Problem in setting up CT sensor with arduino

Hey I am using the arduino based setup as explained on the website. The input source that I am feeding is via a voltage source having 50 V with a resistance and the current value in the ammeter being 1 Amps

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When i test the output values on the pins of 3.5 mm jack, the values do change when I change the voltage. But I had kept the voltage as 50 V and then fed this data to the arduino, the current value which was controlled by the resistance being 1 Amp.

I had also changed the calibration values and the voltage in the code for current only.

The problem now is that the values on the serial monitor is same even before plugging the CT sensor and after plugging in the ST sensor.

Should I increase the current value and ultimately the voltage value from the voltage source to get some values ?

Can any one help me with this ??

calypso_rae's picture

Re: Problem in setting up CT sensor with arduino

Is your 50V source AC or DC?  The CT will not detect anything if only DC current is flowing.

There are a couple of simple sketches on my Summary Page which you could try.   MinMaxAndRangeChecker.ino will show the values that the ADC is generating for the first four analog inputs.  RawSamplesTool_4ss_2.ino will give you a graphical display of your voltage and current waveforms (but only if you have a voltage sensor and have an AC signal present)

Hope this helps.

hmaggo92's picture

Re: Problem in setting up CT sensor with arduino

My 50V source is AC source. I tried measuring voltage on the analog pin with and without switching on the CT sensor and the voltage remains the same,but when I try to read the change in voltage on the pins of 3.5mm jack and vary the voltage on the source the voltage on the pins of 3.5mm also vary. 

Robert Wall's picture

Re: Problem in setting up CT sensor with arduino

Are you copying this page: http://openenergymonitor.org/emon/buildingblocks/ct-sensors-interface ?  What value burden resistor are you using? The c.t. is a current source and will give you 50 mA per 100 A of primary current.  In your case, if your primary current is 1 A, you should expect a secondary current of 0.5 mA.  That current flows in the burden resistor and generates a voltage at the analogue input. If you have a 33 Ω burden resistor, you will get only 16.5 mV.

The analogue input requires 2.5 V peak - approx 1.77 V rms - for a 5 V Arduino for full scale. Your input is less than 1% of this. You should get a reading, but it will not be very accurate - the error may easily be worse than 10%.

What you can do for testing is multiply the current. To do that, you use a multi-turn primary winding - you pass the wire through the c.t. several times. If you want your Voltbox to give the equivalent of 10 A, make a 10-turn coil and clip the c.t. over the bundle of 10 turns. Explanation: transformers work on ampere.turns (amperes × turns) so 1 pass through the core carrying 10 A is the same as 10 passes carrying 1 A, or 2 passes carrying 5 A.

hmaggo92's picture

Re: Problem in setting up CT sensor with arduino

Yes. I am following that page with the burden resistance of 33 ohms. I am using 5v from arduino and hence the calibration value in the code being 60.6.

Also I should measure the AC current on the 2 pins of the 3.5mm jack ? right ?? because it came out to be 0 when I earlier tested it and I get that it was due to very small values of current.

Also without plugging in the CT sensor and only supplying the 5 V from arduino, when I measured the DC voltage at the wire being sent as analog input and the ground, it cam out to be 2.6 V. Is this normal ?

So now what I will do is that without changing anything, I will just increase the no. of turns in the CT, keeping everything else the same.

 

calypso_rae's picture

Re: Problem in setting up CT sensor with arduino

Also I should measure the AC current on the 2 pins of the 3.5mm jack ? right ??

My suggestion is that you use the ADC in your Arduino to take your measurements.  For this, you will need to build one or more of the standard sensor circuits that are well described in the Building Blocks section.  You can inspect the values that the ADC is generating by using the sketches that I suggested earlier.  Taking direct measurements from the jack-plug of the CT is unlikely to give you any results that we can help with. 

With a properly set up system, there is no need for multiple turns around the CT.   You should be able to detect a small flow of AC current when your is wire going through the CT only once.  Multiple turns is a useful technique to see what happens when a large current flows. 

Robert Wall's picture

Re: Problem in setting up CT sensor with arduino

You can find which are the connections to the secondary winding of your c.t. by measuring the resistance. You should see about 100 Ω. The standard connection is to tip and sleeve, there is no connection to the ring.

Look at the diagram on the page you are copying, and you will see that 2.5 V d.c is the correct quiescent value for the input. 2.6 V is close enough, though it will restrict very slightly the maximum current that you can measure.

hanguyen's picture

Re: Problem in setting up CT sensor with arduino

Hi Robert

My sct013 only show the voltage AC , I measure it with Digital meter. I can read the current with the schematic on the tutorial http://openenergymonitor.org/emon/buildingblocks/how-to-build-an-arduino...

can you help me how to change the hardware so that I can measure current by Sct013

Robert Wall's picture

Re: Problem in setting up CT sensor with arduino

I do not understand your question.

A current transformer measures current. It has no knowledge of the voltage on the wire and it cannot measure voltage.

"SCT-013" is a family of current transformers, there are 10 different variants. The SCT-013-000 gives an output current of 50 mA for a current of 100 A. You must add a burden resistor to convert that current to a voltage. All the others have the burden resistor inside the casing and give you a voltage output of 1 V at the rated current.

Which CT do you have - the exact part number?

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