a querry about circuit

[zeerum]

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

 

[yjkwon57]

Simple. In the figure, all the current flows are to be reversed. According to the value of R, the current flowing through the transistor will be adjusted.

 

[Slayerza]

I agree, the current flow of current is logically counter productive. If the current passes through the zener to ground and also into the base of the transistor the circuit will make musch more sense.

The fact that the zener maintains a constant voltage means that as the load requires more current the current delivered via the resistor and the zener combination also increases. This in effect drives the transistor harder (maybe into saturation) and as such increases the flow of current to the load.

Cheers
Slayer

 

[zeerum]

agreed with you guys but what i wonder is why doesnt this cant be explained on basis of electron flow(which is the actual one) when it is better understood by conventional flow

all i wanted to know was the understanding of this circuit on the basis of electron flow direction of current

 

[mujee]

yeah me too would love to know how does this operates keeping electron flow in mind
actually its very disappointing to see that every where they say the actual direction is of electron flow but when it comes to explaining matters in circuits they opt for the conventional flow
its just so confusing and makes matter worse

 

[Kral]

zeerum,
In a simple Zener shunt regulator, there are 2 current paths; part of the input current flows thru the zener, and the rest flows thru the load. As the load current decreases to zero, all of the current flows thru the Zener. Suppose the maximum load current is 1 ampere. Then under no-load conditions, 1 ampere must flow thru the zener. If the Zener is a 10 volt device, then the Zener dissipation at no load will be 10W.
.
In the transistor circuit you have shown, let's assume that the load requirements are the same as above. Let's further assume that the Beta (Hfe) of the transistor is 100. All the shunt regulator consisting of R and the zener has to supply is the base current. Under maximum load conditions, the base current = 1/100 = .01 Amperes. The shunt regulator consisting of the Zener and R only have to supply the base current. The Zener can be a low powered device. The Zener dissipation at no load will only be 0.1W (.01A A 10V). You don't get something for nothing, however. In this circuit, the transistor will have to dissipate power. If the unregulated voltage is 12V, then the transistor dissipation at full load would be (12-10) X 1A = 2 W. The transistor dissipation at no load would be close to zero.
Regards,
Kral