Hackaday Prize 2023: Abuse A Reference Chip For A Cheap Instrument

A Rogowski coil is a device for measuring AC current that differs from a conventional current transformer in that it has no need to encircle the conductor whose current it measures. They’re by no means cheap though, so over time we’ve seen some interesting variations on making one without the pain in the wallet. We particularly like [Stephen]’s one, because he eschews exotic devices for an interesting hack on a familiar chip. He’s taken the venerable TL431 voltage reference chip and turned it into an op-amp.

We had to look at the TL431 data sheet for this one and shamefacedly admit that since we’d only ever used the chip as a voltage reference, we hadn’t appreciated this capability. In this mode, it’s a op-amp with the inverting input connected to a fixed rail, so it can accept a feedback network to its non-inverting input just like any other. He’s using it as both integrator and amplifier, as well as, of course, in a more conventional power supply.

We like the instrument, and the use of the TL431 in an unexpected manner is the cherry on the cake. Here’s a previous Rogowski circuit using more conventional parts. You can dive a bit more into the theory, too.

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.

Measuring The ~10 Kiloamp Output Of A Large Capacitor Bank

[Norman] put together a rather impressive 22,500 uF capacitor bank. In addition to find things to torture with the strong magnetic field generated by a sudden discharge, he’d like to measure the current pushed from the device. He’s found a way to do this using a digital storage oscilloscope. To protect the oscilloscope [Norman] built his own interface box that includes a 50x voltage divider, and interfaces a current sensor called a Rogowski coil. When it comes time to run the experiment, he turns the safety lock-out key on the bank charger, then discharges the stored potential with the flip of a switch.

Take a look at the video after the break to see soda cans and hard drive platters mangled by the device. The oscilloscope measures the output near 10 kA, giving [Norman] the data he set out to capture. He’s entered this project into the Tektronix contest where it’ll compete with the piano tuner and laser light show tester just to name a few.

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