# What is the minimum current a current transformer can sense?

Datasheet of AS-100 mentions a maximum current it can sense before saturation, but what is the minimum current it can sense.

For instance, if I put 10 mA 50Hz through a conductor passing through the sensor AS-100 (1:50 ratio), will it produce 0.2mA through a 50 Ohm (suggested) terminal resistance on it's secondary?

• How much money are you prepared to spend on readout electronics? Theoretically, you can measure any quantifiable amount of current, with the right readout electronics (and perhaps some cooling of the transformer). – Connor Wolf Aug 18 '13 at 3:28

If you look at the data sheet, the secondary DC resistance is 0.6 ohms. The recommended load resistance is 50 ohms. As the data sheet says, this value was selected to give a 1 volt output for 1 ampere flowing through the primary. As a check, 1 ampere in the primary will yield (for a 50:1 ratio) 20 ma in the secondary. With 20 ma through it, the 50 ohm load resistor will indeed produce 1 volt of output. With that said, the minimum current that can be sensed is probably limited by the noise level of your sensing circuit. With 10 ma in the primary the secondary level will be 0.2 ma which will yield 10 millivolts across the 50 ohm load resistor. This should be well above any electronic noise in your sensing circuit.

Sensing is not performed by the transformer -- it is performed by the circuitry attached to the secondary. The minimum amount is determined by the equipment measuring it -- the load it places on the secondary, significant digits of precision and the reference voltage.

Current transformers are like any other transformers and obey the laws of Faraday and Lenz. What is important to understand are the sources of error and how real world materials act. Before a transformer can actually transmit energy, the poles in the magnetic molecules must be first aligned in the same direction. This is the initial magnetizing current. Additional losses based upon degree of saturation are also present. These IR losses are also subtracted from the potential output. The material of the core chosen and its size determine the amount of these losses. For the application chosen, AS-100, the CT will produce a non-linear output and probably will not give the expected signal, as the magnetizing current from the wire through the hole is not enough to excite the core enough to transmit energy through the core. A different CT should be chosen, one with a highly specialized core that is designed to excite with a minimum of excitation current. Core such as nickel and the new nanocrystal cores would provide a much better result. Please refer to this treatise that explains fully the model of a CT - http://www.crmagnetics.com/assets/technical-references/analysis_of_ct_error.pdf