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

Zener-diode voltage, $V_{z},$ is constant at its threshold-voltage. $R_{p}$ acts as a potential divider. Wiper of $R_{P}$ controls the value of pedestal voltage $V_{p} .$ Diode D allows C to be quickly charged to $V_{p}$ through the low-resistance of the upper portion of $R_{p} .$ The setting of wiper on $R_{p}$ is such that, this value of $V_{p}$ is always less than the UJT firing point voltage $\eta V_{z}$ . When wiper setting is such that $V_{P}$ is small voltage $V_{z}$ charges C through R. When this ramp voltage $V_{C}$ reaches $\eta V_{z}$ , UJT fires and voltage $V_{T},$ through the pulse transformer, is transmitted to the gate circuits of both thyristors $T_{1}$ and $T_{2}$ .

During first positive half-cycle, SCR $T_{1}$ is forward biased and is therefore, turned-on. After this, $V_{c}$ reduces to $V_{P}$ and then to zero at $\omega t=\pi$. As $V_{c}$ is more than $V_{P}$ during the charging of capacitor C through charging resistor R , diode D is reverse-biased. Thus, $V_{P}$ does not effect in anyway the discharge of C through UJT emitter and primary of pulse transformer. From period 0 to $\pi,$ SCR $T_{1}$ is forward biased and is turned- on. From $\pi$ to $2 \pi$ ; $T_{2}$ is turned-on. In this manner, load is subjected to alternating $e_L$ as shown in fig.2

With the setting of wiper on $R_{P}$ pedestal voltage $V_{P}$ on C can be adjusted.

With low pedestal voltage across C ramp charging of C to $\eta V_{z}$ 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 $\eta$ $V_{z}$ 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 $V_{P}$ to $\eta V_{z}$ can be obtained from the relation,

$$\eta V_{z}=V_{P}+\left(V_{z}-V_{P}\right)\left(1-e^{-\mathrm{T} / R C}\right)$$

Note that $\left(V_{z}-V_{P}\right)$ is the effective voltage that charges C from $V_{P}$ to $\eta V_{z}$ .

From above, $$T=R C \ln \frac{V_{z}-V_{P}}{V_{z}(1-\eta)}$$

and the firing angles are given by

$$\alpha_{p}=\omega R . C \ln \frac{V_{z}-V_{p}}{V_{z}(1-\eta)}$$

and $$\quad \alpha_{R}=\omega \cdot R C \ln \frac{1}{1-\eta}$$

where $\omega$ is the input frequency and $\eta$ is the intrinsic stand off ratio of the UJT.