Diode Applications: Rectification

Applications of Diodes

• Diodes function as "one-way" devices in electrical circuits, similar to check valves in pneumatic or hydraulic systems.
• Diodes are prevalent in electrical and electronic applications.
• Common applications: rectification, voltage regulation, flyback protection, reverse voltage protection, voltage doubling, light emission (LEDs), and sensing (photodiodes).

Rectification

• Rectification is converting a waveform into a unidirectional one, commonly alternating current (AC) to direct current (DC) in electronic devices.
• Virtually every electronic device that plugs into a standard electrical outlet requires rectification.
• Diodes and capacitors are used in steps to convert an AC source into one resembling a steady-state DC source.

Half Wave Rectification

• Half-wave rectification is the simplest form of rectification using only one diode.
• During the positive alternation of a sinusoidal input, the diode is forward biased, and the output matches the input across the resistive load.
• During the negative alternation, the diode is reverse biased, resulting in zero output across the resistive load.
• The output waveform frequency is the same as the input waveform, with the cycle repeating at the same interval.
• Half of the input is wasted, making it inefficient for general use.

Full Wave Rectification

• Full-wave rectification improves on half-wave rectification by using two diodes to utilize the full input waveform.
• The input is fed into a transformer with a center tap.
• During the positive alternation, one half of the output circuit is active through one diode, while during the negative alternation, the opposite half is active through the other diode.
• The negative alternation is effectively flipped to positive, resulting in an output frequency that is double the input frequency.
• A main drawback is the need for a transformer

Full Wave Bridge Rectification

• The full-wave bridge rectifier is the most common type, using four diodes in a "bridge" configuration.
• The four diodes ensure a complete circuit from the input through the source, doubling the output frequency.
• During the positive alternation, diodes D1 and D2 are forward biased and pass current, while D3 and D4 are reverse biased and block current.
• During the negative alternation, diodes D3 and D4 are forward biased, while D1 and D2 are reverse biased.
• The current always flows through the load in the same direction, achieving rectification.
• Active diodes are always parallel, and blocking diodes are also always parallel.

Capacitor Smoothing

• The output waveform of a rectifier circuit is a pulsating DC.
• A smoothing capacitor in parallel with the resistive load is added to achieve steady-state DC.
• When the input voltage drops, the capacitor discharges into the circuit, providing temporary output; it recharges near the peak alternation.
• A sufficiently high capacitance reduces the ripple effect, making the output closely resemble a steady-state DC suitable for use.

Voltage Regulation

• Zener diodes are commonly applied in DC voltage regulator systems due to their predictable avalanche breakdown voltage.
• It is used to drain current during an overvoltage.
• A voltage source supplies current through a resistor to power a resistive load.
• The resistor and Zener diode create a voltage divider, providing an output voltage to the load.
• The Zener diode is reverse biased during normal operation, blocking current and forcing it through the resistive load.
• If the voltage exceeds the Zener diode's reverse breakdown voltage (e.g., 5.1V), it avalanches, draining virtually all current and protecting the load from overvoltage and overcurrent.
• The Zener diode ensures output voltage never exceeds its reverse bias voltage, protecting the load.

Light Emitting Diodes

• Light Emitting Diodes (LEDs) are a special type of diode radiating energy from recombination as visible light.
• The color depends on the LED's chemical composition, and each combination has its specific forward bias voltage.
• Common examples include: Red (1.6V), Orange (2.0V), Yellow (2.1V), Green (1.9V), Blue (2.4V), and White (3.5V).
• Cover a wide range of forward bias voltage values.
• Different value current limiting resistors would be needed in similar circuits, depending on the colour of LED used.
• If a blue LED is replaced with a red LED without changing the current limiting resistor, the new LED could be damaged.

Freewheeling Diode

• Diodes are used in electrical systems with high-current inductors, such as motors, relays, and solenoids, to protect components during switching.
• These diodes are called freewheeling or flyback diodes.
• During normal circuit operation, the diode is reverse biased, blocking current.
• The purpose becomes apparent when power to the device is disconnected.
• In accordance with Lenz's Law, an inductor will oppose a change in current and attempts to continue driving current, when switched off.
• The freewheeling diode provides a short circuit path for this current.

Freewheeling Diode Operation

• A circuit with a freewheeling diode operates in two states.
• In the first state, with normal power, the current flows through the resistive load, unable to pass through the reverse-biased diode.
• In the second state, immediately after power is turned off, the inductor attempts to drive current.
• The arrows show the current path which flows through the now forward biased diode
• The diode, when forward biased, is very low resistance in comparison to the resistive load.
• Almost all the current flows through the diode rather than the resistive load.
• Without the diode, the current would have no path other than the resistive load, potentially causing damage.

Diode Testing

• Diodes can be tested using an ohmmeter or digital multimeter in resistance mode.
• In a forward-biased condition, the resistance reading should be very low (effectively zero) for a functional diode.
• In a reverse-biased condition, the resistance reading should be very high (effectively open-circuit).
• Functional if it meets both conditions.
• Low resistance in both directions indicates a short-circuited diode that cannot be used.
• High resistance in both directions indicates an open circuit, rendering the diode useless.