# Voltage scaling

Discussion in 'Homework Help' started by jrekha, Apr 11, 2010.

1. ### jrekha Thread Starter New Member

Mar 31, 2010
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I have voltage measured in range 3.2V -4.2V.
I need to convert this to range 0-10V using opamp circuit.Can anyone help me realise this.

2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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So input voltage change from 3.2V to 4.2V or from - 4.2V to 3.2V ?

3. ### jrekha Thread Starter New Member

Mar 31, 2010
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input will be 3.2V to 4.2V

Actually my original circuit is like i ve 32V to 42V which need be scaled to 0-10V. I used a differential opamp circuit to reduce the voltage to 3.2V-4.2V range.Now i need to change to 0-10V.

I need to use opamp, and in the first stage i had to use differential opamp .so did like that. anythng wrong?or is ther an alternate way?

4. ### Jony130 AAC Fanatic!

Feb 17, 2009
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And what is the supply voltage

5. ### jrekha Thread Starter New Member

Mar 31, 2010
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15V is the supply

6. ### Jony130 AAC Fanatic!

Feb 17, 2009
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So you need this circuit:

Resistors R3 and R4 need to be very accurate.

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7. ### jrekha Thread Starter New Member

Mar 31, 2010
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can i use lm324?

8. ### Jony130 AAC Fanatic!

Feb 17, 2009
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LM324 is a quadruple version of a LM358.
So yes you can use it.

9. ### jrekha Thread Starter New Member

Mar 31, 2010
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is ther any steps to arrive at these Resistor values?i mean design, so that i cud know the circuit more.

10. ### jrekha Thread Starter New Member

Mar 31, 2010
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how do u come with those Resistor values, please tell me too..

11. ### Jony130 AAC Fanatic!

Feb 17, 2009
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The voltage gain mus be equal :
Au = (10V - 0V) / (4.2V - 3.2V) = 10V/1V = 10[V/V]
So gain of a non-inverting amplifier is equal:
Ku = 1 + (R1/R2) then R1/R2 = 9
R1 = 9 * 20K = 180KΩ.

And we wont for Vin = 3.2V Vout = 0V and this can only be done when non-invent input "+"V is equal inventing input voltage "-"V.
So we have this situation:

So voltage divider voltage must be larger then 3.2V by voltage drop on a R2.

Vd = 3.2V + I * R2

I = 3.2V /180KΩ = 17.77uA then

Vd = 3.2V + 17.77uA * 20KΩ = 3.2V + 0.355V = 3.555V

So we must design voltage divider that deliver 3.555V

Voltage on R3 = 15V - 3.555V = 14.445V
14.445V/ 3.555V = 3.2194 = R3/R4
R3 = 3.2194 * R4 = 470Ω * 3.2194 = 1.513K = 1.5K

I start with low value for a R4 because current that is flow through R3 and R4 must be much large then current that is flow through R1, R2.

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Last edited: Apr 12, 2010