# Homework Help: Voltage difference

1. Feb 18, 2015

### gracy

1. The problem statement, all variables and given/known data:What will be potential difference across the sensitive instrument?

2. Relevant equations:

3. The attempt at a solution:I think the potential difference across the sensitive instrument would be 7.5v.

2. Feb 18, 2015

### Staff: Mentor

Is the diagram correct and complete? It shows that all the components are connected in parallel, yet states that the potential difference across the zener diode differs from that of the supply voltage. That's not how parallel components behave.

3. Feb 18, 2015

### gracy

That's what I was thinking.But take a look at this video

from time 1:22 to 1:39

4. Feb 18, 2015

### Staff: Mentor

They are taking certain liberties with circuit theory, omitting complicating details in order present a concept. In the context of way the material is presented your answer is fine.

5. Feb 18, 2015

### gracy

What are the needed corrections?

6. Feb 18, 2015

### Staff: Mentor

The major omission is a resistance between the source voltage and the zener that would host the potential drop between them.

7. Feb 18, 2015

### gracy

What does line voltage variation means?I have googled it but didn't find anything.

8. Feb 18, 2015

### Staff: Mentor

It's just what it says: variation in the line voltage. "Line voltage" typically refers to a primary voltage supply, such as as that of the power lines entering your house. It can also refer to the supply voltage presented to a circuit from some unspecified source (maybe a separate power supply).

9. Feb 18, 2015

### gracy

I don't understand one thing.
resistance between the source voltage and the zener causes potential difference between these two ,so why load resistance RL not causes potential drop?

The load is connected in parallel with the zener diode, so the voltage across RL is always the same as the zener voltage,how connecting resistance in series and parallel differs?

10. Feb 18, 2015

### gracy

In my textbook it is written
With no load connected to the circuit, the load current will be zero, ( IL = 0 ), and all the circuit current passes through the zener diode which in turn dissipates its maximum power. Also a small value of the series resistor RS will result in a greater diode current which in turn dissipates its maximum power.That's why we connect load and greater value of RS so that it's power is not dissipated.But Why we want to save zener power?
I think it is because greater current(greater than max current of zener will cause it to damage.Right?

11. Feb 18, 2015

### Staff: Mentor

It does. But that drop is the same as that across the zener (they are in parallel). It is the desired operating condition of the load.
The resistor Rs in your diagram is in series with the zener//load combination. Its voltage drop depends upon the current through it. The current through it is Is in the diagram, and is comprised of Iz + IL.

12. Feb 18, 2015

### Staff: Mentor

Yes, to protect the components you want to remain within their safe operating parameters. It is also good engineering practice not to waste power unnecessarily. Batteries are expensive.

13. Feb 18, 2015

### gracy

In parallel ,there is a point where current divides .Current divided with a specific pattern,where there is a larger resistance current will be less,where resistance is less current will be more so that voltage across them remains same.But in series ,there is no such point where current divides so whole current flows through ,so voltage depends on current through resistance and resistance itself.Right?
I tend to express my views about concepts and then say Right?because I think it is useful and wise to take corrections from experts on your concepts than growing up with wrong concepts.

14. Feb 18, 2015

### Staff: Mentor

Right. That is in essence Ohm's Law.

15. Feb 18, 2015

### gracy

Is my post 13 completely correct?

16. Feb 18, 2015

### Staff: Mentor

Yes it is correct.

17. Feb 18, 2015

### gracy

Can you please explain how zener maintain constant voltage?I have gone through many sites but no one explains this.

18. Feb 18, 2015

### Staff: Mentor

The video you linked to showed the I-V characteristic curve of the zener. It is the heart of the explanation. The voltage across the zener simply cannot go (much) beyond its reverse breakdown voltage. At and past the breakdown voltage very tiny increases in voltage lead to relatively massive increases in the current through the device. That increased current passes through the Rs of your previous diagram and causes a potential drop there. So the voltage across the zener remains steady and the "excess" voltage is dropped across Rs.

19. Feb 18, 2015

### gracy

is there any resistance of zener diode?

20. Feb 18, 2015

### gracy

The value of "excess" voltage which is dropped across Rs is same as that tiny increase in voltage,right?

21. Feb 18, 2015

### Staff: Mentor

Yes. When the diode is conducting in breakdown mode there is some resistance; the V-I curve for a real device is not perfectly vertical there as it would be for an ideal one. It is generally a fairly small value of a few Ohms to a few tens of Ohms. Values in the teens are typical.

22. Feb 18, 2015

### gracy

Is this voltage (voltage of primary source when massive increase in current takes place) obtained by the formula
high zener current (Iz)+IL(load current)*total resistance[/QUOTE]

Last edited: Feb 18, 2015
23. Feb 18, 2015

### lightgrav

best to not treat a zener like a resistor, because its IV curve is so non-linear.
Vsource - Vzener = (Izener + Iload) Rs

24. Feb 18, 2015

### Staff: Mentor

You should define what "total resistance" is in this case.

Note that the "massive increase in *zener* current" I refered to takes place if a voltage greater than the zener breakdown voltage is impressed across the zener. This behavior can be read right off the V-I characteristic curve.

The resistor Rs is in place to moderate such currents when the source voltage rises. Remember I said that Rs and the zener resistance form a voltage divider that greatly reduces the magnitude of the voltage change that appears across the zener compared to a change in the source voltage.

25. Feb 18, 2015

### Staff: Mentor

A useful equivalent model for purposes of design and analysis is the series connection of an ideal diode, resistance, and fixed voltage supply to set the zener voltage):