# Zener Diode

Discussion in 'Homework Help' started by celect, Jun 8, 2005.

1. ### celect Thread Starter Member

May 31, 2005
18
0
I was wondering how this zener got it voltage, I attached the problem I working on, I assume this to be a voltage regulator circuit.

If this zener is regulating 15V is my output to the load going to be 15V.

How does this work, why are the transistors in the circuit if this zener will work
with out them.

2. ### Brandon Senior Member

Dec 14, 2004
306
0
Zener's are used a bit differently than a typical diode. They are normally used in reverse, like the one in the circuit is designed, but this zener is not regulating the output at all.

This zener is indirectly regulating the top xtr which is the actual voltage source through its effect on the bottom xtr.

In typical AC/DC converters the zener is used to regulate along with a cap.

3. ### David Bridgen Senior Member

Feb 10, 2005
278
0
The zener is in series with the base-emitter of Q2. The voltage at the junction of the 2k5 and 1k5 resistors will therefore be 15 + 0.7V, so the current through the 2k5 is 6.28mA.
This current also flows through the 1k5. Also flowing through the 1k5 will be the base-emitter current (negligible) and the current through the resistor between the base and emitter of Q2. Since its value is not stated all we can say is that the output voltage will be the sum of the voltage at the junction of the 2k5 and 1k5 resistors (15.7) and at least 9.42 (given by at least 6.28mA x 1k5.) i.e. output voltage is at least 25.12.

4. ### davidand Active Member

Jun 2, 2005
43
0
Thanks for explaining this to me. So if R4 shorts or is removed and they since no vaule was given for R4 how would I determine the voltage output?

5. ### mozikluv AAC Fanatic!

Jan 22, 2004
1,437
1
hi

if you remove R4 then Q1 will not conduct since that resistor is you base bias of Q1. likewise with R3 to Q2

6. ### David Bridgen Senior Member

Feb 10, 2005
278
0
The labelling of the resistors is unreadable, so I don't know which one you mean.

7. ### celect Thread Starter Member

May 31, 2005
18
0

I re-labeled the drawing, in this voltage regulating circuit how would I determine the Voltage out if R4 is open or shorted do I need to know the vaule of R4?
or is this resistor a standard for circuits

8. ### David Bridgen Senior Member

Feb 10, 2005
278
0
R4 provides the base current for Q1, the series pass transistor (the actual regulator.)
If R4 was replaced by a short circuit, Q1 would be permanently "on", like a switch, and the output voltage would be full and unregulated output of the basic supply.
If R4 were removed, the Q1 would effectively be open circuit and there would be no output.
The value of R4 has no direct influence on the output voltage and you don't need to know it.

The resistor to which I referred, without a value, is the one between base and emitter of Q2, apparently R3.
Q2 will have about 0.6V across its base-emitter junction. The current through R3, due to the 0.6V, will also flow through the 1k5 and therefore have an influence on the voltage across the 1k5.

The information you need to state the output voltage of the circuit with any accuracy is incomplete.
The best you can do with the information which is given is to say that it is about 25.
My guess is that it is a poor attempt at a 24V supply.

9. ### celect Thread Starter Member

May 31, 2005
18
0
if xtr on the bottom Q2 conducts more, what would happen to xtr on top Q1 would it conduct more also?

Oct 11, 2004
628
3
What is xtr?

11. ### celect Thread Starter Member

May 31, 2005
18
0
I assumed it to mean transitor

12. ### pebe AAC Fanatic!

Oct 11, 2004
628
3
I've never heard of it before. Why not just refer to Q1 or Q2?

13. ### celect Thread Starter Member

May 31, 2005
18
0
I will for now on, somebody replied back to me and I thought that was how they were refering to the transistors.
thanks

14. ### David Bridgen Senior Member

Feb 10, 2005
278
0
That's the danger of using stupid jargon and silly buzzwords.

I've said it on other forums and I'll no doubt say it again:

If I use jargon or buzzwords and you don't understand, it will be my fault.

If I use plain English and you don't understand, it will be yours.

15. ### David Bridgen Senior Member

Feb 10, 2005
278
0
Let's take it one step at a time.
When and/or why will Q2 conduct more?

16. ### davidand Active Member

Jun 2, 2005
43
0
I think if the zener were to short then Q2 would conduct more,

17. ### David Bridgen Senior Member

Feb 10, 2005
278
0
I thought it might be more beneficial if you were thinking of normal operation.
Do you understand how the circuit works?

18. ### davidand Active Member

Jun 2, 2005
43
0

No I'm sorry celect
I was just going over some of the posting.
and put my foot in my muth again.