# voltage scaling

Discussion in 'Homework Help' started by Hardik Vamja 1, May 12, 2016.

1. ### Hardik Vamja 1 Thread Starter New Member

May 11, 2016
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i'm making college project and my project is about scaling of voltage, i want to scal voltage range from "1.65v-2.75v into 1v-5v" with accuracy, so it can be easyly use to convert into 4-20ma current and use by PLC, I know that it can possible with opamp but i dont know how its work (i mean equations and all)

2. ### blocco a spirale AAC Fanatic!

Jun 18, 2008
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1. Start by calculating the required gain; this is just output voltage range / input voltage range.
2. Design a non-inverting amplifier to achieve the calculated gain.
3. Calculate the offset voltage required at the - terminal of the op-amp so that an input of 1.65V results in an output of 1V.
4. Design a simple resistive voltage divider to provide the calculated offset voltage. (You will need to buffer the output with another op-amp to provide the low impedance reference voltage)

Last edited: May 12, 2016
Hardik Vamja 1 likes this.
3. ### Hardik Vamja 1 Thread Starter New Member

May 11, 2016
9
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i already do what you said & its working but the output voltage are slightly deferr, it gives 4.5 v instead of 5v at max input
dont know why
any suggestions?

Jun 18, 2008
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357

5. ### dannyf Well-Known Member

Sep 13, 2015
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I like your definition of "working".

Why is that this forum has a lot of posts like that?

Hardik Vamja 1 likes this.
6. ### Hardik Vamja 1 Thread Starter New Member

May 11, 2016
9
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i don't have software simulator circuit but i have diagram of circuit same as main
here it
and one more thing now i made some changes in circuit due to heating problem so input valtage range is become 1.9v to 3.5v
i make calculation for that circuit for 0-5v instead of 1-5v Because it easy and dosent need Vref voltage

if you have solution for 1.98v - 3.53 into 1-5v than it'll be great

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7. ### DGElder Member

Apr 3, 2016
313
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What is the output impedance of your signal source and did you take into consideration the loading by R5?

Also is a linear amp what you need or are you translating between logic levels, in which case you would need a comparator or switching circuit of some sort instead.

Last edited: May 12, 2016
8. ### Hardik Vamja 1 Thread Starter New Member

May 11, 2016
9
0
i
i don't understand!!

9. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Hardik Vamja 1 likes this.
10. ### Hardik Vamja 1 Thread Starter New Member

May 11, 2016
9
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i used your solution see my attechment above
but there is some points
1) is there capacitor are assessor?
2) what does R5 do
3) practically how resistors should be accurate ( it's hard to find out accurate value)
4) dose it works for offset (1v-5v not 0v-5v)

11. ### Jony130 AAC Fanatic!

Feb 17, 2009
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1) Yes, decoupling capacitors are needed also if the signal is noisy or signal wires are long, low pass filter at the input would help.
2) pull down resistor
3) This depends on the application requirements.
4) Yes, but supply voltage must be stable. And your supply voltage is ? And the op amp is ?

12. ### Hardik Vamja 1 Thread Starter New Member

May 11, 2016
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supply is 12v & opamp is 741

13. ### WBahn Moderator

Mar 31, 2012
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What does "with accuracy" mean?

14. ### Hardik Vamja 1 Thread Starter New Member

May 11, 2016
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i'm getting those voltage from rtd ( through amplifier) and and i want to convert it into 4-20ma signal

15. ### WBahn Moderator

Mar 31, 2012
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That still tells me nothing about what you by the phrase "with accuracy".

If I asked you to design an amplifier to scale something with accuracy, would YOU know what degree of accuracy I was asking for? If not, then how do you expect US to know what YOU mean?

16. ### Jony130 AAC Fanatic!

Feb 17, 2009
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With E24 resistors try R1 = (150kΩ + 10kΩ) ; R2 = 100kΩ; R3 = 6.8kΩ ; R1 = 1.8kΩ
And this old 741 is completely not suitable for this application.

17. ### Hardik Vamja 1 Thread Starter New Member

May 11, 2016
9
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+-10mv that is okay?

18. ### Hardik Vamja 1 Thread Starter New Member

May 11, 2016
9
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ok got it
1) this resistor value is for 1.98v to 3.53 input right?
2) what should i use instead of 741

19. ### WBahn Moderator

Mar 31, 2012
17,461
4,701
That is at least a spec. Whether it is okay is for you to decide. Keep in mind that if a 100 mV tolerance would have been fine that you are paying for performance you don't need, while if it really needs 1 mV tolerance then you are paying for something that is not good enough.

By asking for 10 mV tolerance at 5 V you are asking for an accuracy of 0.2%.

Are you prepared to pay for the kind of precision components that are needed to achieve that?

That kind of performance is going to be very susceptible to signal source impedance and load impedance. It's also going to be very susceptible to variations in things like temperature and power supply voltage. Are you prepared to deal with those things? Or do you want to perhaps spend a bit of time figuring out what kind of accuracy you NEED instead of what might WANT?

20. ### HW-nut Member

May 12, 2016
43
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Ignoring if the gain/ offset calculations are correct, the design has no power supply rejection.