Circuit : Andy Collinson
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Description
In this circuit a zener diode is "amplified" by a set of power transistors. The transistors substantially increase the current and power to the load.
Notes
In this circuit D1 is the zener diode (whose power will be amplified). In this circuit a 47V zener of the BZX84 series of zener diodes was chosen. Q1 and Q2 form a Darlington pair and greatly increase current drive. The final stage is a set of six parallel connected power transistors, Q3 - Q8. The output configuration ensures that each power transistor deliver 1/6 of the total output load power.


Load Voltage

The input voltage of the circuit must exceed the output voltage by at least 10 Volts. In addition the nominal value of the zener voltage will be three base-emitter voltages lower at the output. This is because Q1, Q2 and Q3-Q8 all buffered the zeners output current. In addition to the voltage drop, an added advantage is that the effective value of C8 is also amplified by the current gain product of Q1, Q2 and Q3-Q8. The graph above is extreme and shows an output voltage of 42V delivering a little under 24 Amp to a resistive load.

Power Dissipation

The power dissipated in each power transistor is the product of Collector-Emitter Voltage and emitter current. For a 24 amp load this equates to about 56 Watts per transistor. A generous heat-sink is therefore required. In addition because of the triple Vbe drop in Q1, Q2, and Q3 the circuit is not as efficient as a dedicated voltage regulator IC, but is shown here as an example of delivering extreme power to a load.