Help choosing CT

Hi all,

I’m new in this forum and I would like to thank you all for so much info posted here.
I don’t now if this was posted before and if so could you please redirect me to that post?
I also google it but didn’t get any answer.

I’m making a small project using a microcontroller to measure up to 1A / 230V. I need the circuit to be insolated from mains so using a shunt resistor solution is a no go.
As the microcontroller have a max input of 3.3V I’m thinking in using a CT to interface. The ouput of this signal would then be added to 1.5 DC so I would have close to 0V in negative peak and 3V to the positive one.
Now there’s were my problem begins: The right CT for the job. I’ve thought in chosing the CST-1005 from Triad Magnetics but it’s rated to 5A. Would a primary multi-turn/multi-winding be the solution to measure up to 1A with this transformer with enough sensitivity? In that case what would be the appropriate burden resistor to do the job?
I’ve bought and tried to use an Murata 56050C and no joy so would the CST-1005 be a solution for my problem?

Thanks for any help you could give me!

Hello Samf, and welcome to the OEM forum.

You are thinking along exactly the right lines. Adding 1.5 V d.c (or 1.65 V) is the correct way to do it, and the CST-1005 looks to be a good choice. And yes, you can have a multi-turn primary winding to improve the sensitivity. But you cannot have the full 5 turns. The reason is hidden deep in the data: “Power Rating: 2.5 mW”.
To use the full range of your input circuit, you need about 1 V rms. (That will give you 1.414 V peak plus a little spare before you hit 1.5 V, which is your maximum (negative) peak.)
At 5 × 1 A, the c.t. secondary current will be 5 mA, and that implies 5 mW of power. Your c.t. cannot do that, it can only do 2.5 mW.

So I suggest an engineering compromise: Have 3 turns for your primary winding, so the secondary current will be 3 mA, and a burden resistor of 270 Ω. That will give you 810 mV rms (roughly 80% of your maximum, so you are not losing very much) and the c.t. will be working very close to its maximum power.

One word of caution: if your 1 A current is not a good sinusoid, but the “crest factor” is greater than √2, then you should use a lower value of burden resistor so that the c.t. does not saturate on the peaks.

The reason that the Murata device did not work for you is it is designed for high frequencies only: 20 kHz - 200 kHz.

You should be able to use emonLib to perform the calculations - possibly with some changes depending on which microcontroller you use. EmonLib will filter out the d.c. component and give you the true rms current.

Thank you for your quick answer.
Regarding the Murata… 50KHz not 50Hz… :grimacing: Upps…
I’m going to order the CST-1005 but i think it will work. I believe my biggest problem was the frequency in the first place. I’m glad you point it to me.
I’ll give feedback regarding the CST one.