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### Topic: Measuring voltage over a resistor (Read 2505 times)previous topic - next topic

#### soulid

##### Jan 29, 2013, 02:53 pm
Hi,
I would like to measure a voltage drop across a resistor. The voltage over that resistor is 0,7V to 1.4V although the voltage at the resistor can vary between 10 to 14V (that's why the voltage drop is so interesting).

How can I do that?

#### retrolefty

#1
##### Jan 29, 2013, 03:00 pm

Hi,
I would like to measure a voltage drop across a resistor. The voltage over that resistor is 0,7V to 1.4V although the voltage at the resistor can vary between 10 to 14V (that's why the voltage drop is so interesting).

How can I do that?

Well it's pretty easy if you have a digital multimeter, you just place the volt meters leads across the two resistor leads and measure it's voltage drop directly. Or you could measure each end with respect to ground and subtract the difference, both would be the voltage drop across the resistor.

Now if you asking how you could have an arduino measure it when that is a different animal in that the voltage to be measured at either end of the resistor must always be less then +5vdc as that is the electrical limit the AI pin can be subjected to.

So what are you trying to accomplish or learn?
Lefty

#### soulid

#2
##### Jan 29, 2013, 03:55 pm
I'd like to measure that with the arduino.

#### retrolefty

#3
##### Jan 29, 2013, 04:02 pm

I'd like to measure that with the arduino.

That's a little more difficult, as I said the arduino will be damaged if subjected to greater then +5vdc. Maybe with voltage divider resistors wired to both side of the resistor to be measured would work but kind of cluggy. Would have to see the wiring you are presently using for the resistor in question.

Lefty

#### larryd

#4
##### Jan 29, 2013, 06:03 pm
Please attach a drawing or schematic of your project.
No technical PMs.
The last thing you did is where you should start looking.

#5
Here you are

#### liuzengqiang

#6
##### Jan 29, 2013, 07:05 pm
If you power arduino with a battery, you can do that measurement. If you use USB, your arduino gnd is tied to the common ground, you will damage arduino. Not sure about ac adapters. They seem to be floating or are they? How large are these resistors you drew. If they are not very large, you can build another voltage divider to get the voltage lowered. But the resolution of arduino will suffer if you lower the voltage to 0-5V since your variation will be scaled down from 0,7-1,4 to about 1/3 of that. Can you swap the locations of the two resistors? That'll be truly useful.
Serial LCD keypad panel,phi_prompt user interface library,SDI-12 USB Adapter

#### retrolefty

#7
##### Jan 29, 2013, 07:07 pm
Quote
If you power arduino with a battery, you can do that measurement.

How? The arduino board must still share a common ground with the voltage source driving the constant current driver so I don't see how that solves the 'too high a voltage for an arduino AI pin, no matter how you power the arduino board?

Lefty

#### larryd

#8
##### Jan 29, 2013, 07:13 pm
See ideas for layout:
No technical PMs.
The last thing you did is where you should start looking.

#### liuzengqiang

#9
##### Jan 29, 2013, 07:21 pm

Quote
If you power arduino with a battery, you can do that measurement.

How? The arduino board must still share a common ground with the voltage source driving the constant current driver so I don't see how that solves the 'too high a voltage for an arduino AI pin, no matter how you power the arduino board?

Lefty

If I use a battery, I would attach floating arduino gnd to the junction between the two resistors and measure the junction between the left resistor and power supply. If the constant current is small, then the junction between it and resistor will have low voltage. Otherwise it's as high as 10-14V. There's no numbers to decide which way it is.
Serial LCD keypad panel,phi_prompt user interface library,SDI-12 USB Adapter

#### mauried

#10
##### Jan 29, 2013, 11:29 pm
You need an opamp specifically designed for this purpose like this.
http://www.analog.com/static/imported-files/data_sheets/AD626.pdf

Operates off 5V , but can measure common mode voltages as high as 50 V.
Has fixed gain of either 10 or 100.

#### dhenry

#11
##### Jan 30, 2013, 12:09 am
Quote
How can I do that?

Find an opamp with common mode voltage range beyond its supplies. Likely an instrumentation opamp.

#### soulid

#12
##### Jan 30, 2013, 01:07 pm
Found a TI chip named INA200...need to order now

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