Circuit : Andy Collinson
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.
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
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.
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.