Op amps and Current Transformer

[pop]

Hi,

I'm working on a current measuring circuit. I must detect a minimum of 5mA and rectify it and feed it into the pic. Now I have the active rectifier (2 opamps with 2 diodes) done, but I am not entirely sure if I should connect the input of rectifying circuit directly to the current transformer. Do I need a current-to-voltage amplifier between current transformer and active rectifier, or can I just directly attach rectifier to transformer?

Thanks

 

[Russlk]

What is the turns ratio in your current transformer? A ratio of 1:1 would be OK for such a low current. The active rectifier should work down to a few hundred millivolts but if the signal is smaller, I would use an amplifier. Perhaps the rectifier can have gain?

 

[pop]

The current transformer that i'm using is not a conventional current transformer. It's a toroid removed from the ac motor and it's ratio is close to 1:1 (I still have to measure it precisely). However , I still might use a conventional CT and in that case it will be 50:5A (I'll probably adjust the ratio to 30:5A).

I've designed the active rectifier (full-wave) for a gain of 100. The design also minimizes the voltage and current input offsets.

Do you mean current to voltage amplifier?

Also will my active rectifier load the CT too much? I know that it is best to match high impedance output source with high impedance input amplifiers but how will this be affected in using CT and active rectifier?

Thanks

 

[JimB]

Your current transformer should have a low value load resistor across the secondary winding. So if you use a 1 ohm resistor, 1 amp in the secondary of the CT will give 1 volt across the resistor.

You will have to choose a value of resistor based on:

the minimum current you want to detect

the sensitivity of you rectifier

the highest current you will measure.

JimB

 

[pop]

Do you suggest using a precision resistor?

Thanks for the help.

 

[JimB]

You could use a precision resistor, depends on how accurate you want your current measurements.
Also be aware that this resistor could get hot depending on the current flow.

JimB

 

[StupidDum]

Is a currrent transformer necesary? As only 5mA is required.
why dont you use difference op-amp to convert current to voltage, and feed into ADC of PIC.

 

[pop]

Yeah, CT is required. The minimum current is 5mA (on secondary). I'm expecting max of 12 amps on primary. I'll definately use a resistor with a large enough power rating. My only concern with that is how accurate my readings are going to be, because of the resistor tolerances and other inaccuracies.

Thanks for the help though! It definately clarified some issues that I had with this project.

 

[Nigel Goodwin]

Yeah, CT is required. The minimum current is 5mA (on secondary). I'm expecting max of 12 amps on primary. I'll definately use a resistor with a large enough power rating. My only concern with that is how accurate my readings are going to be, because of the resistor tolerances and other inaccuracies.

How accurate are you hoping to achieve?.

Assuming you're looking for 5mA resolution on a 12 amp range, that's about 0.04% accuracy!.

I think that's pretty unlikely?, and if it was, what would you calibrate it aagainst?.

 

[pop]

I would like to have the best precision that I can get

I haven't made my intentions clear before.
I will have two current transformers. One will be a part of GFI and that is where I would like to detect really small currents. I know that I can get a GFI, but for this project I find it fun to build my own.

The second CT will be used to measure the current passing through the main line. I would like to keep the measurements as accurate as possible. I'm expecting the line to carry a maximum of 12A. And if I use 50:5 Amp transformer this will be a maximum of 1.2 A on secondary.

Anyway I just wanted to make clear what I'm doing. Although I wish that I could get 0.04% accuracy