Preregulator for power supply

Posted on Nov 21, 2012

This SCR pre-regulator keeps the filter capacitor Vc, in a variable output power supply, a few volts above the output voltage V0. The benefits include: less heat dissipated by the pass transistor and therefore small heatsink, cooler operation and higher efficiency, especially at low output voltages. Ql, Rl, R2, Dl and D2 form a constant current source for zener Zl, so that the contribution to the output current is always a few mA (2-3 mA). The Darlington pair Q2, Q3 keeps the SCR off.

Preregulator for power supply
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The voltage Vc decreases until Vc = V0 = V at which point the Darlington pair fires the SCR, charging the filter capacitor to a higher voltage VC1 in less than half the period of the input voltage. The component values, shown are for a 0 - 250-V, 3-A power supply.

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Nolan   Apr 9, 2018

Schematic has Failure: Connect D2 only to Vo not Vc, connect R2 to Vr not SCR, connect Cathode SCR direct to Vc

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