Construct the AC-to-DC Conditioner Circuit

Step 3: Construct the AC-to-DC Conditioner Circuit

The main drawback with using an AC clamp is that the mV output is also AC. This isn't a problem for a multi-meter since it can be set for measuring AC voltages. However, the ioBridge module is expecting a DC voltage on its analog input pin. Therefore a little signal conditioning is required to convert the AC RMS value into a DC equivalent. The circuit I used was my own design and used components I had on hand, so I'm almost certain that it's less than ideal. After I was done soldering it up, I found a simpler circuit here. You may want to try it instead.

The main component in the circuit is an Analog Devices AD8220 instrumentation op-amp. This part only comes in surface mount style packaging. I needed to use a tiny SMT-to-DIP adapter board for my circuit. Don't feel like you need to use the exact same part. Any rail-to-rail instrumentation op-amp will work just fine. For example, the AD627 (Analog Devices) will work too and it comes in a breadboard friendly DIP package. In my circuit, I used a 16k resistor to achieve a gain of 4. Feel free to use a different resistor to get to a particular gain you need. However, any change in gain must be compensated for in the JavaScript of the webpage.

The other part of the circuit is a leaky peak detector made with a diode, resistor and capacitor. Its purpose is to translate the AC wave peaks into a DC voltage level. The 47k resistor causes the leakiness. Since the voltage level updates 60 times per second, adding that resistor increases the system's response time.

The potentiometer is used for offset calibration. I used a 15-turn pot for the best accuracy. To calibrate, power the circuit, make sure the AC clamp is not on any wire and measure the circuit's voltage output.

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paulstich says: Feb 15, 2010. 6:38 PM

Good stuff! Thanks! I built this using the AD627 as you suggested. Works great. I'm using a homemade ammeter coil (based on the info in hydronics' $2 Carabiner ammeter) and an Onset Hobo U12 data recorder. I would like to improve the rectification, and am thinking about trying a true-rms chip, like the AD737.

jeremiahthebullfrog says: Apr 21, 2009. 3:15 PM

Don't know what country this is but talk about dangerous!!! That's like a load of spagehetti in there! and why are your incoming cables exposed instead of being covered where they terminate??? Another thing is how do your supply company measure your usage? I can't see a meter.

dylanwinn in reply to jeremiahthebullfrogNov 24, 2009. 11:26 AM

1) In the US (California) at least, the terminals are left exposed inside the breaker box by the utility company, and covering them is unlawful because it could present danger when removing the covers.

2) In most places, the Meter is OUTdoors (where it can be easily read), while the breaker box is INdoors (where it is difficult for you neighbors to tamper with).

jasonT (author) in reply to jeremiahthebullfrogApr 22, 2009. 1:26 PM

Since the voltages on the wires coming out of breaker box to my custom circuit are in the millivolt range, I didn't feel like I needed to cover them. Besides, this was a weekend project and I ran out of weekend.

uberdum05 says: Jan 23, 2009. 11:52 AM

GET THEM LIVE TERMINALS COVERED!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! DONT risk an electric shock when you could be enjoying your life

dalesd in reply to uberdum05Jan 30, 2009. 9:42 AM

Correct me if I'm wrong, but there are no exposed 110 or 220v connections in the completed project.

uberdum05 in reply to dalesdJan 30, 2009. 10:14 AM

No I meant the main incoming live power terminals which you said were live

jasonT (author) in reply to dalesdJan 30, 2009. 10:09 AM

Correct. The AC clamp wires that are coming out of the bottom of the panel only carry a few volts at the most.

uberdum05 in reply to jasonTJan 30, 2009. 11:06 AM

No the red and black terminals inside the actual breaker box were live werent they??

dalesd in reply to uberdum05Jan 30, 2009. 12:35 PM

uberdum, Once the cover is put back on the breaker box, there are no exposed 110 or 220v wires. All the AC voltage stays inside the breaker box.

uberdum05 in reply to dalesdJan 30, 2009. 1:07 PM

Yeah I know but I meant be careful while the panel was off because he might have isolated all the ring mains but the input terminals are still live.

orsty3001 says: Jan 22, 2009. 2:25 PM

That's got to be one of the cleanest breaker boxes I've ever seen and who has a red hot leg running in like that? What kind of crazy building codes do they have where this was made.