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Ramp and Pedestal Triggering Circuit of SCR
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Figure 1 shows the circuit for ramp-and-pedestal triggering of two thyristors connected in anti-parallel for controlling power in an ac load. Ramp and pedestal triggering is an improved version of Synchronized UJT Oscillator triggering. The various voltage-waveforms are shown in Fig.2

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Zener-diode voltage, Vz, is constant at its threshold-voltage. Rp acts as a potential divider. Wiper of RP controls the value of pedestal voltage Vp. Diode D allows C to be quickly charged to Vp through the low-resistance of the upper portion of Rp. The setting of wiper on Rp is such that, this value of Vp is always less than the UJT firing point voltage ηVz . When wiper setting is such that VP is small voltage Vz charges C through R. When this ramp voltage VC reaches ηVz , UJT fires and voltage VT, through the pulse transformer, is transmitted to the gate circuits of both thyristors T1 and T2 .

During first positive half-cycle, SCR T1 is forward biased and is therefore, turned-on. After this, Vc reduces to VP and then to zero at ωt=π. As Vc is more than VP during the charging of capacitor C through charging resistor R , diode D is reverse-biased. Thus, VP does not effect in anyway the discharge of C through UJT emitter and primary of pulse transformer. From period 0 to π, SCR T1 is forward biased and is turned- on. From π to 2π ; T2 is turned-on. In this manner, load is subjected to alternating eL as shown in fig.2

With the setting of wiper on RP pedestal voltage VP on C can be adjusted.

With low pedestal voltage across C ramp charging of C to ηVz takes longer time and firing angle delay is, therefore, more and output voltage is low. With high pedestal on C voltage-ramp charging of C through R reaches η Vz faster, firing angle delay is smaller and output voltage is high. This shows that output voltage is proportional to the pedestal voltage.

The time T required for capacitor to charge from pedestal voltage VP to ηVz can be obtained from the relation,

ηVz=VP+(VzVP)(1eT/RC)

Note that (VzVP) is the effective voltage that charges C from VP to ηVz .

From above, T=RClnVzVPVz(1η)

and the firing angles are given by

αp=ωR.ClnVzVpVz(1η)

and αR=ωRCln11η

where ω is the input frequency and η is the intrinsic stand off ratio of the UJT.

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