# Measuring 0-250V and 10A (AC) with arduino

So,

I've decided to make a "homemade variac", but for simple things, such as lightbulbs, etc. Just to try things. So I'm going to use a voltage regulator to regulate AC voltage between 0 and 230V, where current can be up to 8A (according to voltage regulator's "max. power", but I think never will go higher than 3-5A).

Now, the question here is: How can I properly measure AC voltage and current, which I intend to show on LCD display? Which modules would be proper to use?

( I intended to use these voltmeters, but couldn't find one for AC current 0-10A, so I decided, to make it on a display and show also input voltage, input current, power and Wh.)

• Note that this thing and variacs do entirely different things. Variacs keep the waveform more or less intact and reduce Vpp as well as Vrms but these things chop up the waveform and spit it out as something with a changed Vrms but mostly unchanged Vpp. This can lead to dire results. Jun 1, 2016 at 10:57
• Thanks for reply, but I think you missunderstood. If I use this to power lightbulbs and simmiliar things..I don't think there might be a problem. I won't power supply things such as osciloscopes etc. :) Just soldering iron or something as this, where's no or very small amount of electronic components). Jun 1, 2016 at 11:01

By using that cheap SCR "chopper" you have created a measurement problem for yourself. Since you have a non-sinusoidal waveform, you need a "true RMS" measurement scheme for BOTH voltage and current.

You can certainly find RMS conversion chips that will allow you to construct a circuit that accurately reports equivalent RMS voltage and current. But that is not a trivial thing you can throw together with an Arduino. The measurement of your "dirty" power has now become the primary project vs. a simple measurement task.

It seems unlikely that you can find any of those inexpensive digital voltage/current meters on Ebay that will read true RMS. Even if you could find one that is powered separately (so that it will measure down to zero).

If you want to use a cheap, noisy chopper SCR controller like that, then you must pay at the other end with RMS metering. There are reviews on YouTube for popular-price, commodity DMMs which have true RMS, and they are relatively inexpensive. For example you could start here with Dave Jones' Digital Multimeter Buying Guide for Beginners: https://youtu.be/gh1n_ELmpFI

If you are going to be experimenting with SCR choppers and mains power, then you need a couple of decent DMMs with true RMS anyway. And you would need at least one anyway in order to calibrate your Arduino circuit with the RMS chips.

• Hmm...okay. But, how about if I change the way it's going at the very beggining? What if I create something simmiliar to variac or at least something, that acts like one? How would that go? (it's not much of a expenses talk here, it's more of a education and curiosity) Jun 1, 2016 at 15:32
• You can use a variable RESISTANCE to reduce the power into the load. That will preserve the sine waveform, but at the expense of dissipating ALL the unused power as HEAT. The unique thing about a real Variac is that it doesn't waste the unused power. Jun 1, 2016 at 17:11

Most cheap voltmeters and ammeters will display entirely the wrong numbers when dealing with non-sine outputs like that will produce. You need a true RMS meter.

I would suggest just dropping the arduino, and using something like this.

If it needs to run at low voltages, a meter like this with a separate supply will work - but ouch on the price.

I'd try getting the first one - it might work to lower TRMS voltages if there's still the high peaks; it depends on what they're doing in the power supply. You might also be able to separate the voltage input from the power supply.

• That's a great idea, thanks! I've thought about it too, but here's one single problem. It measures only 80VAC up and nothing below, but I'd really like to measure from 0-250(or 300)V. Jun 1, 2016 at 13:03
• @Jakey see edit. Jun 1, 2016 at 13:38
• Okay, another question. What if I use analog instrument or measuring current? Would that be proper for this? I've used the analog voltmeter and worked just fine. Jun 4, 2016 at 0:20
• I think analog meters average, so you'd still get incorrect readings. They'd look OK-ish, but they won't reflect reality. Jun 4, 2016 at 11:11
• Okay, I understand the concept now. But here's another question (which I hope, isn't stupid one). What about if I would add a capacitive load or a capacitor for smoothering the "ripple"? Would it be more "look-alike" to a variac? Jun 5, 2016 at 22:28