Hello
I want to measure the current of any circuit. So i make my own current transformer. its a toroidal type CT which have primary turn 3 and secondary turn 20. i connect a 0.5A load at primary.
so secondary side current should be Is = (N1/N2)*Ip = (3/20)*0.5 = 75mA
which approximately i get it.
but the problem is that the voltage at secondary side. i use 68 ohm Burdon Resistor. so output voltage should be = 75mA*68 = 5.1V
but i get only 0.002V. why its happen i dont know.
please tell me the reason why its happen and how i get 5V at secondary side.

 

Welcome to AAC!
Perhaps the core is saturating. What is its size/rating?

 

What is the inductance of the secondary and the resistance of the secondary + the resistance of the leads to the burden resistor? What is the frequency of the current in the primary and what is its waveform? With what kind of instrument are you observing the output?

Below is a link to a project on this site in which simple current transformers for oscilloscopes are explained.

https://forum.allaboutcircuits.com/threads/project-oscilloscope-ac-current-probe.14911/

 

If the core is saturating, you could use a op amp transimpedance amp configuration with a 68Ω feedback resistor to reduce the burden voltage to essentially zero volts.
For 75mA you will need a high current op amp or add a buffer amp at the op amp output.

 

Hello
I want to measure the current of any circuit. So i make my own current transformer. its a toroidal type CT which have primary turn 3 and secondary turn 20. i connect a 0.5A load at primary.
so secondary side current should be Is = (N1/N2)*Ip = (3/20)*0.5 = 75mA
which approximately i get it.
but the problem is that the voltage at secondary side. i use 68 ohm Burdon Resistor. so output voltage should be = 75mA*68 = 5.1V
but i get only 0.002V. why its happen i dont know.
please tell me the reason why its happen and how i get 5V at secondary side.

Your formulae is correct for an ideal transformer however; type of core and it's behaviour at a given frequency will create a transmission factor.
This factor will have a negative effect at output ( factor x N1/N2.)
How to figure out what is going on.

Measure the un-terminated output and put a changeable load from let say 1000 Ohm to 100Ohm.
Change this load until you reached 1/2 the open voltage. Doing so you identify the output impedance = impedance load.
This allows you to calculate the resistor needed to reach the voltage wanted. (or never reach it when losses are huge.)

Picbuster

 

What is the frequency of the signal? I recommend that you do not use three turns in the primary winding. Usually use 1 turn. To obtain 1 turn, it is sufficient to pass the conductor through the core ring. Do you necessarily get a big signal? With a large signal, a bad current meter is obtained (large internal resistance).

 

It would not be bad for you to specify the size and material of the core. I can then calculate your current transformer using LTspice.

 

What is the frequency of the signal? I recommend that you do not use three turns in the primary winding. Usually use 1 turn. To obtain 1 turn, it is sufficient to pass the conductor through the core ring. Do you necessarily get a big signal? With a large signal, a bad current meter is obtained (large internal resistance).

its 50Hz AC Sine Wave.

 

You will need a very permeability core with no gap so you can get a high inductance with ow wire resistance on primary and secondary.

See the calculations in the referenced thread:
https://forum.allaboutcircuits.com/threads/project-oscilloscope-ac-current-probe.14911/

If you have some way of measuring inductance you can determine the inductace coefficient (AL) of your core. Read the article.

For reference, I made one with a 0.49 ohm primary
2.7 ohm burden
11.5 mH secondary inductance.
The low frequency cutoff frequency is 44 Hz.
A 50 Hz square wave does not look much like a square wave until the corner frequency gets down to below 10 Hz.