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SCR Gate-Triggering Circuits
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Triggering circuits provide firing signal to turn
on the SCR at precisely the correct time.
Firing circuits must have following properties1. Produce gate signal of suitable magnitude and sufficiently short
rise time.
2. Produce gate signal of adequate duration.
3. Provide accurate firing control over the required range.
4. Ensure that triggering does not occur from false signals or
noise
5. In AC applications, ensure that the gate signal is applied when
the SCR is forward-biased
6. In three-phase circuits, provide gate pulses that are 120apart
with respect to the reference point
7. Ensure simultaneous triggering of SCRs connected in series
or in parallel.
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Types Of Gate Firing Signals
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1. DC signals
2. Pulse signals
3. AC signals
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(a) DC Gating Signal From Separate
Source
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Disadvantage of DC gating Signals
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1. Constant DC gate signal causes gate power
dissipation
2. DC gate signals are not used for firing SCRsin AC applications, because presence of
positive gate signal during negative half cycle
would increase the reverse anode current and
possibly destroy the device.
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(2) Pulse Signals
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1. Instead of continuous DC signal, single pulse
or train of pulses is generated.
2. It provides precise control of point at whichSCR is fired.
3. It provides electrical isolation between SCR
and gate-trigger circuit.
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SCR trigger circuits using UJT
oscillator
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Circuit A
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90 Phase Control of SCR.
(R-Triggering)
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In ac circuits, the SCR can be turned on by the gate at any angle a with respect to the
applied voltage. This angle is called the firing angle.
Power control is obtained by varying the firing angle and this is known as phase control.
In the phase-control circuit given in fig. 1, the gate triggering voltage is derived from the ac
supply through resistors R1,R2and R3.
The variable resistance R2limits the gate current during positive half cycles of the supply.
If the moving contact is set to the top of resistor R2, resistance in the circuit is the lowest and
the SCR may trigger almost immediately at the commencement of the positive half cycle of
the input.
If, on the other hand, the moving contact is set to the bottom of resistor R2, resistance in thecircuit is maximum, the SCR may not switch on until the peak of the positive half-cycle.
By adjusting R2 between these two extremes, SCR can be switched on somewhere
between the commencement and peak of the positive half-cycle, that isbetween 0and 90.
If the triggering voltage VT is not large enough to trigger SCR at 90, the device will not
trigger on at all, because VThas the maximum value at the peak of the input and decreases
with the fall in voltage. This operation is sometimes referred to as half-wave variable-resistance phase control. It is an effective method of controlling the load power.
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180 degree Phase Control.
(RC-Triggering)
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The circuit shown in figure, can trigger the SCR from 0 to 180ofthe input waveform.
In the circuit shown here, the resistor R and capacitor C determine
the point in the input cycle at which the SCR triggers. During the negative half cycle of the input, capacitor C is charged
negatively (with the polarity shown in the figure) through diode D2tothe peak of the input voltage because diode D2is forward-biased.
When the peak of the input negative half cycle is passed, diodeD2 gets reverse-biased and capacitor C commences to discharge
through resistor R. Depending upon the time constant, that isCR, the capacitor C may
be almost completely discharged at the commencement of thepositive half cycle of the input, or it may retain a partially negativecharge until almost 180of positive half cycle has passed.
So long as the capacitor C remains negatively charged, diode D1, is
reverse-biased and the gate cannot go positive to trigger the SCRinto conduction.
Thus R and /or C can be adjusted to affect SCR triggering anywherefrom 0to 180of the input ac cycle.
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Pulse Control of an SCR.
(UJT-Triggering)
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The simplest of SCR control circuits is shown in figure.
If SCR were an ordinary rectifier, it would develop half-wave rectified
ac voltage across the load RL.
The same would be true if the gate of the SCR had a continuous bias
voltage to keep it on when the anode-cathode voltage VAK goes
positive.
A trigger pulse applied to the gate can switch the device at any timeduring the positive half-cycle of the input.
The resultant load waveform is a portion of positive half cycle
commencing at the instant at which the SCR is triggered.
Resistor RG holds the gate-cathode voltage, VG at zero when no
trigger input is present. The instantaneous level of load current can/be determined from the
following relation
VT
=VD
+ VG
+
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Unijunction Transistor
UJT does not belong to thyristor familybecause itsdoes not have a four-layer type ofconstruction and term unijunction refer to itshas one pn junction.
UJT being used mainly as a triggering deviceor switchin thyristor circuits and can also beused in oscillatorcircuits.
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Unijunction Transistor
The symbol is almost similar to a JFET. Notethe angle of the emitter. The other terminalsare called B1 and B2. The characteristics arequite different than any other transistor.
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Unijunction Transistor
Even though UJT is a switching device it
works very differently from SCR variety ofdevices.
The equivalent circuit indicates that UJT islike a diode and a resistive voltage dividercircuit.
rB2
rB1
E
B2
B1
+
VEB1
+
Vpn
+VBB
VBB
IE
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UJT Operation:
Unijunction transistor can trigger larger thyristors with a
pulse at base B1.With the emitter disconnected, the totalresistance RBB, a datasheet item, is the sum of RB1and RB2.
RBBO ranges from 4-12k for different device types. The
intrinsic standoff ratio is the ratio of RB1 to RBBO. It varies
from 0.4 to 0.8 for different devices.
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Unijunction Transistor
The resistance exhibited by rB1 is variable; it
is dependent on the value of current IE. Voltage is applied across UJT, VBB and to
emitter input, VEB1. Once VEB1 reaches a peak value (Vp) the UJT
begins to conduct (pn junction forwardbiased).
At point where VE = Vp, current IE is at
minimum. This is the threshold value of VEthat puts the UJT into conduction. Once conducting, IE increases and VE
decreases. This is called negative resistance.
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Unijunction Transistor
Beyond the valley point (VE= VV and IE = IV),
the device is in saturation and VE increasesvery little with an increasing IE.
UJT characteristics curve for a
fixed value of VBB
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Unijunction Transistor
The resistive equivalent circuit of a UJT
shown makes it easier to understand itsoperation.
rB1represent the internal dynamic resistancebetween emitter and base 1, B1 and rB2represent the dynamic resistance betweenemitter and base 2, B2.
Total resistance orinterbase resistance,
rBB= rB1+ rB2.
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Unijunction Transistor
rB1 varies inversely with IE with value from
several thousand ohm to tens ohm. Thevoltage across resistance,
rB1,VrB1= (rB1/rBB)VBB
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Unijunction Transistor
UJT is used almost exclusively as a triggercircuit for SCRs.
UJT Application Trigger Circuit
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Unijunction Transistor
UJT can be used as a trigger device for SCRand triac (other application : nonsinusoidaloscillator, sawtooth generator, phase control)
To ensure turn on, R1must not limit IEat peakpoint to less than IP. To ensure this, VR1 atpeak should be greater than IPR1.
VBBVP> IPR1 or R1< (VBBVP)/IP
UJT Application Trigger Circuit
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Unijunction Transistor
To ensure turn-off, of UJT at valley point, R1must be large enough that IE can decreasebelow the specified value of IV. So VR1 at
valley point must be less than IVR1. So for turnoff ;
VBBVV< IVR1R1> (VBBVV)/IV
UJT Application Trigger Circuit
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Unijunction Transistor
UJT Application Trigger Circuit
R l ti ill t
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Relaxation oscillator
waveforms
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