Newbie_ Question on CT's and Sensors

Just about to start on my home energy monitoring system. Being that I have access to commercial building control components I was wondering if I could use 0-5V CT's ? I have been out of the automation field for awhile now. We use to just use a simple table to convert the volt reading to an amperage. I also have access to thermistors (10K ohm etc...) and was curious about using them also. 

I just received my Pi2 and am starting my research and component purchases shortly. 

My goal is to monitor as much as possible. Main incoming,heatpump,hotwater tank, plug loads, etc...

Jeff in Ontario    

Robert Wall's picture

Re: Newbie_ Question on CT's and Sensors

Do you intend to add an analogue input to your PI, or use the Emon system?

For the emonTx/Arduino, then if you mean CTs with an inbuilt burden resistor giving 5 V rms at your maximum current, then yes, but you'll need to divide the output down to give you 5 V p-p (for an Arduino) or 3.3 V p-p (for an emonTx), less say 5% for tolerances and a sniff of headroom; and you'll need to remove the on-board burden resistor in the emonTx. The software sketches calculate rms real power (given an ac voltage also) or true rms current, from which you can get apparent power if your voltage is known and stable.

You could use your thermistor if you can feed it some current and have a look-up table or the equation to convert the output, as there are a couple of spare ADC I/O on the emonTx, though I've never tried to use them for that. We tend to use the DS18B20 for temperature measurements.

cormad's picture

Re: Newbie_ Question on CT's and Sensors

Thanks for the reply, 

                               I will using the emonTX/Arduino system as there are alot of sensors both wired and wireless senors I want to add. Can you explain what p-p means ( 5 V p-p (for an Arduino)? I am leaning towards real power for the accuracy and added graph and data point. 

Here is a link to the CT's I have access to, i will call and get pricing tomorrow.

Could I use an wireless arduino board with 10 CT's to communicate with RFM69Pi V3? I will do do more research on your kick starter in the morning. I very much want to support you and your site. 

Priority will be connecting to 200amp service 220v/1phase. 

Robert Wall's picture

Re: Newbie_ Question on CT's and Sensors

Those ACI devices won't work with the emonTx. You could use those with an Arduino, the output is a dc signal proportional to current, so you have lost much of the information that the output of a pure CT provides. You'd know only the magnitude of the current, so only apparent power is available to you, not real power nor direction of power flow (important if you have on-site generation), as you would have with a CT and emonLib.

And it's not clear how accurate the output of the ACI unit would be in the face of a non-sinusoidal current wave, there's a critical piece of information - the type of rectifier - missing from both the web page and the data sheet.

"p-p" is peak to peak, the (in this case) voltage between the most negative and the most positive parts of the wave.

If you want a reasonably accurate CT good to 200 A, I normally recommend one from the Magnelab range, if only because we know a fair bit about them and can recommend the correct unit once we know details of your set-up. They are not cheap, accuracy in things like this costs money, but from memory they're much cheaper than your ACI unit.

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