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A tungsten filament, similar to
that found in an electric light bulb is heated by an electric
current, in a glass envelope containing a vacuum. This produces
a cloud of electrons around it, which are negatively charged. If
a positively charged metal plate is positioned near the
filament, it will attract electrons from the cloud and a current
will flow in the circuit. If the plate is charged negatively the
electrons will be repelled and no current will flow. This is the
basis of the valve rectifier. This type of valve is called a
diode, the filament is called the cathode and the plate is
called the anode. The diode valve was developed by Sir John
Ambrose Fleming in 1904, while working for the Marconi Company.
A diode with a filament as the cathode is called
a directly heated valve, but most modern valves are indirectly
heated. An indirectly heated cathode consists of a nickel tube
that is coated with an oxide consisting of barium, strontium and
calcium. Inside the tube is the heater which consists of an
insulated tungsten filament. This type of cathode has two
advantages. It produces more electrons and can be operated from
an AC or DC voltage.
Electronic symbols for the diode
The graph below shows that voltage
current relationship is not a straight on off, in reality the
curved bit at the bottom is where more and more of the electrom
cloud is attracted across this is called the non saturated mode
then when all the electons boiled off are attracted as quickly
as they are boiled off the graph is flat
Voltage current graph for a typical valve
The diode is used as a rectifier
to convert an AC voltage to DC. In the circuit below, the diode
only conducts when the anode is positive with respect to the
cathode, and so only conducts on each positive half cycle of the AC
input. The voltage at the cathode consists of just the positive half
cycles.
The capacitor charges to the peak of these half
cycles to produce a smooth DC voltage.
DC output voltage shown in the graph, is not
very smooth because the capacitor starts to discharge between
each positive peak. This is called the ripple voltage, and it
can be reduced by increasing the size of the smoothing capacitor
The diagram below shows how four diode rectifiers may
be arranged in a "bridge" to produce a direct current from an
alternating current
The graph below shows the unsmoothed output waveform
from a bridge rectifier circuit
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Diode