# Using CST-1020

Hi Everyone,

I'm Nuno from Portugal and I have started to use the arduino quite ago.

Now I decided to build an energy meter for the plugs at home.
I want to measure the current so I bought the CST-1020 current sense transf. ( Datasheet: http://triadmagnetics.com/pdf/Page%2013-14.pdf check on the bottom of the document)
I don't really understand this sensor. How do i compute the current on the primary? Does it has a burden resistor already? My guess is no...
How the the hell do I compute the ratio from the voltage on the secondary winding and the current on the primary changes for a burden resistor that is not listed?

Hi Nuno!

RL seems to be outside of the CST1020 according to the picture…
Let RL=100, then appr. 100mV on RL translate to 1A… 200mV to 2A…

I hope u took sufficient care to uphold proper insulation…
How did u do it?

Bye

I didn't used any insulkation so far. I am still at the begining of the project. Should I use it?

That's exactly the maths i did, but if you took a RL=500 you should get 500mV on RL to 1A, but acoording to the datasheet you get 394mV....
And if you choose the 5kohm for the RL you should get 5V for each amp, but according to the datasheet you get 766mV.

How is that possible? Shouldn't that be approximate linear?

coils/capacitors have logarithmic/exponential curves...

how do u plan to make the primary side of that sensor?
it seems to b a little bit dangerous for my taste...
do u know an electrician?

RIDDICK:
1.
coils/capacitors have logarithmic/exponential curves…
Inductor
http://en.wikipedia.org/wiki/Inductor#Inductance_formulae

how do u plan to make the primary side of that sensor?
it seems to b a little bit dangerous for my taste…
do u know an electrician?

What’s the diference from the one I’m using (CST-1020) and this one? ( Non-Invasive Current Sensor - 30A - SEN-11005 - SparkFun Electronics? )

I've looking for more sensors like the one i'm using and i found tihis on: http://www.soselectronic.com/a_info/resource/a/pdf/AC1050.pdf.
It looks its almost the same and has a more detailed datasheet.

I have had a burden resistor at the output of the sensor to measure current, but if i use the value given for 100 ohm at the output it doesn't give even close to the right result. (I am measuring the current with another equipment.)
Does anyone have used this type of sensors?

how do u get a mainspower plug through that tiny hole?

that sparkfun sensor can b integrated easier, i guess...
one just snaps it on the mains power line...

possibly u just measure the reactive power?
i have no idea how that sensor works, when there r 2 wires with the same current (just the direction is the oposite)...
i think u should wait for the high voltage experts... they should b here within a day...

RIDDICK:
1.
how do u get a mainspower plug through that tiny hole?

that sparkfun sensor can b integrated easier, i guess...
one just snaps it on the mains power line...

possibly u just measure the reactive power?
i have no idea how that sensor works, when there r 2 wires with the same current (just the direction is the oposite)...
i think u should wait for the high voltage experts... they should b here within a day...

I have a cable with a plug i can easily remove. I don't need the plug to go through the hole. I only attach it after the cable passing the hole.

I think the basis of work are the same, but i have this one and it was rather expensive. I want to see if i understand it...

I'm not measuring power. I'm just looking for the current. Only one of the cable go in the hole, otherwise the sensor would give zero Volts at the output.

ok - that sounds safe…

i c

ok - then it should work… how many measurements do u take? since it is AC, the voltage changes quite rapidly and strongly, i guess…

RIDDICK:
1.
ok - that sounds safe...

i c

ok - then it should work... how many measurements do u take? since it is AC, the voltage changes quite rapidly and strongly, i guess...

I'm looking to the current wave in an osciloscope at the current transformer terminals. I have 100 ohms conected to that terminals too (the burden resistance).
Then i take the rms value of the wave and after dividing by 0.097 (according to the datasheet) I should get the current value. It happens to give a greater value than it should.

In the osciloscope I get 0.42 V (rms at the burden resistance terminal) and after dividing by 0.097 i get 4.3 Amps, when i should get something like 2.63 Amps (a resistance with 1 kW )

I can't find any example on the web with this sensors. This is driving me crazy!!!!

uhm
why do u take the RMS value (it contains square values)?

i think the arithmetic mean would b the right value...

have u seen this:
Arduino Forum ?

RIDDICK:
uhm
why do u take the RMS value (it contains square values)?
Root mean square - Wikipedia

i think the arithmetic mean would b the right value...
Arithmetic mean - Wikipedia

have u seen this:
Arduino Forum ?

I'm working in AC. If I use the aritmetic mean i get zero or something near zero...

I have just found that topic. I will try ti find something there too.

Thanks

Let RL=100, then appr. 100mV on RL translate to 1A... 200mV to 2A...

That "sensitivity" figure is specified at the rated current (of 25amp). So running 25amp Ip, you should expect about 2.5v (=100mv * 25amp) on the secondary. All rms of course. The datasheet doesn't say if that relationship holds if Ip is other than 25amp.

dhenry:

Let RL=100, then appr. 100mV on RL translate to 1A... 200mV to 2A...

That "sensitivity" figure is specified at the rated current (of 25amp). So running 25amp Ip, you should expect about 2.5v (=100mv * 25amp) on the secondary. All rms of course. The datasheet doesn't say if that relationship holds if Ip is other than 25amp.

But check this data sheet http://www.soselectronic.com/a_info/resource/a/pdf/AC1050.pdf.
It gives you all the information we need. We can see it is approximate linear. So it should be the same value used to calculate the current, for a given burden...

what if u use a variation of “average mean”, that uses absolute values?
sumi=1n (ai)/n
that would almost give u the wanted result…

EDIT: oops - that would not really help (just 10%)… lol - sorry - i have no idea anymore what could cause that error…

r u sure ur oscilloscope is sufficiently accurate?
although errors of 60% would b funny… XD

I found the error.
The datasheet is right. I have to divide the voltage by 0.98 for a burden resistor of 100 ohm.
My mistake was calculating the current for the load. I was adding a sqrt(3) on a monophasic load..

yay!

What sort of accuracy are you expecting with this energy meter?
Current only sensing energy meters can be highly inaccurate as they dont measure the voltage.
So with loads that have a power factor of less than 1 , the meter will read incorrectly.
Loads that have switch mode power supplies in them , like computers or plug packs will read even more
innacurately, due to the current waveform not being sinusoidal.

mauried:
What sort of accuracy are you expecting with this energy meter?
Current only sensing energy meters can be highly inaccurate as they dont measure the voltage.
So with loads that have a power factor of less than 1 , the meter will read incorrectly.
Loads that have switch mode power supplies in them , like computers or plug packs will read even more
innacurately, due to the current waveform not being sinusoidal.

I'm starting by the current. The next step is to measure the voltage. How do you recomend to measure the voltage?

If you really want to measure energy accurately, you are far better off using a device specifically
for this purpose like a ADE5569.

It does all the hard work for you.