# UJT Triggering Circuit

```POWER ELECTRONICS
UJT Triggering Circuit
R, RC, UJT Triggering Circuit
Introduction
Triggering of SCR by using three methods
 R Triggering
 RC Triggering
 UJT Triggering
R Triggering:
It includes one fixed resistor, variable resistor, diode, SCR (Silicon
The circuit diagram of an R Triggering is shown below(Figure1).
Simple resistor, diode combinations trigger and control SCRs over
the full 180 electrical degree range, performing well at commercial
temperatures. These types of circuits operate most satisfactorily when
SCRs have fairly strong gate sensitivities. Since in a scheme of this type
a resistor must supply all of the gate drive required to turn on the SCR,
the less sensitive the gate, the lower the resistance must be, and the
greater the power rating.
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D1 = IN4007
D1 = IN4007
D1
D1
Figure
Half-wave
variablevariable
Resistor Phase
Control
– Circuit
Schematic
Figure1:1:Simple
Simple
Half-wave
Resistor
Phase
Control
– Circuit
Schematic
Figure 1 shows a very simple variable resistance half-wave circuit. It
provides phase retard from essential zero (SCR full “on”) to 90 electrical
degrees of the anode voltage wave (SCR half “on”).Diode D1 blocks
reverse gate voltage on the negative half-cycle of anode supply voltage.
It is necessary to rate blocking to at least the peak value of the AC
upply voltage and the trigger voltage producing the gate current to fire
IGF are in phase. When EAC = Em, at the peak of the AC supply voltage,
the SCR can still trigger with the maximum value of resistance between
anode and gate.
Used for the phase control application such as converter circuits
(half &amp; full controlled).
RC Triggering:
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It includes variable resistor, two diodes, SCR (Silicon Controlled
The circuit diagram of an RC Triggering is shown below (Figure 2).
Figure 2 shows an R-C-Diode circuit giving full half-cycle control (180
electrical degrees). On the positive half-cycle of SCR anode voltage the
capacitor charges to the trigger point of the SCR in a time determined
by the RC time constant and the rising anode voltage. The top plate of
the capacitor charges to the peak of the negative voltage cycle through
diode D2 on the negative half-cycle, resetting it for the next charging
cycle.
D1, D2 =IN4007
D2
D1
Figure 2: Simple Half-wave RC Phase Control – Circuit Schematic
Working of Capacitor:
During negative half cycle capacitor charges in reverse
direction when the supply voltage increases towards positive side the
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capacitor voltage also recharges in opposite direction. When this
capacitor voltage reaches threshold voltage SCR will turn on and
capacitor discharges through diode D2 and its voltage become very
small positive voltage.
Firing angle can be varied from 0 to 180 degree.
UJT Triggering:
It includes UJT triggering, SCR (Silicon Controlled Rectifier), Load
resistor.
UJT
A unijunction transistor (UJT) is an electronic semiconductor
device that has only one junction. The UJT has three terminals:
an emitter (E) and two bases (B1 and B2). The base is formed by
lightly doped n-type bar of silicon. Two ohmic contacts B1 and
B2 are attached at its ends. The emitter is of p-type and it is
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heavily doped. The resistance between B1 and B2, when the
emitter is open-circuit is called interbase resistance.
Working of Capacitor:
Initially the capacitor charges through R whose voltage is applied
to the emitter of UJT.
When the capacitor voltage reaches peak point voltage of UJT. the
UJT will switch to on condition. Now the capacitor discharges through
the output resistance. Thus the pulse is generated in the circuit.
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