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Current and Voltage Sensing - Parts/Techniques?

z.s.tar.gz

Nov 21, 2009
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I need a way to monitor voltage on a scale of 0-10V and current on a scale of 0-2A.
I would like the resolution to be pretty good, but it isn't crucial. (As long as it's close)

What parts/techniques would you recommend for doing this?
I'm currently looking at this for current sensing but I don't think it has the correct scale or resolution.
I don't know which of the multitude of voltage sensing techniques is the best.

Also, I'm kind of limited to through hole components as I do not have the tools or money to do surface mount at this time.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Here is one simple way

http://cgi.ebay.com/Universal-Digit...emQQptZLH_DefaultDomain_0?hash=item518fd0756f

Seems to do all you want, as long as a digital output is acceptable.

Otherwise you could do it the traditional way with an analog meter and appropriate series and shunt resistors to give you either 0-10V or 0-2A.

I guess this is the wrong answer, but you don't specify what form the output is to be -- I just assumed "human readable".

Is the input impedance for the voltage measurement going to be an issue? Is the voltage drop across the current measuring device going to be an issue?
 

z.s.tar.gz

Nov 21, 2009
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I guess I should have specified that in the first place.
This is going to be read by an AVR Microcontroller to monitor the charging of LiFePO4 batteries.

So I guess human readable is actually not an option...:)
 

(*steve*)

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So I guess we can assume that the microcontroller has a separate power supply.

What are you trying to do?

a) measure an absolute voltage from 0V (to, say, 10V)
b) measure an absolute voltage within limits (e.g. 3.7 to 4.3 volts)
c) determine if the voltage is above or below a limit (e.g. 4.15 volts)

(and then the same questions for current)

For options 1 and 2, how much accuracy do you require? Do the analog input on your microcontroller have sufficient resolution?

How much voltage drop across the current sensor is permitted?

My first thought would be to have a low value resistor to measure current (let's say 1 ohm) and use an op-amp to amplify the voltage across this (if required) so that the max current becomes the max analog input voltage, and then use a voltage divider (if required) to bring the battery voltage into range of the microcontroller's analog input.

Issue's you'll need to address include the need for a common supply rail for the controller and the battery, with all the noise implications that may come from that. In addition, you may need a separate + & - supply for an op-amp. An op-amp that allows the inputs to go right to the -ve supply rail may eliminate the need for a split power supply.

The simplest solution would be to place the current limit in the ground lead (thus making the "grounded" end of the battery float slightly above 0V (depending on charge current). With a 1 ohm resistor you would have a full scale reading of about 5A (but your resistor power dissipation would be a massive 25W and it would be dropping 5V) and assuming an 8 bit resolution, your resolution is around 19.5 mA. I would read the battery voltage from the + terminal of the battery (remembering that the voltage of the battery plus the voltage drop across the current measuring resistor would have to be < 5V). A voltage reading would have to subtract the "current" value, and -- assuming zero charge current -- the resolution would be 19.5 mV. If you need a voltage divider, the resolution decreases by the inverse of the factor that the voltage divider divides. (e.g. if it divides by 2, the resolution is now 39 mV).

So as not to cause damage to your controller you'd want to have series resistors and diodes to the supply rail to prevent the inputs from exceeding 0-5V.

If your A-D has higher resolution, then your voltage and current resolution will be correspondingly better.

If you need more resolution than is easily available from your controller, things get a little more complex -- an option is to measure the time taken to charge a capacitor.
 

z.s.tar.gz

Nov 21, 2009
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Number 1 is what I wanted to do.
I've decided to just buy a premade IC for doing this, as that's the easiest option.
 

(*steve*)

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Out of interest, what did you choose?
 
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