ebay Current Transformer

Current Transformer AC Load Indicator LED

A current transformer (CT) is a simple, practical and inexpensive means of detecting the presence of AC current. Detecting load current rather than applied voltage is a superior method of determining normal operation because the load (e.g. motor or heater) could be either disconnected or otherwise defective. Described here are two circuits – the 1st may be tailored for a known and fixed load current, while the 2nd is suitable for a wide range of currents.

What is a current transformer (CT)?

A CT is generally a toroidal core transformer with an opening for a single turn (or multiple turns if required) rather than a primary winding. The secondary winding often has 1000 or 2000turns. Some that are intended for switch panel instrumentation using panel meters have a 5A secondary. Current transformers require a resistive load (ballast) in order to output the desired signal voltage. Avoid operating them without a load resistor as they can generate self-destructive voltages in the order of several kV. Like all magnetic transformers, they operate only on AC.

For sensing DC current, refer to the circuit in this link.


Wonderful properties of the current transformer

  • Minimal voltage drop – runs cool even with a 100A load
  • Inherent isolation – use even on high voltage applications – avoids electrical shock
  • Robust and reliable
  • Inexpensive if purchased as surplus – otherwise not cheap
  • Simple support circuitry


My CT is an odd ball out of my junk box – it has a 550 turn secondary and was originally intended for current feedback in AC drives.
Surplus CTs often have a 1000 or 2000turn secondary. This means that it divides the input current by that figure (referred to as “N”).

Buy this CT on eBay – every experimenter needs one in his bag of tricks

ebay Current Transformer

Avoid CTs with a 5A secondary unless you have a 5A moving vane AC ammeter to use with it.

Minimum current threshold

While they work great at high currents, I could not get mine to light an LED with below 1.5A (1.5 ampere-turns) on the primary. A CT that is intended for instrumentation would probably be better. The reason for this is that a little current is required simply to magnetize the steel magnetic core, and this level needs to be exceeded before any appreciable output is realized.

Circuit 1

This is the most simple application, but it is generally limited to detecting a fixed AC current. The ballast resistor (R1) must be calculated for the desired primary current. A single LED may be driven via full-wave rectifier, or two LEDs may be connected anti-parallel. White LEDs are recommended due to their high efficiency.

Circuit 2

This circuit has an active ballast so that it may be used over a broad range of currents. The active ballast is simply a shunt voltage regulator that shunts the excess current so that the voltage does not get too high for the LED. Transistor Q1 enables the shunt regulator when the voltage across R4 exceeds 0.6V. This way, the shunt regulator voltage divider (R1 & R2) does not load down the signal under light load conditions. When Q1 turns on, it applies full voltage to the divider and the regulator uses the Vbe of Q2 as a reference voltage. This is on the crude side, but very practical and simple for this application. The current rating of the composite transistor connection (Q2 & Q3) is roughly 100mA without a heatsink. In the oscillographs, the two modes of operation are visible – one clamped at high current and the other unclamped at low current.

Interface possibility

Interface the CT current sensor to a microcontroller or Arduino via an optical coupler.



Undocumented words and idioms (for our ESL friends)

bag of tricks –noun –literally, a magicians bag –in electronics, it refers to perhaps obscure solutions to difficult problems

odd ball –noun –unusual or different than others –in electronics it refers to non-standard components. An individual could also be called an “odd ball,” especially an engineer…LOL

LOL –idiom –slang acrostic often used in texting –Laugh Out Loud

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  • Philip Cook

    The fixed ballast circuit with two LEDs worked a treat for me. I have used it to monitor current flowing in an electrical emersion heater circuit. I used a 1000:1 CT with fixed 2.7k resistor and 470R resistor in series with two LEDs. Remember that the two LEDS never conduct at the same time so the current through the resistor does not divide in two through the LEDs. It flows either through one LED or the other, never both. In my circuit the LEDs light from about 5A in the primary. Typical full flow is 12A at which the LEDS are comfortably bright. Remember to choose resistor power rating so that the circuits never overheat even under a component falure. I used 0.5W resistors.