Current through coil

A

alphacat

New Member
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.

---
I'm not sure whether Is equals Vo / Req or Vo / Rb.
Where Req = Rb || Rs
 
Last edited:
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**
 
Last edited:
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.
 
alphacat said:
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**
Click to expand...
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.
 
Last edited:
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.
 
alphacat said:
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.
Click to expand...
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.
 

Attachments

Boncuk said:
Hi alphacat,

you might consider using an Allegro-micro chip ACS758xCB. ...
Click to expand...

Can you buy these things anywhere? I'd like to stick one between the alternator and the battery in the airplane.
 
Last edited:
alphacat said:
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.
Click to expand...
You are correct, of course. My thinking was backwards.
 
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