A Current Sensing Coil That’s Open Ended

One of the joys of writing for Hackaday comes in learning new things which even after a long engineering background haven’t yet come your way. So it is with the Rogowski coil, an AC current sensing coil which is unlike conventional current transformers in that it’s open ended — in other words not needing to be closed around the conductor it’s measuring. [Weston Braun] has an interesting introduction to the subject, as part of his open source Rogowski coil based current probe.

The project itself is an amplifier and integrator that provides a voltage output proportional to the current sensed by the coil, but the real meat is in discovering the coils themselves. They’re a many-turn coil wound on a flexible former, forming in effect a toroidal inductor with a gap in it when bent into a circle. They’re for high frequencies only though, with the one in this project having a bandwidth from 888 Hz to 25 MHz. We don’t have any immediate need to non-intrusively measure current at those frequencies, but it’s something to know that we could.

This isn’t the first time a Rogowski coil has turned up on Hackaday though, back in 2011 we saw one used to measure a steep current impulse.

9 thoughts on “A Current Sensing Coil That’s Open Ended

  1. 888Hz is NOT a high frequency. Might be higher frequency than most of the currents one would want to measure (motor power and such), but in the grand scheme of things it is a very low frequency indeed. I’ll agree that 25MHz is high frequency, though expect many would still describe itas only medium.

    1. If you’re trying to measure mains current, 888Hz might as well be .888THz, lol. I also don’t know who considers 25MHz not HF but ITU HF is the decameter range at 3-30MHz so 25 qualifies.

    2. They are used in a lot of high voltage (11kV) switchgear at 50Hz, then main advantage being no magnetic saturation that a conventional CT would experience. Also a lot lighter without the magnetic core.
      Schneider electric ring main units for example use them for operating the transformer protection relay.

  2. Rogowski coils are also commonly used to measure high currents in power grid related stuff, which is 50 or 60 Hz. In our lab for example we have rogowskis for up to 6000 A.

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