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AS Level     Semiconductor     Diode     >Rectifier and PSU<     Zener Diode    

Semiconductor Rectifier and PSU


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  AS Level    Semiconductors  0 of 4    Question 606    Device Linearity 
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Rectifier diodes convert AC into DC by passing current in one direction only.

In the image below ...

Various Diodes

Most electronic circuits need low voltage direct current (DC).

To minimise energy loss in power lines, the power companies use high voltage alternating current (AC) for power distribution. This is because it's possible to step the voltage up or down using transformers. High voltage transmission is more efficient. Power supply circuits step the mains voltage down to a suitable value and produce direct current.

Here are some examples. The transformer steps the voltage down to a suitable low value. The diode/s only pass current in one direction so the AC is converted to DC. The capacitor stores charge and smooths out the raw DC. To get really good smoothing, electronic regulation is needed in addition to the capacitor used to store charge.

DC Power Supply with a Half Wave Rectifier

Power Supply - Half Wave Rectifier

Smoothing - Use a Big Capacitor

These sketches show ...

Power Supply Smoothing - Graphs

The half wave rectifier is the simplest and least satisfactory. It is called a half wave rectifier because the diode only conducts for half the time. 0.7 Volts is lost across the diode. Here are some improved circuits.

DC Power Supply with a Full Wave Rectifier

Power Supply - Full Wave Rectifier

This is a full wave rectifier using a transformer with a centre connection to the secondary coil. 0.7 Volts is lost across each diode.

Bridge Rectifier

Power Supply - Full Wave Bridge Rectifier

This is a bridge rectifier. This is the best solution. It has a simple transformer and full wave rectification. 1.4 Volts is lost across the diodes.

Smoothing - Use a Big Capacitor

These sketches show ...

Power Supply Smoothing - Graphs

Voltage Regulators

Here is a voltage regulator circuit using the 7805 chip. This produces very smooth DC at exactly 5 Volts. Other voltages are possible too.

Power Supply with a Voltage regulator    7805 Voltage Regulator

Op-Amp Voltage Regulator

Using a high power version of an op-amp, this circuit could (left) provides a stable 5.1 Volt output up to 10 Amps or whatever the op-amp rating is.
The resistor and Zener diode combination, on its own, can only stabilise a few milliamps of load current.
The right  hand circuit uses a standard op-amp with a MOSFET source follower or a bipolar transistor emitter follower.

Op-Amp Voltage Regulator    Op-Amp Voltage Regulator with Source Follower

Line Regulation

Power supplies should produce a constant output voltage over a range of line input voltages. For example many lap-top power supplies and phone chargers will work correctly for any line voltage between 100 and 250 Volts. This means they can be used in most countries of the world without any worries.

Load Regulation

Power supplies should maintain a constant output voltage, even if the output load current varies. Load regulation is a measure of how well a power supply manages to achieve this.

This formula calculates the percentage output change between minimum and maximum loads where Vmid is the nominal rated output.

%Regulation = 100 x ( ( Vmin - Vmax ) / Vmid )

 

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