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 Author Topic: Measuring voltage over a resistor  (Read 1054 times) 0 Members and 1 Guest are viewing this topic.
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 « on: January 29, 2013, 08:53:08 am » Bigger Smaller Reset

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?
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 « Reply #1 on: January 29, 2013, 09:00:37 am » Bigger Smaller Reset

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
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 « Reply #2 on: January 29, 2013, 09:55:37 am » Bigger Smaller Reset

I'd like to measure that with the arduino.
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 « Reply #3 on: January 29, 2013, 10:02:49 am » Bigger Smaller Reset

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
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The last thing you did is where you should start looking.
 « Reply #4 on: January 29, 2013, 12:03:00 pm » Bigger Smaller Reset

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The way you have it in your schematic isn't the same as how you have it wired up! That goes for me too.

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 « Reply #5 on: January 29, 2013, 12:53:48 pm » Bigger Smaller Reset

Here you are
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 « Reply #6 on: January 29, 2013, 01:05:09 pm » Bigger Smaller Reset

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.
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Measurement changes behavior
 « Reply #7 on: January 29, 2013, 01:07:48 pm » Bigger Smaller Reset

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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

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 « Reply #8 on: January 29, 2013, 01:13:36 pm » Bigger Smaller Reset

See ideas for layout:
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The way you have it in your schematic isn't the same as how you have it wired up! That goes for me too.

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 « Reply #9 on: January 29, 2013, 01:21:46 pm » Bigger Smaller Reset

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.
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 « Reply #10 on: January 29, 2013, 05:29:18 pm » Bigger Smaller Reset

You need an opamp specifically designed for this purpose like this.

Operates off 5V , but can measure common mode voltages as high as 50 V.
Has fixed gain of either 10 or 100.
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 « Reply #11 on: January 29, 2013, 06:09:25 pm » Bigger Smaller Reset

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How can I do that?

Find an opamp with common mode voltage range beyond its supplies. Likely an instrumentation opamp.
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 « Reply #12 on: January 30, 2013, 07:07:25 am » Bigger Smaller Reset

Found a TI chip named INA200...need to order now
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