[ANSWERED] Voltage sensor to Arduino, but with isolated grounds.

[gabrielcassio]

Hi everybody, I'm new to this forum, however I think it's possible to have my doubts answered.

I'm trying to measure the voltage of a battery (around 45V), however I wouldn't like to use a voltage divider with resistors, because the voltage for the analog input of the Arduino will have the same ground (GND) of my battery, and this is my concern. So my questions:

• Already has any IC to do this kind of measure, with isolation that you recommend?
• Or is there a way to use the voltage divider with resistores with an isolated IC?

Thank you so much.

[Andreas]

Hello,

isolation amplifiers are gone rare (and costly) these days.

The usual way nowadays would be to use a small micro-controller on the isolated side
and some isolation barrier (e.g. photo coupler) between the 2 parts.
Of course you have also to deliver some energy to the isolated side. (e.g. DC/DC converter).

You do not write about resolution and accuracy requirements.
If 10 bit resolution and a voltage divider on the isolated side would be ok
you could use a little PIC or ATMEL processor with its ADC on the isolated side and
as isolation a ADUM part from Analog devices.
(there are some parts who transfer also up to 50mW energy to the isolated side).

With best regards

Andreas

[Zero999]

Isolation amplifiers still exist. The last time I did this I used the ACPL-790A with a potential divider.
http://docs-europe.electrocomponents.com/webdocs/14ca/0900766b814cadf3.pdf

How are you powering the Arduino? If it's off the 45V battery, via a non-isolated voltage regulator, then you might as well go with the potential divider.

[Seekonk]

You have left out a lot of details, how accurate and hoe low do you want to measure.  If checking for general voltage, an opto is pretty good.  Throw in a zener and you have an expanded scale. Cheap and dirty.

[gabrielcassio]

Maybe some edit in my post for better understanding:
This is how I am measuring the values of the battery (45V), and the value of 12V is with other battery. My concern is about having two grounds connected. Whit this way, using the 10 bit resolution of ADC of the Atmega2560, I got decent result.
So this was the question, how to separate them, for a little of security.

[Circlotron]

I would suggest joining all your grounds together at one point, including the bottom of R2. Then run a single ground wire to the Arduino. This stops any high current running through your board.

[Zero999]

Maybe some edit in my post for better understanding:
This is how I am measuring the values of the battery (45V), and the value of 12V is with other battery. My concern is about having two grounds connected. Whit this way, using the 10 bit resolution of ADC of the Atmega2560, I got decent result.
So this was the question, how to separate them, for a little of security.

Is the 12V battery's 0V connected the 45V battery's 0V? If the answer is no, then configure it as per your previous schematic and it'll be fine. As long as the only part where 12V and 45V circuit's earths meet is the bottom of R2, then there won't be any problems.

Trouble starts when the 0V of the 12V circuit is connected to the 45V circuit somewhere else, resulting in a potential difference between the Arduino 0V and 45V circuit's 0V; if the two were connected a large current could flow. On the other hand, if the only connection between the two circuits is the Arduino's GND pin, then no current can flow between the two circuits and there will be no problems.  This is similar to how the battery powering a multimeter is fine measuring circuits connected to the mains because the only part where the circuits are joined is the common pin on the meter.

[Seekonk]

First off, that 10 bits is a joke. Use an opto isolator and a TL431 to create an offset voltage.  That will expand the voltages in question. The current transfer will be linear enough for what you want to do.