scr triggering

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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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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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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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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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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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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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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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UJT is used almost exclusively as a triggercircuit for SCRs.

UJT Application Trigger Circuit


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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



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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



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R l ti ill t


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Relaxation oscillator

waveforms

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scr triggering

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