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"A zener diode voltage regulator is inefficient when the supply is used with equipment that draws high current. When a supply must deliver a lot of current, a power transistor is used along with the Zener diode as shown below"

After reading this statement,I still not understand the benefit/purpose of transistor in this circuit. Please can you explain further this statement?


1 Answer 1


If the load requires high current and it is attached directly to the zener then resistor R must be very low because all the load current must pass through it. The result is the current through the zener will also be quite high, making it hot and/or requiring a high wattage zener (for extra money).

Adding the transistor separates the zener current from the load current. So resistor R can be high, zener power can be low, and the heat becomes the problem of the transistor, not the zener.

  • 1
    \$\begingroup\$ The key is that if the transistor is working in forward active region the output voltage is the zener voltage minus a Vbe, that's about 700mV. \$\endgroup\$ Jun 13, 2015 at 14:11
  • \$\begingroup\$ Can you also tell what is the capacitor doing here \$\endgroup\$
    – ema
    Jun 13, 2015 at 15:19
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    \$\begingroup\$ As with most power supply outputs, the capacitor is added for smoothing the output to the load. The transistor may not react quickly to changes in the load, so a capacitor is placed at the output to attempt to provide the extra current and hence smooth the output voltage. en.wikipedia.org/wiki/Capacitor#Power_conditioning \$\endgroup\$
    – Fuzz
    Jun 13, 2015 at 15:44
  • \$\begingroup\$ Don't forget that the zener voltage changes with current. \$\endgroup\$
    – PlasmaHH
    Jun 14, 2015 at 22:17

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