Bridge rectifiers

There are several ways of connecting diodes to make a rectifier to convert AC to DC. The bridge rectifier is one of them and it is available in special packages containing the four diodes required. Bridge rectifiers are rated by their maximum current and maximum reverse voltage. They have four leads or terminals: the two DC outputs are labeled + and -, the two AC inputs are labeled

Bridge rectifier

Example: Circuit symbol:

Function

Diodes allow electricity to flow in only one direction. The arrow of the circuit symbol shows the
direction in which the current can flow. Diodes are the electrical version of a valve and early diodes were actually called valves.

Forward Voltage Drop

Electricity uses up a little energy pushing its way through the diode, rather like a person pushing through a door with a spring. This means that there is a small voltage across a conducting diode, it is called the forward voltage drop and is about 0.7V for all normal diodes which are made from silicon. The forward voltage drop of a diode is almost constant whatever the current passing through the diode so they have a very steep characteristic (current-voltage graph).

Reverse Voltage

When a reverse voltage is applied a perfect diode does not conduct, but all real diodes leak a very tiny current of a few µA or less. This can be ignored in most circuits because it will be very much smaller than the current flowing in the forward direction. However, all diodes have a maximum reverse voltage (usually 50V or more) and if this is exceeded the diode will fail and pass a large current in the reverse direction, this is called breakdown.

Signal diodes (small current)

Signal diodes are used to process information (electrical signals) in circuits, so they are only required to pass small currents of up to 100mA.

General purpose signal diodes such as the 1N4148 are made from silicon and have a forward voltage drop of 0.7V.

Source : http://robosapienv2-4mem8.page.tl

Zener Diode Tutorial - 23

Zener Diode Tutorial

The Zener diode is operated in reverse bias mode (positive on its cathode).
It relies on the reverse breakdown voltage occurring at a specified value.
This value is printed on it.

It has two main applications.

1. as a reference source, where the voltage across it is compared with another voltage.

2. as a voltage regulator, smoothing out any voltages variations occurring in the supply voltage across the load.

When being used a voltage regulator, if the voltage across the load tries to rise then the Zener takes more current.
The increase in current through the resistor causes an increase in voltage dropped across the resistor.
This increase in voltage across the resistor causes the voltage across the load to remain at its correct value.

In a similar manner, if the voltage across the load tries to fall, then the Zener takes less current.
The current through the resistor and the voltage across the resistor both fall.
The voltage across the load remains at its correct value.

Voltage Doubler Tutorial - 22

Voltage Doubler Tutorial

It's best to read the page on the half-wave rectifier, first.

Point A is doing the opposite of point B.

As A increases in a positive direction, B increases negatively, and vice-versa.

When A is positive, D1 is forward biased and charges C1 to the peak voltage, as in diagram 2.
D 2 is reverse biased and does not conduct.

When A goes negative, D1 is reverse biased and does not conduct.
D2 is forward biased and charges C2 to the peak voltage, as in diagram 3.

We now have two capacitors in series, each charged to the peak voltage.
The voltage across the load is. therefore, twice the peak voltage.

The voltage has been doubled.

Varicap Diode Tutorial - 21

Varicap Diode Tutorial

When the junction diode is reverse biased, the insulating barrier widens.
The higher the reverse voltage the wider the barrier becomes.

The barrier forms the dielectric, of variable width, of a capacitor.
The N and P type cathode and anode are the two plates of the capacitor.

In the diagram, the diode and coil form a resonant circuit.
The capacitance of the diode, and thereby the resonant frequency, is varied by means of the potentiometer controlling the reverse voltage across the varicap.
The capacitor prevents the coil shorting out the voltage across the potentiometer.

Steering Diode Tutorial - 19

Steering Diode Tutorial

If the AC supply fails then the equipment is automatically powered by the standby battery.

While the AC supply is present, D1 has +15 volts on its anode, and is forward biased.
Since there is 0.6 volts across the diode, there is +14.4 volts on its cathode.
This voltage powers the equipment.
D2 is reverse biased since its anode is less positive than its cathode.
D2 is non conducting and the battery is isolated.

If the supply fails then the +15 volts disappears, and D2 becomes forward biased and conducts, to power the equipment.

The diodes STEER the voltages