CT Design not clean on the signal end

[samuelr]

Hi All,

I am trying to build an AC current monitoring device which would allow me to sample AC incoming current using a MURATA 1:100 current transformer. When I use the scope to see the signal I can make out the waveform of the lamp I am using as load but its not clean it has alot of noise riding on it. Here is what my circuit looks like.



Regards,

Samuel

 

[Warpspeed]

One thing you can try is placing a Faraday screen between the wire and the CT. Just pass the wire through a short length of metal tubing where it goes through the hole in the CT, and ground this metal tube.

The way its scaled, 10A will produce 45mV across your 0.45 ohm resistor, and the amplifier amplifies that to 250mV.
The voltages are all fairly low which may make it more susceptible to noise.

Another way to do it might be to make R1 10 ohms, that will give you 1v for 10 amps across R1.
Then give the amplifier a gain if 0.25 by making R3 56K and R8 14K.

You could also try placing a capacitor across R8 to roll off the high frequency gain.

 

[KlausST]

Hi,

I like to use Opamps.
But an Opamp is not always the best solution.
It will introduce noise, offset, and other errors.

By changing R1 (as Tony said) you could adjust output voltage.
Then there is no need for amplification.

You could use RC filters to suppress HF noise.

****
What lamp are you using? Maybe the "noise" you see is the true current waveform.

Klaus

 

[ankitvirdi4]

Hi,

Klaus is correct; no need to amplify the single, rather increase the burden across your CT.

Although, if you wish to use an opamp, I suggest place a small capacitor across R8 (100pF should suffice) this will attenuate HF noise.

 

[Warpspeed]

A thought has just occurred to me,,,

What are you measuring, and is this current transformer one of these ?
https://www.murata-ps.com/data/magnetics/kmp_5300c.pdf

If so, the quoted frequency response is 50Khz to 500 Khz, it will not measure 50/60Hz mains current through your "lamp".