# voltage scaling

#### hardik vamja

Joined Oct 15, 2015
13
Hi I need to convert 3.7 to 4.7 volt into 1 to 5 volt
I know it can be done with opamp but I don't have any idea what can I do in this
Plz any one can help me..!!!

#### crutschow

Joined Mar 14, 2008
24,758
What supply voltages do you have?
How accurate does the shift need to be?

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#### hardik vamja

Joined Oct 15, 2015
13
What supply voltages do you have?
How accurate does the shift need to be?
Actually I'm converting RTD resistance into 4to20 ma current and output of bridged & instrumentation amplifier is 3.7 to 4.7 volts so I need 1 to 5 that can easily converted into 4-20ma
My power supply will be 30v(+15 -15) to opamp

#### MikeML

Joined Oct 2, 2009
5,444
So what is the RTD min and max resistance over your range of interest? There may be a way of doing it more directly.

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#### hardik vamja

Joined Oct 15, 2015
13
So what is the RTD min and max resistance over your range of interest? There may be a way of doing it more directly.

View attachment 92989
for RTD pt100
at o c' -> 100 ohm
and at 100 c' -> 138 ohm
plz help if can

#### hardik vamja

Joined Oct 15, 2015
13
@MikeML hi id try to simulate your ckt but dont getting exact results
which one is input v1(you show 4.6v) or v2

#### hardik vamja

Joined Oct 15, 2015
13
hay hay hay thanxxx its working
thanxx a lot

#### MikeML

Joined Oct 2, 2009
5,444
@MikeML hi id try to simulate your ckt but dont getting exact results
which one is input v1(you show 4.6v) or v2
V1 is a fixed offset of 4.600V. Note that this must be very precise. A 1mV error causes a 3mV error at the output. Watch it if it is a trim pot off +15V.
V2 is the input that varies from 3.7 to 4.7V

#### #12

Joined Nov 30, 2010
18,210
this must be very precise.
May I suggest using fixed resistors to make a trim pot work in a small range of voltages. That's one way to get a more accurate voltage setting.

For instance, if you're trying to get 4.6V out of a 5V supply, put a 4.7 k resistor under a 1 k pot. That way the pot works from 4.1 volt to 5 volts.

#### MikeML

Joined Oct 2, 2009
5,444
May I suggest using fixed resistors to make a trim pot work in a small range of voltages. That's one way to get a more accurate voltage setting.

For instance, if you're trying to get 4.6V out of a 5V supply, put a 4.7 k resistor under a 1 k pot. That way the pot works from 4.1 volt to 5 volts.
Yeah, but if the 5V supply drifts from 5.000 to 5.050 (1% change), the output of the scaling opamp shifts 3%, i.e., it is ratiometric. In the OP's case, he is likely trying to get 4.600V from his +15V supply. How stable is that?

In any case, I would use a voltage divider, followed by an opamp voltage-follower to drive the left end of R1 (see post #4). Or use a real voltage reference like (wait for it) TL431 to make the 4.600V.

#### MikeML

Joined Oct 2, 2009
5,444
As promised, here is the basis of producing your voltage directly using only one opamp. You would have to tweak the gain and offset a bit, but you could do it

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#### hardik vamja

Joined Oct 15, 2015
13
@MikeML thanks for sharing your experiences.
Do you have circuits for making 4.600v power supply
Like you say to use TL431

#### hardik vamja

Joined Oct 15, 2015
13
Thanks @MikeML I'll try it tomorrow morning..

#### MikeML

Joined Oct 2, 2009
5,444
Try this for a relatively stable adjustable voltage reference.

If you keep the instrumentation amplifier, you will need a stable voltage reference for it, too...

#### #12

Joined Nov 30, 2010
18,210
Oh look! R4 is a trim pot in series with a fixed resistor. I wish I had thought of that!

#### hardik vamja

Joined Oct 15, 2015
13

#### hardik vamja

Joined Oct 15, 2015
13
H
So what is the RTD min and max resistance over your range of interest? There may be a way of doing it more directly.

View attachment 92989
Hi now my inputs is changed to 3.2 to 4.8 into 1-5volts so if I use your calculations than I'm getting some variation about 1 volt can you fix it?

#### hardik vamja

Joined Oct 15, 2015
13

#### MikeML

Joined Oct 2, 2009
5,444
@MikeML pllzz help me..
I am away from the computer where I did the sim before. I'll be over there tomorrow and redo it with you new specs.