# Current transformer for current sensing at Low Voltage AC

I have 230V/50HZ AC to 12V/5A step down transformer. Is it possible to measure the current at the secondary (Low Voltage AC)side of the transformer? is it possible to use regular CT coil(which is used to measure the current @230V AC)

Measuring the current on the secondary side of the transformer can be accomplished in three possible ways.

1. 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).

2. 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.

1. A third way could be to use a hall effect sensor with corresponding electronics such as the NA25 or NAP25 both from FW BELL.

The least intrusive method would be to use a current transformer which simply clamps around your secondary cable. Here's one that should work well: http://cdn.sparkfun.com/datasheets/Sensors/Current/ECS1030-L72-SPEC.pdf

You can also get Hall effect sensors, and these are particularly easy to interface to a microprocessor: https://www.sparkfun.com/datasheets/BreakoutBoards/0712.pdf

These are readily available on Ebay as breakout boards

A current transformer may be used to measure current at the primary side of your step down transformer as well as on the secondary side. You want to measure current, not voltage, it is no problem if the voltage is 230 V or only 12 V. But the current transformer should never be used without a load resistor. A CT rated for 5 A should be used for current up to 5 A but not more. But such a CT for use on 230 V wires should not be used for measurement on high voltage like several kV, only up to the rated voltage of the datasheet.