component
Emphasis on interpreting I–V graphs and understanding directionality
Includes diode in non-linear component studies and practicals
Focus on circuit diagrams and diode behaviour in forward/reverse bias
Investigates diode function and current flow direction
Strong emphasis on I–V graphs, definitions, and practical understanding

Student Tips for a diode question

  • Memorise: Diodes allow current in one direction only
  • Sketch the I–V graph: Show threshold behaviour and no reverse current
  • Use correct terms: “Forward bias”, “reverse bias”, “threshold voltage”
  • Explain non-ohmic: Diodes don’t follow ( V proportional to I )
  • Revise practicals: Be confident describing method, apparatus, and expected results
  • Think real-world: Diodes are essential in chargers, power supplies, and LED lighting

Keywords, phrases and learning objectives for a diode device in an electrical circuit

Know how a simples diode circuit device works.

Be able to describe what is diodes are used for e.g. rectifies a.c. to d.c., signal limiters, voltage regulators, switches, signal modulators, signal mixers, signal demodulators and oscillators.

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