One Bjt series pass linear regulator

[dkace]

The schematic is what the title describes. What I can't understand is why I have a voltage drop higher that 0.7 in the output and this is changing linear with the raise of the r load. I.E, for RLoad= 10 I have Vout = 6.144V ( Zenner voltage is 7.03V) and for RLoad= 2M I have RLoad= 6.536V.
Of course Iout is dropping accordingly.
Regulation means that I will have a steady output voltage. What is going on and I have this voltage drop in the BJTs' emmiter?

Thanks,
D.

 

[IanP]

I assume the resistor between 24Vdc and zener(base) ia 10kΩ.
Transistors such as 2N... have rather moderate β so to drive them you need some current to be fed to the base.
One solution will be to decrease the value of that resistor to, say, 1kΩ or less, the other option will be to add another small signal transistor to build the darlington configuration and drive the power transistor with a decent current ..

 

[dkace]

My question is why I have this output variation on the voltage, variation that is not following the Vz-0.7 . Why is that ?
D.

 

[Borber]

That is because Vbe varies with Ic. Look at the Ic/Vbe curve of transistor. Next reason is that with increasing Ic increases Ib on account of Iz. Vz is thus lower.

 

[dkace]

And how can you cancel this variation on this particular circuit? How can you compensate for the variations of the current?
D.

 

[Borber]

In this circuit you can do nothing to compensate thiv variations. You must use different circuit with negative feedback to improve regulation.

 

[obrien135]

Maybe this pdf file will help. There arre some circuit diagrams that could be simplified and probably work for you, depending on your load requirements. The darlington configuration in the feedback may not be necessary. You might be able to connect the voltage divider directly to the op amp - input, depending on the current gain and output current capability of the op amp. and the reference can be a zener diode. There is an equation that would help. I think it is as follows:

Vout = Vz(1 + Rf2/Rf1)

Please see both attachments. Hopefully this will help.

obrien135

 

[FvM]

Maybe this pdf file will help.

As the original poster is still active in the forum, you have the opportunity to see, that he has improved is knowledge over the last 6 years. I guess, he won't need an advice for this basically simple problem now.

 

[angel18]

I really want to know One BJT series pass linear regulator anyone can help me to explain it in detail, thanks.



**broken link removed**

 

[biff44]

If you really need this simple circuit to work better, then use a darlington transistor in place of the BJT. More gain = less voltage variation with load variation.