# Thread: Measuring current and/or wattage with Arduino

1. ## Measuring current and/or wattage with Arduino

I've been trying to figure out how to set this up. I want to be able to read current or wattage of 3 different DC lines (3v, 5v and 12v) and displaying them to an LCD screen.

This will be used in conjunction with my LCD voltmeter and project Power house. I want to be able to push a button and switch the display between volts and current or volts and amps or maybe even all 3.

I've been looking into precision resistors and also hall effect sensors. what would be the best way to go about hooking this up?

2. ## Re: Measuring current and/or wattage with Arduino

What you want to do is get a current sense resistor (also called a shunt). It will have a very, very low resistance. Then, measure the voltage drop across the shunt, apply I=V/R, and you're done. Also, if you want to show power use, just multiply the voltage by the current (P=IV).

EDIT:
If you want to do it really cheaply or need to pass a very large amount of current, you could also just drop in a length of large-gauge wire. Just measure the resistance and use that in your calculations. High current shunts are just sheets of metal of a known resistance, so it's basically the same thing and doesn't require you to buy more components.

EDIT2:
Originally Posted by SXRguyinMA
volts and current or volts and amps
Same thing. Current == phenomenon. Amps == unit.

3. ## Re: Measuring current and/or wattage with Arduino

ok so that will work for the 3.3v line as it's well within the Arduino's reading range. According to this guy, I'll need an opamp stage or two for both the 5v and 12v sections (as I needed voltage dividers for both) to get the readings within spec. Off to Google I go lol

This is all the stuff I'll be learning once I get my prerequisites out of the way and actually get into my Electrical Engineering course

oh and I meant volt and amps or volts and watts, but ideally, be able to cycle through all 3

4. ## Re: Measuring current and/or wattage with Arduino

Opamp stages for what? Preamp? Sorry, I'm not familiar with Arduino parts ... aren't these functions built into the ADC inputs?

5. ## Re: Measuring current and/or wattage with Arduino

The link was saying that the voltage drop on the heftier rails probably won't be enough for the arduino to pick up, thus the signal amplification.

6. ## Re: Measuring current and/or wattage with Arduino

Ah. Choosing a better Arduino part (or a better non-Arduino part) isn't an option?

If you're gonna stick a bunch of extra logic in there anyhow then maybe cannibalize an inexpensive DMM instead ... achieving basic DCV accuracy of <±0.5% across the full DMM range is easy for even the lousiest cheap DMM, plus you get the display and whatever logic is needed to drive it (and some other cool parts like the rotary lock switch and probes and jacks) ... check out this cheap beauty (\$18 at Wal-Mart, \$12 at Amazon) or even this ultra-cheap Shenzhen special (\$3.49) ... these would probably work well for this application and might cost less than proper Arduino parts. Then again, an LM324 (or whatever) probably only costs two bucks ... still, if you've got an old junkmeter lying around ...

7. ## Re: Measuring current and/or wattage with Arduino

well I'm going to be integrating a multimeter into the project (see Power House in my sig), but I would like to have a real-time readout on the LCD of exactly what is being used by each output without having to probe them. The Arduino components are the easiest for me, as it's the only thing I'm familiar with at the moment

8. ## Re: Measuring current and/or wattage with Arduino

This could be a cool addition to this. 640x200 monochrome display, \$2 at Electric Goldmine.
http://www.goldmine-elec-products.co...?number=G14876

I did some searching the other day and found a guy who wrote an AVR driver for it...unfortunately it needed, iirc, 32KB of SRAM to drive the whole display, so you would need to figure something out with a display buffer. I'm planning on picking up a few of them in a week or two to see how they work out.

9. ## Re: Measuring current and/or wattage with Arduino

awesome idea but I'm already cramming everything in spacewise lol

10. ## Re: Measuring current and/or wattage with Arduino

well I think I've found a simple and inexpensive current monitoring device

http://www.goldmine-elec-products.co...?number=G16976

I'll pick up 3 of these and hook them to the arduino, and run one power supply line through each

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