### How to interfacing CT with 5A secondary current

Hi there

I want to measure 1 phase current.

I want use my standard industrial CT with 5A secondary curent, i have 250/5A CT trafo, were my country used 220VAC/50hz 1 phase. For this project max current i want to measure is 100A.

Is anyone experiencing similar project? Any tips and wat must i do to start my project?

Thanks

djatie

### Re: How to interfacing CT with 5A secondary current

You need to load your c.t. with a burden resistor that will give the correct voltage for your input circuitry - a little less than 5 V peak-peak ( = 1.75 V rms) for an Arduino using a 5 V reference for the ADC, or 3.3 V peak-peak ( = 1.15 V rms) for an emonTx using a 3.3 V reference for the ADC - at whatever maximum current you require to read.  Do be sure NEVER to open-circuit the current transformer when a primary current is flowing.

You can purchase a standard emonTx kit from the shop and construct that exactly according to the instructions except for the burden resistor. With 100 A primary current flowing, the current in the burden resistor will be 2 A. The plug and socket will not carry 2 A, you must mount your burden resistor off the printed circuit board. I have calculated your burden resistor to be 0.55 Ohms and rated at 2.5 W for an emonTx. You could use 2 x 1 Ohm resistors in parallel, which would allow you to measure a little over 100 A. You can leave the existing 18 Ohm burden in place if you wish, it will affect the calibration only slightly and you can correct that in the software.

### Re: How to interfacing CT with 5A secondary current

I should have added that you need to make sure that your existing c.t. is rated at 2.5 VA or greater for the emonTx, or 3.5 VA or greater for the Arduino. Alternatively, you could short-circuit the existing c.t. and use the standard split-core 100 A c.t from the shop.

### Re: How to interfacing CT with 5A secondary current

OK thanks. But now im confused to chose the arduino board.

On my country i just found

- arduino uno rev 3 clone

- arduino duemilanove clone

- arduino mega 2560 clone

can you recomended about the arduino must i use?

thanks

### Re: How to interfacing CT with 5A secondary current

I would have thought that any version of Arduino would be suitable for this task providing that the output of the CT is scaled correctly for the range of the input circuit.

Many people on this forum are using the Uno Rev 3.  I have two of these.

### Re: How to interfacing CT with 5A secondary current

Thanks calypso_rae, tomorow i will buy this board  and get started my own energy monitor.

### Re: How to interfacing CT with 5A secondary current

You're welcome, djatie .  Good luck with your project and let us know how you get on.

### Re: How to interfacing CT with 5A secondary current

#robert wall

Sorry many questions. I just graduated from technical high scholl, electrical installation, and not so familiar with the microcontroller.

For CT I think it's too long distance to buy.

I count the existing CT ratio is 2000:1 at your store, then I look for in a store around here and I found a product with a ratio of 3000:1,  the datasheet can be viewed at:
multimic.com/e/sensor/ac1/index.html

CTF-100A
Applicable Current: AC 1 ~ 100Arms
Output Example: AC33.3mA ± 1% (100A)
Phase Accuracy: -0.7 ° ± 0.5 ° (100A)
Nominal CT Ratio: 3000:1
CT Inside Diameter: 16mm × 14.5mm
Max. Input Current: continuous 200Arms
Open Circuit Protection: 7.5V
Applicable Frequency: 10Hz ~ 5kHz

Can This CT be used?
Thank you.

### Re: How to interfacing CT with 5A secondary current

Yes, I think that c.t. can be used. You know the maximum current that you want to measure, therefore you can calculate the current out of the c.t. From that, you must calculate your burden resistor to give you a peak - peak voltage that is about 5% smaller than the input range of your processor, to allow for component tolerances. (Therefore, if you have an Arduino with a 0 - 5 V input range, and you want to measure 100 A, you need a burden resistor of 51 Ohms.  How did I get that? 5 V peak - peak = 2.5 V peak = 1.768 V rms, - 5% ~= 1.68 V.  1.68 V / 33.3 mA ~= 51 Ohms).

For the input circuit, see http://openenergymonitor.org/emon/buildingblocks/ct-sensors-interface

You might have read that increasing the burden resistor will give you better sensitivity at low current. Do not increase the burden resistor too much, if you do you will risk distorting the shape of the wave which will give you inaccurate measurements.

### Re: How to interfacing CT with 5A secondary current

Hello all, thanks for the guide, its works but its have some difference measurement with my hioki clamp power hi-tester (i use this device measurement for my calibration).

The difference  point of power factor from my  arduino and on my hioki is being 0.05 ( arduino 0.83X, on hioki 0,78X), the current is same as my hioki, but voltage difference between arduino and my hioki max 2v (arduino 198v than on hioki 200v or vice versa), and im stil search for good ones calibration.

Whether the value of phase shift affects the value that we measure?

I have purchased 3 different product for ct's, i just use 2A load and not more, some ct i use voltage calibration with 262.255 and current calibration 35.055, is the value reasonable and save?

How to see the power factor is LEADING or LAGGING?

But i don't know how to implementation it on the arduino programing, can some body help me?

Thanks :D

### Re: How to interfacing CT with 5A secondary current

When measuring voltage and current, you will need to calibrate your own system.  If you believe that some other piece of equipment is giving good results, then you can just alter your Arduino's calibration values to give similar results.

After you have calibrated your system in that way for one setting of power, it will be interesting to see what happens at a different setting of power.  If everything is working well, then your two systems will always keep in line with each other.  That means they are both linear.  So if you double the size of the current, your measurement of current should also double in size when using either system.

'Power' gets more interesting because your measurement system will certainly be introducing some phase-shift on each of your inputs.  When your system is set up correctly, it should measure a resistive load as having a Power Factor of 1.0.  To  make it do this, you may need to experiment with PHASECAL.  That is an algorithm which phase-shifts the voltage signal relative to the current signal.  There's lots of information about this on the thread at http://openenergymonitor.org/emon/node/870

### Re: How to interfacing CT with 5A secondary current

It's hard to me to understand but, i will try.