I would like ask what is the basic function of the compoent in the dashed box as shown in the diagram, and I have attached a file.
Thanks for all of you. Please help.

 

I have moved this post into the homework section as it is apparently a class assignment.

Before we provide you with answer, you would do well to take a stab at the answer yourself. Afterwards the members will be more than happy to help correct any weakness you may have in your explanation.

hgmjr

 

Hi hgmjr,

I notice the post is closed ....was connie happy with the outcome or did they no longer need help?

 

Since they were the same, one was close and one stayed open, both being moved to the homework area. It has to do with the forum rules.

 

Thanks Bill.

 

can any one help?

 

Hi hgmjr,

I notice the post is closed ....was connie happy with the outcome or did they no longer need help?

you cannot see the post?

 

Did you have any thoughts so far?

 

Greetings tnk,

As Bill has indicated, the moderators purge the forum of duplicate posts as rapidly as possible to prevent clutter and confusion. That is why the other post was locked.

hgmjr

 

Cheers hgmjr!

connie just dropped by with another request for assistance ...

Things move fast at AAC!

 

Did you have any thoughts so far?

of course!

 

of course!

Go ahead and post your write up for the members to review and comment upon.

hgmjr

 

There are 5 components inside the box. Some have obvious functions - it's the overall function that makes the regulator work.

If you can describe how each one works, you may discover how they all work together.

 

It is such a simple circuit that the function of the parts is obvious.
The zener diode is drawn backwards.

 

That is an interesting analysis - how would the zener function in the circuit with the cathode and anode swapped?

 

I think the zener is probably drawn correctly for this circuit design. I believe it's purpose is for temperature stability to compensate for the base-emitter voltage change of -2.5mV per deg C in the feedback control transistor.

 

This looks rather like an old RCA design, except I agree the zener appears to be the wrong way round.

Also forming a darlington from two 2N3055 transistors is not a good idea.

http://forum.allaboutcircuits.com/showthread.php?t=13594&highlight=current+limit

Post #17

 

Greetings connie,

I would first apologize for the comments of those members who have belittled your request for assistance. It is easy for those of us who eat, sleep, and drink this stuff to state that the operation of the circuit in question is obvious. However, it is not so obvious to those who are encountering such a circuit for the first time.

I would recommend as have others that you take each of the elements contained in the dashed area and recall what you have learned about that element and consider how it might contribute to the overall behavior of the circuit.

The circuit elements to which I refer are:

1. The potentiometer
2. The zener diode
3. The NPN transistor

One important hint is that the circuit in the dashed block takes a portion of the output voltage and feeds it back to the remaining portion of the voltage regulating circuit in order to enhance the performance of the regulator. Without this feedback, the output voltage would be prone to change as the current drawn by the load changed.

I too believe that the zener diode was inadvertantly drawn backward.

hgmjr

 

Hello hgmjr, connie & other interested folk,

Apologies from me also to connie if I appear to have implied the understanding of the circuit is in any way simple or obvious. I hope my sole intention was to challenge connie to make an attempt at stating what they thought about how they would "solve the problem" by explaining the operation of the sub-circuit.

I must admit when I first laid eyes on the diagram, I thought the indicated zener orientation was the wrong way around and that in its current form, the circuit would not function as intended.

In fact, I'm of the opinion that the analysis and understanding of the circuit operation is quite subtle and ingenious - and worthy of some consideration.

If one takes the time to draw the circuit and do a careful analysis, making some reasonable assumptions - the entire circuit and not just the feeedback part - it becomes much clearer that it does work and works quite nicely.

If you "build" a simulation of this circuit it also provides several valuable insights.

In my opinion this is not a trivial question that connie was asked.

Cheers.

 

Apologies to you tnk. My comments were not directed at you. Quite the contrary. I believe your measured comments were enlightening to the OP without revealing more than a hint at a path to a solution.

Well Done,
hgmjr