# a circuit querry

#### zeerum

Joined Nov 15, 2006
4
hi guys
can anyone explain how does this circuit solve the solution of load having high current requirements
the document says that in absence of transistor , the zener diode regulates a constant voltage across itself by allowing maximum current through the series resistor
and when we have a load with greater current requirements, we use this configuration of a transistor to solve that problem

what i dont get is how in this circuit transistor solves the problem. please answer according to the flow of current. dont give mathematical equations for solution.
i want to know the flow of current that occurs in it and how it increases with placement of a transistor from the one in its absence through the resistor

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#### nomurphy

Joined Aug 8, 2005
567
The emitter of the xstr will be one Vbe drop below the zener value. So, if the zener is 5V, the emitter will be at ~4.3V. If properly designed, one would use a xstr with an Ice and power dissipation proportional to the circuit requirements. This means the xstr is routing most of the current thru it's CE path rather than thru a small, say 1/2W, zener and it's ballast.