Simple Current Transformer

The other day, I needed to measure the AC current of my water heater without having to disconnect the wiring. Typically, you would need a clamp meter to do this. Since I don’t have a clamp meter, I thought I would just make a simple current transformer myself.

In fact, making a current transformer is a very easy task. All you need is a magnetic core and a single winding. A ferrite bead is a convenient choice for the magnetic core as the opening allows the core to wrap around the wire and thus enable easy measurements.

I used some thin magnetic wire (35 AWG) and made a 50-turn winding on half of a ferrite bead (see picture below):

Current Transformer Winding

Current Transformer Winding

And the two halves of the ferrite bead can be snapped onto wires to measure the AC current:

Current Transformer

Current Transformer

The reading was fairly linear for the range I was testing with. when the primary winding current is 1A the current reading on the secondary winding is around 3mA and when the primary winding current is 10A the current reading is around 30mA.

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  1. Dick Cappels says:

    The use of a snap-on split core ferrite bead is very clever! The fact that the current ratios were not the same as the winding ratios indicates that the cutoff frequency of the transformer (1/(2Pi*L/R) of the secondary was quite a bit higher than the AC line frequency. This difference, if known can be used as a correction constant. Now, I need to start lookin for one of those snap-on cores. Thank you for the idea.

  2. Stanley says:

    Very nice and creative idea to make a CT..

    I enjoy reading yr previous articles on AVR/LC meter & DS1052 oscilloscope…

    I hv a question, how do you generate the current of 1A to 10A on the primary winding ? Are you using some sort of power supply with adjustable current ?


  3. Dick Cappels says:

    I use a power supply, a resistor, and a MOSFET in series then drive the MOSFET with a pulse generator.
    Recently, I have been using a high current voltage follower driving the primary of the transformer through a low value resistor so I could use sine waves for excitation when I want. Get a good heatsink!

  4. Dick Cappels says:

    I recently bought some of those snap-together ferrite beads and made a current probe out of it. This is really nice. Thank you for the idea!

  5. Joao says:

    Hi there.

    I really liked the simple and cleaver idea that you had. Genious!
    But i have some questions. You said that the CT you’ve made, was only with 50 turns. And that 1A = 3mA and 10A = 30mA. I assume that the 1A and the 10A are the current of your water heater??
    Another question i have is, can i convert that reading in the CT to voltage and than use that voltage to activate some circuit with the help of an APAMP? For example: i only want that circuit to turn on only when the current used by what i’m measuring with the CT, is greater than 2A!!
    I never did a CT and i don´t know if what i want the way i want is possible.

    Thank you very much!
    (Sorry my grammar)

    • kwong says:

      Thanks for your comments. Yes the CT was made to measure the current flow through my water heater (current up to 15A), but it is a generic CT so it can be used for pretty much any AC current measurement (the test sensitivity is 3mA through the CT when the current to be measured is 1A).

      You can convert the measured current to voltage via a small resistor. Typically you want the resistor to be small so that it doesn’t impact the measurement accuracy. And most likely you will need to follow up with an amplifier as the output voltage will likely be very small.

      One thing to keep in mind is that the induced current is AC, so you may need to rectify it depending on how your detection circuit is built.

      As an example, for testing a 2A current, you could use a 1 Ohm shunt resistor so that the output voltage across the shunt will be 60mV with 2A.

  6. Misha says:

    Can I connect LED that will turn on when certain current is reached, I understand
    that I need some rectification to AC current.
    My goal to hook 3 CT for 3 phase motor, and turn on red led when there is difference in current between phases, otherwise green Led will be always ON , without using any battery to power the device, you thin it’s possible?
    Current in every phase around 25amps., this way I can predict motor failure before it happens.

  7. […] The first experiment I did was to make a current transformer, based on the design here: […]

  8. Tee says:

    Hi Kerry,

    Thank you for all you do for the DIY EE Community. It is much appreciated.

    What do I need to do to make this usable with an oscilloscope… Add a BNC connector… But, what about a termination resistor? Or, amplification? If the additional circuit is powered, I would think that I would want to use a battery, so that there are absolutely no grounding issues. Please be specific as to the design, as I am still noobish…

    Also, I have this exact Ferrite Bead. Please tell us what you did to get it apart, without breaking the tiny retainer clips holding the Ferrite halves in the cover shells.

    What did you do to prevent the wire from building up under the bead (which would make it impossible to close the cover)? Drill a whole in the cover and take the turns outside the cover? The photo doesn’t show what you did.

    What did you do to secure the wire? I see the tape, but I would think that it is more for insulation than securing things… A little glue, perhaps?

    Thank you for your help. I look forward to your reply.

    • kwong says:

      Thanks for your comment. You will need a resistor (e.g. a few Ohms) to convert the current to voltage so you can use an oscilloscope to observe the waveform. Whether or not you need further amplification totally depends on the current you are trying to measure and the turn ratio of the current transformer. You will need to use a ferrite core large enough to wrap around the wiring.

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