Potential Difference Across Sensitive Instrument: 7.5V

In summary, the potential difference across the sensitive instrument in this circuit would be 7.5v. However, the diagram shown in the conversation is incorrect and incomplete as it does not account for a resistance between the source voltage and the zener diode. This causes a potential drop between them, which is why the load resistance does not cause a potential drop. The purpose of using a greater value of resistance in series is to protect the zener diode from damage and to avoid wasting power. The zener diode maintains a constant voltage by utilizing its reverse breakdown characteristic, which causes a small increase in voltage to result in a relatively large increase in current. This excess voltage is then dropped across the resistance in the circuit.
  • #1
gracy
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Homework Statement

:What will be potential difference across the sensitive instrument?[/B]

Homework Equations

:
upload_2015-2-18_20-40-12.png
[/B]

The Attempt at a Solution

:I think the potential difference across the sensitive instrument would be 7.5v.[/B]
 
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  • #2
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
gneill said:
. That's not how parallel components behave.
That's what I was thinking.But take a look at this video

from time 1:22 to 1:39
 
  • #4
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
gneill said:
They are taking certain liberties with circuit theory, omitting complicating details in order present a concept
What are the needed corrections?
 
  • #6
gracy said:
What are the needed corrections?
The major omission is a resistance between the source voltage and the zener that would host the potential drop between them.
 
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  • #7
What does line voltage variation means?I have googled it but didn't find anything.
 
  • #8
gracy said:
What does line voltage variation means?I have googled it but didn't find anything.
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).
 
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  • #9
I don't understand one thing.
gneill said:
resistance between the source voltage and the zener that would host the potential drop between them.
resistance between the source voltage and the zener causes potential difference between these two ,so why load resistance RL not causes potential drop?
diode24.gif

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
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
gracy said:
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?
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.
diode24.gif

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?
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.
 
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  • #12
gracy said:
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?
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. :smile:
 
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  • #13
gneill said:
The resistor Rs in your diagram is in series with the zener//load combination. Its voltage drop depends upon the current through it.
gneill said:
But that drop is the same as that across the zener (they are in parallel).
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
gracy said:
But in series... ...so voltage depends on current through resistance and resistance itself.Right?
Right. That is in essence Ohm's Law.
 
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  • #15
Is my post 13 completely correct?
 
  • #16
gracy said:
Is my post 13 completely correct?
Yes it is correct.
 
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  • #17
Can you please explain how zener maintain constant voltage?I have gone through many sites but no one explains this.
 
  • #18
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.
 
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  • #19
is there any resistance of zener diode?
 
  • #20
gneill said:
the "excess" voltage is dropped across Rs.
gneill said:
At and past the breakdown voltage very tiny increases in voltage lead to relatively massive increases in the current through the device
The value of "excess" voltage which is dropped across Rs is same as that tiny increase in voltage,right?
 
  • #21
gracy said:
is there any resistance of zener diode?
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.
gracy said:
The value of "excess" voltage which is dropped across Rs is same as that tiny increase in voltage,right?
It is the difference between the supply voltage and the voltage across the zener. If the current through the zener changes (say due to a change in the supply voltage), then the bulk of the change will be dropped across Rs. Rs and the zener's small resistance form a voltage divider, and Rs being the larger resistance expresses the bulk of the change.

Small changes in the voltage across the zener due to the zener's own resistance can't be avoided, but can be minimized by choosing a modest operating current for the zener.
 
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  • #22
gneill said:
. At and past the breakdown voltage very tiny increases in voltage lead to relatively massive increases in the current through the device.
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]
 
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  • #23
best to not treat a zener like a resistor, because its IV curve is so non-linear.
Vsource - Vzener = (Izener + Iload) Rs
 
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  • #24
gracy said:
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
You should define what "total resistance" is in this case.

Note that the "massive increase in *zener* current" I referred 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.
 
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  • #25
lightgrav said:
best to not treat a zener like a resistor, because its IV curve is so non-linear.
Vsource - Vzener = (Izener + Iload) Rs
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):

Fig1.gif
 
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  • #26
lightgrav said:
Vsource - Vzener =
Vsource - Vzener it can be small as well as very large
when it is small how this equation holds?
Vsource - Vzener= (Izener + Iload) Rs[/QUOTE]
 
  • #27
Does something make you believe it wouldn't hold?

The formula will hold so long as Izener is sufficient to hold the zener past its breakdown "knee". Refer to the discussion in your video about the minimum zener current.
 
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  • #28
When Vsource = Vzener or Vzener> Vsource
Vsource= Iload* (Rs+Rzener diode)
Right?
 
  • #29
gracy said:
When Vsource = Vzener or Vzener> Vsource
Vsource= Iload* (Rs+Rzener diode)
Right?
Not quite. If Iload*Rs drops the voltage below the zener voltage then the zener is cut off (stops conducting) and effectively disappears from the circuit. The load voltage becomes unregulated at that point.

Vsource being equal to Vzener is not practical as there will be some voltage drop across Rs due to the load drawing current through it, even if the zener is cut off. Again the zener would be ineffective as a regulator.

A correct operating point for the zener is to maintain a current through it that exceeds its "knee" current by a comfortable margin when the source voltage is at its lowest anticipated value and the load drawing its maximum current. This is why you'll often see source voltages specified as a nominal value with a +/- percentage associated with it. It tells you the minimum and maximum values to expect during operation.
 
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  • #30
gneill said:
If Iload*Rs drops the voltage below the zener voltage then the zener is cut off (stops conducting) and effectively disappears from the circuit.
So,If Iload*Rs drops the voltage below the zener voltage Vsource= Iload*( Rs+RL).Right?
 
Last edited:
  • #31
Yes.
 
  • #32
gneill said:
If Iload*Rs drops the voltage below the zener voltage then the zener is cut off (stops conducting)
Why?When Iload*Rs drops the voltage below the zener voltage ,does zener offer a large amount of resistance so that at current dividing point no current goes in zener?Right?
 
  • #33
Yes
 
  • #34
One more question
I had asked you a question
gracy said:
when it is small how this equation holds?
Vsource - Vzener= (Izener + Iload) Rs
gneill said:
Refer to the discussion in your video about the minimum zener current.
So,when Vsource - Vzener is small Iz is minimum.If we maintain This v source so that this small Vsource - Vzener remain constant,will there be constant minimum Iz?
 
  • #35
Yes, it looks as if you're now the forum zener expert.
 
<h2>1. What is potential difference?</h2><p>Potential difference, also known as voltage, is the difference in electric potential energy between two points in an electric circuit. It is measured in volts (V).</p><h2>2. What is a sensitive instrument?</h2><p>A sensitive instrument is a device that is designed to measure small changes or variations in a physical quantity. In this case, it refers to an instrument that can accurately measure potential difference across a circuit.</p><h2>3. Why is the potential difference across a sensitive instrument important?</h2><p>The potential difference across a sensitive instrument is important because it allows us to measure and monitor the flow of electricity in a circuit. This information is crucial in understanding how the circuit is functioning and can help us troubleshoot any issues that may arise.</p><h2>4. How is the potential difference across a sensitive instrument measured?</h2><p>The potential difference across a sensitive instrument is measured using a voltmeter, which is a type of electrical measuring instrument. The voltmeter is connected in parallel to the circuit and displays the potential difference in volts.</p><h2>5. What does a potential difference of 7.5V mean for a sensitive instrument?</h2><p>A potential difference of 7.5V across a sensitive instrument means that there is a difference of 7.5 volts in electric potential energy between the two points being measured. This could indicate the presence of a power source or the flow of electricity through the circuit.</p>

1. What is potential difference?

Potential difference, also known as voltage, is the difference in electric potential energy between two points in an electric circuit. It is measured in volts (V).

2. What is a sensitive instrument?

A sensitive instrument is a device that is designed to measure small changes or variations in a physical quantity. In this case, it refers to an instrument that can accurately measure potential difference across a circuit.

3. Why is the potential difference across a sensitive instrument important?

The potential difference across a sensitive instrument is important because it allows us to measure and monitor the flow of electricity in a circuit. This information is crucial in understanding how the circuit is functioning and can help us troubleshoot any issues that may arise.

4. How is the potential difference across a sensitive instrument measured?

The potential difference across a sensitive instrument is measured using a voltmeter, which is a type of electrical measuring instrument. The voltmeter is connected in parallel to the circuit and displays the potential difference in volts.

5. What does a potential difference of 7.5V mean for a sensitive instrument?

A potential difference of 7.5V across a sensitive instrument means that there is a difference of 7.5 volts in electric potential energy between the two points being measured. This could indicate the presence of a power source or the flow of electricity through the circuit.

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