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UJT Firing Circuit
It is the most common method of triggering the SCR because the prolonged pulses at the gate
using R and RC triggering methods cause more power dissipation at the gate so by using UJT (Uni
Junction Transistor) as triggering device the power loss is limited as it produce a train of pulses.
The RC network is connected to the emitter terminal of the UJT which forms the timing circuit.
The capacitor is fixed while the resistance is variable and hence the charging rate of the capacitor
depends on the variable resistance means that the controlling of the RC time constant.
When
the voltage is applied, the capacitor starts charging through the variable resistance. By
varying the resistance value voltage across the capacitor get varied. Once the capacitor voltage is
equal to the peak value of the UJT, it starts conducting and hence produce a pulse output till the
voltage across the capacitor equal to the valley voltage Vv of the UJT. This process repeats and
produces a train of pulses at base terminal 1.
The pulse output at the base terminal 1 is used to turn ON the SCR at predetermined time intervals
Figure: 1. 26. UJT Firing circuit for SCR and corresponding waveforms
Series and Parallel connections of SCRs
In many power control applications the required voltage and current ratings
exceed the voltage and
current that can be provided by a single SCR. Under such situations the SCRs are required to be
connected in series or in parallel to meet the requirements. Sometimes even
if the required rating is
available, multiple connections are employed for reasons of economy and easy availability of SCRs of
lower ratings. Like any other electrical equipment, characteristics/properties of two SCRs of same make
and ratings are never same and this leads to certain problems in the circuit. The mismatching of SCRs is
due
to differences in
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(i)
turn-on time
(ii)
turn-off time
(iii)
Leakage current in forward direction
(iv)
Leakage current in
reverse direction and
(v)
Recovery voltage.
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