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

**leonel** · 30th August 2005 08:48 AM

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

**checkmate** · 30th August 2005 09:20 AM

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.

**Sebi** · 30th August 2005 12:12 PM

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.

**leonel** · 30th August 2005 01:18 PM

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?

**Sebi** · 30th August 2005 07:34 PM

Analog Devices made a precise transmitter. The receiver is simple.

**leonel** · 31st August 2005 11:37 AM

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

**instruite** · 1st September 2005 12:43 AM

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

**leonel** · 1st September 2005 05:25 PM

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?

**checkmate** · 1st September 2005 05:36 PM

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

**instruite** · 1st September 2005 11:39 PM

leonel wrote:

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?

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.

**leonel** · 2nd September 2005 07:21 AM

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

**Thunderchild** · 4th September 2005 03:16 PM

Originally Posted by checkmate

Anyway, what you need is an opamp in non-inverting configuration with gain of 0.5.

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 !

**rl_east** · 18th October 2008 08:00 AM

Originally Posted by Sebi

Analog Devices made a precise transmitter. The receiver is simple.

thanks for your attachment files.

in your disign, there areresistor zero and resistor span.

please tell me about resistor zero and resistor span.

thank you before..

**ericgibbs** · 18th October 2008 08:14 AM

Originally Posted by rl_east

thanks for your attachment files.

in your disign, there areresistor zero and resistor span.

please tell me about resistor zero and resistor span.

thank you before..

hi,
The thread is years old.

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.?

**rl_east** · 18th October 2008 08:20 AM

how we can set resistor to be zero or span?

thank

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