current measurement
I have made a current sense transformer to sense primary current maximum 2A(of my electric iron) and secondary output is 1A but there is no current in the secondary. I have used a iron "Nut" as a torroidal core(I suspect this may be the problem) and used normal wire with plastic insulation for winding. I took 2 turns for secondary, and burden resistor is 1.4 Ohm. As a newbie I am getting problems, experts please guide me.
update: Thank you experts for helping now I can develop a CT for me.
For the AC component you can use a simple current transformer. You can directly wrap a few loops of wire around the current carrying wire (i.e. use air as the core material) to test the concept, or you can buy a pre-existing solution.
However, a buck converter will have some AC+DC component, and any transformer setup cannot measure the DC level.
There are only 2 solutions for measuring the DC level: hall effect sensors and current shunts. I don't understand why you think a hall effect sensor will necessarily be expensive (possibly a locality issue?): take for example the DRV5053RAQDBZR, which digikey sells for about a dollar in 1-off quantities.
As PlasmaHH meantioned, there are current shunts available which can measure these currents with ~100mV or lower burden voltage, however they are usually somewhat expensive. If you don't need to worry too much about tempco, you can just use a length of wire with a known resistance (measured before-hand using 4 terminal sensing).
Measuring the current on the secondary side of the transformer can be accomplished in three possible ways.
place a small resistor (less then 1 ohm) in series with the transformer connection and measure the AC voltage drop across the same. Thereafter it is simply Ohms law (I = V/R).
The other way is placing a miniature current transformer (see picture) and measure the voltage across the transformer load resistor ( Do not use such a transformer without a load resistor). Look at the data and application sheet of such transformers for further information.
Best Answer
Current transformers normally have hundreds to thousands of turns in their secondaries, since the idea is usually to turn a sensed current into a measurable voltage.
Keep in mind that a current transformer is also a voltage transformer. With a 1:2 ratio, the output voltage will only be at most twice the input voltage. In this case, the input voltage is simply the voltage drop caused by the primary current interacting with the impedance of your 1-turn primary winding, which is very tiny.
Just to put some numbers to this, let's say that your nut has an inductance on the order of 10 µH. At 50 Hz, this will have an impedance of 2πfL = 3 milliohms. Your 2A primary current will produce a voltage drop of about 6 mV, and your secondary will produce a maximum open-circuit voltage of 12 mV.