## measuring voltages greater than 3.3V

Questions on other types of hardware and getting it talking to the ARM CPU
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### measuring voltages greater than 3.3V

I could swear I saw a sample project to do this somewhere... I have a power
source that will always read between 11 - 14, or 0 volts. (It's the output of a
charge controller for a solar powered WiFi tower.) I'd like to periodically
read the output voltage, and send it to my desktop computer.

I already have the means to send small data samples from a SuperPro over the
network (via TTY session on network radio's serial port) but I'm not sure how to
read voltage. Ideally resolution would be 0.01V but 0.1V would be workable.

I see that the AD pins read voltage between 0 and 3.3V, so would I need to drop
the voltage with resistors? How do I calculate the resistance value? The
highest quality resistors are 5%, right? So I'd just have to calibrate the
values returned against a DMM?

One [more] thing that confuses me, I put a few K ohms resistor between my meter
and 12 volts and it still read 12 volts?

Any help appreciated.

-Mark McGinty

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### Re: measuring voltages greater than 3.3V

Hi Mark,

Basic Ohms Law is what you need.

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### Re: measuring voltages greater than 3.3V

I did not know that "pick your own value" resistors are available:
http://www.digikey.com/product-search/e ... esistors/6\
6806?k=resistors

1% resistors are very common.

Resistors in IC's can be passivated first and then "trimmed" using a laser
through the passivation.

Standard 1% values available are different than 5% and are different than 10%

Take a look here:
http://hyperphysics.phy-astr.gsu.edu/hb ... oldiv.html

A voltmeter gives you a good result because the input Resistance is much greater
than the Source resistance.Â A battery might have a internal resistance < 1
ohm, and a typical meter may have a resistance of 10 M ohms.Â 1 ohm || 10 M
ohms is extremelyÂ close to 1 ohm and thus the meter doesn't disturb the
circuit.

In electronic devices, there is a input bias current associated with the device
and it could be on the order of 1e-12 Amps or pA (picoamps).Â This affects a
measurement too.Â Input bias current has all sorts of dependencies on such
things as temperature etc.

You might be better with an expanded scale voltmeter and say a comparator.Â
e.g. 10-16 V, and when the voltage is say <9.5 volts you get a digital
indication of that.

With simple A/D converters there is something called a quantization error which
is easy to see if you hypothetically set the full scale count to 100.Â If the
read 3, it's less and if you read 50 it's even less.Â That's also why some
measurements take a zero of the process variable and shift it to 1V of the
measured value.Â e.g. 1- 5V.

When the outside world is measured all sorts of things can happen,Â One of the
most common is a difference in ground potentials and prtection against a
reversed battery or an alternator out of control, for instance.Â Automotive
transients can easily be from -200 V to + 50 Volts.

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### Re: measuring voltages greater than 3.3V

I agree with Will, but want to clarify how the resistor divider works. You would
connect the 12K 1% resistor to your voltage source, then connect the 3K 1%
resistor to the return (ground) of your source. Connect the remaining ends of
the two resitors together, and also connect that junction to the input of your A
to D Converter. The two resistors will make a voltage ladder between Ground (0
Volts) and the Input voltage (~15 Volts). The junction voltage can be calculated
by the formula.

Junction Voltage = 3000 x (Input Voltage/15000).

1/8 watt resistors or larger are sufficient to dissapate the power.

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### Re: measuring voltages greater than 3.3V

Thanks Dan,

I should have made that clear.

"KeepIt SimpleStupid" I have standard value 0.1% resistors from 1950's and used
special value 0.01%? from Vishay 25 years ago.

Cheers
Will

Vernmalloy
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### Re: measuring voltages greater than 3.3V

I have tried it but the connection between the two resistors was giving a problem. So can anyone tell me what would be the problem..??
Other than kids the onesies for adults are also available in the markets.

basicchip
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### Re: measuring voltages greater than 3.3V

Below is a sample circuit to measure a 12V signal. It divides the input by 4, so a max of 3V to th ARM.

It's not required but a zener diode can be used to limit the voltage if there is the potential of spikes on the input.
divider.JPG (15.88 KiB) Viewed 12246 times