# 0-10V, 4-20mA conversion to 0-5V

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by leonel, Aug 30, 2005.

1. ### leonelNew Member

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Hi...
I'm building an electronic project that has to control 4 relays.
I've one input sensor that could be 0-5V, 0-10V or 4-20mA selected with jumpers. My input microcontroller only accept 5V. What i have to do to the others inputs to have accurate results? If i use a voltage divider to the input 0-10V i´ll have unprecision results due to error associated to resistances. In my input 4-20mA how i turn in an accurate signal 0-5V?
There are any IC that does that?
Regards

2. ### checkmateNew Member

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Resistors comes in precisions of 5%, 1% ir even lower. I don't think resistors are your problem, more like output impedance.
Anyway, what you need is an opamp in non-inverting configuration with gain of 0.5.

3. ### SebiNew Member

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If i understand right, You have 3 outputs from sensor. Why not connect the 0-5V output directly to microcontroller? I mean, the 0-5V is a basic output and the other two contains additional circuits (opamp for 0-5V to 0-10V and 0-10V to 4-20mA). Using the 4-20mA output only reasonable when the distance between devices too long.
So the most precise output is 0-5V.

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5. ### leonelNew Member

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inputs

Sebi, my problem it´s not choose what input shoul i use... My problem it´s that my circuit has to be prepared to the 3 inputs, that are choosen by jumpers.
I agree with the ideia to use the non-inverting amplifier with a 0.5 gain to the 0-10V input.
But to 4-20mA what configuration shoud i use?

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7. ### leonelNew Member

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What do you think it's best... A simple opamp with a 250 ohm resistor or the upper one?

8. ### instruiteNew Member

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Hi
first of all the 4-20mA should not be converter to 0-5V
using 4mA as base point is with a reason so it should be always converted to 1V-5V
depending on the accuracy requirement you can use a precision resistor of 250 ohm (actually no opamp is required just a current limiting series resistor for AD input will work) but if you can use an opam as buffer it will be good (just it will add some errors so accuracy will reduce a bit depending on what component you use

for 0-10V you can use a precision voltage divider

Just note that even if you use a precision OPAMP the best available its will still have offset voltage and bias current errors

9. ### leonelNew Member

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Hi...
I build one board with a 0-5V and a 0-10V. The 0-5V works very well with my micro, but i did a voltage divider with R=150ohm (1%) and i don't have my voltage output right. For example if my input is 5V, my output it´s supposed to be 2.5V, but i have 2.2V. It's impedance problems? If i put an opamp with a gain i will not have problems with my output impedance?

10. ### checkmateNew Member

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The problem is with input impedance rather than output impedance. You can use an opamp voltage follower, or a half-gain non-inverting opamp.

11. ### instruiteNew Member

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leonel wrote:
150 ohm is too small a value to use for voltage division
You should use a bigger resistor the maximum resistance value will be decided for current requirement for the analog input of the microcontroller.

12. ### leonelNew Member

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Initially i had a 510ohm resistor but the difference was bigger... i will try with opamps to see the result!

13. ### ThunderchildNew Member

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u better go check out your op-amp theory. a non inverting op amp has to have a minimum gain of 1 to obtain 0.5 gain and no inversion a ptential devider on the input must cut the signal in half !

14. ### rl_eastNew Member

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help me

in your disign, there areresistor zero and resistor span.

thank you before..

Last edited: Oct 18, 2008
15. ### ericgibbsWell-Known MemberMost Helpful Member

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hi,

The 'zero' is set to give '0v' on the output for 4mA loop current and the 'span' is set to give +5V output for 20mA loop current.

Is this what you are asking.?

16. ### rl_eastNew Member

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how we can set resistor to be zero or span?

thank

17. ### ericgibbsWell-Known MemberMost Helpful Member

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You have to have 4mA flowing thru the input resistors and adjust the zero to give a 0V on the output of the amplifiers.

You have to have 20mA flowing thru the input resistors and adjust the span to give a +5V on the output of the amplifiers.

EDIT: added a drawing with the settings in Red for 20mA and Blue for 4mA.

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18. ### sr052780New Member

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hai i am ravi,
i want to know the value of span resistor is it 100k or 100 ohms please reply for current to voltage converter circuit

19. ### ericgibbsWell-Known MemberMost Helpful Member

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The Span is 5K, the sense resistor is a 100R [Zero]

Is this what you mean.?

Last edited: Apr 4, 2009
20. ### ccurtisActive Member

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The gain of the circuit is set entirely by the 100Ω pot. The 5K pot sets the offset voltage. The voltage gain of the two amplifiers is fixed at 1 each for a total voltage gain of 1. In my view the 100Ω pot should be called GAIN (or SPAN) and the 5K pot should be called OFFSET (or ZERO).

I think the proper way to adjust the circuit is to apply 4 mA and adjust the "zero" and/or "span" pots as necessary to achieve a convenient 0V reference output. Then apply 20 mA and adjust the "zero" pot for a 5V output. That sets the proper gain. Last, apply 4 mA again and set the "span" pot for a 0V output. That sets the proper offset.

21. ### DiegoANew Member

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Hello! Sebi!
I'm working with the circuit you attached...but I have A problem.. Would you mind if I ask you something?.. I need 0-5 output.
My zero is 1.22V (turning pot 100 ohm to the minimun)..when I fix 5K pot to 5V. What can I do to get zero when de input is 4mA and 5 when 20mA?