The circuit, shown below, uses a UJT to trigger a SCR. The UJT is used to more accurately trigger the SCR. When the source voltage exceeds 20V, the zener diode (DZ) will begin to conduct, applying a DC voltage across the base connections of the UJT. At the same time, diode D1 will be forward biased, and the capacitor will quickly charge through R1 and R2. This represents the left-hand pedestal portion of the of the emitter voltage. Once the capacitor charges to the voltage across R3, D1 will become reverse biased and the capacitor will continue to slowly charge through R4. This represents the ramp portion of the emitter voltage. The capacitor continues to charge until the UJT fires. At this point the capacitor will quickly discharge through R6, and this represents the right-hand pedestal of the emitter voltage. The capacitor discharge is sufficient to trigger the SCR.
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| Figure: RAMP AND PEDESTAL UJT-SCR CONTROL CIRCUIT |
The point at which the UJT fires can be adjusted by varying the pot R3. With a large setting on R3, the capacitor must charge to a larger value before D2 becomes reverse biased. This causes the UJT to fire faster, resulting in more of the source voltage appearing across the SCR. This can be seen graphically as:
