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Current through coil

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alphacat

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Hi.

I'd like to talk again about a CT (current transformer).

I connect a burden resistor (Rb) to the output of a CT.
The secondary coil has a DC resistance (Rs).
I pass a primary current (Ip) through the CT's core.
I measure the voltage (Vo) across the burden resistor.

From these data - Rb, Rs, Ip, Vo - I'd like to derive N (number of secondary turns) and Is (the current flowing through the secondary coil).

How do I do that?

Thank you.

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I'm not sure whether Is equals Vo / Req or Vo / Rb.
Where Req = Rb || Rs
 
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Since you are measuring Vo across Rb then ohms law gives Is= Vo / Rb.
 
But isnt the secondary circuit considered as the picture below?

Moreover, when I measured the resistance at the CT's output, the ohm-meter showed Rb||Rs, and not Rb.

**broken link removed**
 
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You can treat the output of a CT as a transformer and usually ignore its winding resistances.
The higher the sensing resolution of the CT the higher the burden resistance needs to be.
 
But isnt the secondary circuit considered as the picture below?

Moreover, when I measured the resistance at the CT's output, the ohm-meter showed Rb||Rs, and not Rb.

**broken link removed**
No. Rs is in series with the winding and Rb, not in parallel. But an ohmeter will sill see the two resistances in parallel, since L has no resistance.
 
Hi again.

Is there any advantage of having one turns' ratio (N) over another turns' ratio?

I was offered to pick a CT out of two same CTs, that differ from one another only in their ratio of turns - N=500 vs N=3000.
Any advantages that one has over another?

All it means for me is that R_Burden @ N=3000 will be 6 times R_Burden @ N=500.
 
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It also likely affects the minimum and maximum currents that can be monitored. A higher turns ratio will allow sensing lower currents.
 
Got it, thanks!

The device which senses the secondary current is voltage-operated, meaning it only sees the voltage drop on the Burden Resistor.

I think I'll go for the 500 turns, its just a bit scary to deal with such low currents as happen with 3000 turns.
 
Before understanding CTs, I used a DMM to measure the output voltage of the CT (when current passed through its primary core), without connecting a burden resistor at the CT's output, and I got just a few hunderds of mV.
 
I think I'll go for the 500 turns, its just a bit scary to deal with such low currents as happen with 3000 turns.
More turns generates a higher current, not a lower current. The turns ratio is the ratio of the primary current to the secondary current.
 
Having more secondary turns (N) generates lower secondary current (Is), since
Is = Ip / N
Where Ip is the current that passes through the primary core.
 
Hi alphacat,

you might consider using an Allegro-micro chip ACS758xCB. It is fully isolated and the primary (current sensor) can withstand high overcurrent.

The device works with an interior Hall sensor and generates a proportional output voltage which can be used directly connected to an A/C converter.

Available ranges are ±50A, ±100A, ±150A and ±200A. The primary side has a resistance of 100µΩ and the voltage drop is extremely low.

Here is the datasheet.

Boncuk

P.S. I've already made an Eagle device of it.
 

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Hi alphacat,

you might consider using an Allegro-micro chip ACS758xCB. ...

Can you buy these things anywhere? I'd like to stick one between the alternator and the battery in the airplane.
 
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Having more secondary turns (N) generates lower secondary current (Is), since
Is = Ip / N
Where Ip is the current that passes through the primary core.
You are correct, of course. My thinking was backwards.
 
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