# Current transformer with up to 10A DC in the primary...will it saturate?

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#### treez

##### Guest
Hello

I have a 50KHz sinusoidal current which I must detect. The current is full wave rectified, so its actually pulsating DC half-sines.
The current is sinusoidal, but may be an average of 1A, or 2A, or 3A, or 4A…………..to 10A.

I must detect whether it’s a 1A, or a 2A , or a 3A……to 10A, that’s flowing………so I don’t need great accuracy, just to depict which one of these current levels it is that’s flowing.

Here is a picture of the 10A average rectified current that I must detect…..
https://i46.tinypic.com/2mxpjdg.jpg

….Do you think this is best done with a current sense transformer?

…..The DC level for the 10A one, is, exactly that ..10 Amps, and that seems an awful lot to put through a current transformer…transformers don’t like DC, surely.

I was wondering of the B82801B0205A100 (1:100) current sense transformer is up to this job?…..

B82801B series C.T. datasheet:-
https://www.epcos.com/inf/85/ds/b82801b.pdf

….the frequency range for almost all off-the-shelf current sense transformers is either 20KHz-50KHz or 50KHz-1MHz.
…..this makes it awkward for me because I am right on the boundary…i.e. 50KHz.

Also, the equation on page 5 of the B82801B series datasheet has a “duty cycle” parameter in it……..though my waveform, (shown above) is just a continuous train of half-sines…….so I don’t know what to put in for the “duty cycle” parameter?

So anyway, do you think the B82801B0205A100 C.T. will be OK for this job, and not saturate?

#### barry

Could you just pass the current through a sense resistor? Then you can just use a voltage comparator(s). What voltages are you dealing with?

treez

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Points: 2

#### WimRFP

Do you have the possibility to put the CT in the AC input to the rectifier?

It will saturate as the V*s product is just 10uVs with 80uH primary inductance. 10uVs into 80uH results in 125mA.

If you don't have access to the AC current, and you need isolation, you may use a gapped core transformer with very low primary inductance that does not saturate under peak current condition. Of course, this will introduce some minor voltage drop. To get the AC current component you need to use a low pass filter to integrate the output wave form. Instead of a gapped core, you may use a very low permeability ring core (iron powder).

Complete other option is to use a hall type sensor with or without feedback.

treez

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Points: 2

#### crutschow

Because of the saturation problem of a transformer I would suggest measuring the voltage across a shunt resistor in series with the circuit or, if you need isolation, a Hall-effect current sensor.

treez

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Points: 2
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#### treez

##### Guest
thanks, thoughthe hall sensors always need an expensive mini magnet in there aswell, which makes it pricey, and pricey to assemble because the magnet needs mounting

#### crutschow

thanks, thoughthe hall sensors always need an expensive mini magnet in there aswell, which makes it pricey, and pricey to assemble because the magnet needs mounting
No. The magnetic field the Hall sensor detects is generated by the current flow. You don't need any external magnet.

treez

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#### treez

##### Guest
Thanks, the A1220 hall sensor (SOT23) by allegro looks very cheap and interesting...

unfortunately it wouldnt be able to measure our haversine current and say if its 1A, or 2A or 3A ...10A.

however, the A1220 looks like it may be useful in our battery circuit, where we simply have to detect if charger current is flowing or not?

#### picgak

Hi,
If you want to use CT, you should put it before the rectifier stage.
regards ani

treez

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Points: 2

##### Super Moderator
Staff member
A 3914 IC has an internal stack of comparators. It will tell you which of ten levels is the one at which an incoming signal peaks.

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