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The unijunction
transistor (UJT) is made
of a bar of N type
material with a P type
junction (the emitter)
near the centre.
Base 1 is connected to
zero volts and base 2 to
the positive supply.
The resistance between
the two bases (the
INTERBASE RESISTANCE) is
typically 10k.
With the emitter
unconnected, the bar
acts as a potential
divider, and about 0.5
volts appears at the
emitter.
If a voltage is
connected to the
emitter, as long as it
is less than 0.5 volts,
nothing happens, as the
P-N junction is reversed
biased. (see the right
hand diagram).
When the emitter voltage
exceeds 0.5 volts, the
junction is forward
biased and emitter
current will flow.
This increase in current
is equal to a reduction
of resistance between
base 1 and the emitter.
This causes the emitter
voltage to fall.
In the circuit, C
charges via R1.
When the voltage across
C exceeds 0.6 volts, the
b1/emitter junction goes
low resistance and
discharges C.
The result is a sawtooth
waveform across C.
There is also a pulse of
current through R3 ,
giving a pulse of
voltage across it.
This circuit is called a
relaxation oscillator.
The voltage across C
charges up slowly then
suddenly relaxes.
The circuit is often
used to trigger
thyristor circuits.
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