Rogowski coil with integrator - Page 1
Forum section: electronics / projects

author: 13
author_rank: Contributor
timestamp: July 21, 2018, 03:06:55 am
content: Hello,I have recently made several shielded Rogowski coils as shown. The coils themselves are small gauge enameled wire (26-30 AWG) wrapped around the inner insulation of the coaxial cable. One side of coil is soldered to the inner conductor and the other end is wrapped around the exposed shield of the coax. I wrap the entire coil using aluminum tape and connect it to the outside conductor of the coax for shielding. Copper piping to shield the coil could be used; however, copper is rigid and hard to bend. The shield helps a lot with noise as without it you can pick up a spark in another room or even, in my case, lightning from a storm. I also put a wire mesh on the outside of the coil to increase durability of the coil. To test the coils, I fed a 120v 60Hz 8-amp current through the coils with some great results (shown in pictures)! The first coil I made did not have a shield and was too noisy to see even 150 amps. The ultimate goal of this project is to measure pulsed DC current in the order of tens of thousands of amps. I have a good idea on how to make coils but when it comes to quantifying the signal from them I am struggling. I know that to read the current you need some type of integrator. I have done some research into operational amplifier integrator but from what I can tell they only have a narrow frequency span for operation. Does anyone have an idea of how I might go about integrating the signal and eventually attempting to "calibrate" the coil/ integrator?

author: David Hess
author_rank: Super Contributor
timestamp: July 21, 2018, 11:44:03 am
content: Integrators made from operational amplifiers are limited in bandwidth due to frequency compensation requirements. For higher bandwidth, a transconductance amplifier is used with the capacitor on the output to ground. With some cleverness in the form of a resistor in series with the inverting input replacing the feedback resistor, a fast current feedback amplifier can be used as an integrator.Check out figure 58 on page 22 of theOPA860 datasheet.

author: PartialDischarge
author_rank: Super Contributor
timestamp: July 21, 2018, 01:17:21 pm
content: With rogowski coils keep in mind that slew rates of the current are more important than the absolute value of currents itself. Also, since they cannot measure DC, they suffer from droop in the measure of pulses.The following pic shows a 50A 5us pulse with less than 1us rise time (yellow trace), measured with a A6303 and AM503 amplifier. The blue output from the rogowski is in the volts range not in the mV range like yours, not because of the current (50/8 ratio) but due to the rise time.If you want to measure current pulses to measure against your setup get a A6303, maybe a CT-5, or a low ohmic current shunt or coax shunt with amplifier of course.

author: splin
author_rank: Frequent Contributor
timestamp: July 22, 2018, 03:08:10 am
content: PEM make Rogowski coils with bandwidths up to 50MHz or more. A good start might be to have a look at one of their application notes which explains the characteristics and limitations:http://www.pemuk.com/Userfiles/CWT/CWT%20-%20Technical%20notes%20-%20001.PDFThe publications part of their website unfortunately doesn't include any papers but they might provide copies on request.The output of a Rogowski coil is proportional to the rate of change of the current in the conductor it encloses, therefore you need an integrator to convert it to a voltage proprtional to current. You can use a passive integrator, which can be as simple as a single resistor, but an active version using an opamp is likely to give better results. You can use the conventional inverting opamp integrator but it suffers a number of problems which means a non-inverting integrator configuration can offer much better performance as described here:http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.476.7981&rep=rep1&type=pdfYou haven't said what your bandwidth and low frequency requirements are but the large currents help as the sensitivity of the sensor can be very low requiring few turns which have low inductance and capacitance and thus a high bandwidth.An issue with your coil is that the central conductor of most coax is relatively thick which will increase the capacitance to the coil and screen(s) which will limit the high frequency performance. You may be better off winding your coil around silicone tubing which you can buy cheaply off Ebay and using a very thin return conductor wire.An alternative screening/return conductor arrangement which improves performance is briefly described here:http://www.milimsys.com/images/product/PEM/milim_PEM_CWTMini_NextGenerationProbes.pdf

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 22, 2018, 03:46:03 am
content: Uhh... how exactly do you slit a braided shield?You also didn't mention if the foil tape is connecting together or not (where it overlaps itself). Sometimes it connects, sometimes it doesn't (even with "conductive adhesive" tape).Methinks you'll have some response at mains frequencies, but a hard time resolving anything in the >10kHz range?Tim

author: floobydust
author_rank: Super Contributor
timestamp: July 22, 2018, 04:03:45 am
content: TI Designs Active Integrator for Rogowski Coil Reference Design With Improved Accuracy for Relays and BreakersWalks you through the integrator design, for either fast settling or precision.

author: coppice
author_rank: Super Contributor
timestamp: July 22, 2018, 06:50:34 pm
content: Quote from: floobydust on July 22, 2018, 04:03:45 amBe careful with that design. Its phase accuracy is poor. Its a perfectly good design for its intended purposes - relays and breakers - where you only need to measure current. If you want to correlate the signal with voltage, to measure power, is not such a great solution.

author: David Hess
author_rank: Super Contributor
timestamp: July 22, 2018, 07:56:37 pm
content: QuoteThat TI design is intended for AC power line applications.

author: 13
author_rank: Contributor
timestamp: July 23, 2018, 11:22:25 pm
content: Thanks for all the great advice! I am going to order a couple OPA860 to mess around with the circuit they provide. I will post pictures of pulses through the coil as soon as I do another high current discharge (forgot to set the oscilloscope trigger last time).

author: 13
author_rank: Contributor
timestamp: July 23, 2018, 11:34:14 pm
content: Quote from: T3sl4co1l on July 22, 2018, 03:46:03 amThe foil tape is connected electrically along the entire length of the coil underneath the braided shield and is not relying on the tape's glue to conduct. I am pretty new to Rogowski coils and how they are designed especially for high frequencies. Does a slit along the length of the shield allow for the current pulses to be read easier? I will still try to hook the shielded coil up to my pulse generator to see if anything happens.

author: David Hess
author_rank: Super Contributor
timestamp: July 24, 2018, 01:12:40 am
content: Quote from: 13 on July 23, 2018, 11:22:25 pmThe OPA860 is much faster than you are likely to require so a slower part like the Linear Technology 100MHz LT1228 might be easier to manage. There is also the much slower and cheaper 2MHzLM13700for experimenting with the idea.

author: David Hess
author_rank: Super Contributor
timestamp: July 24, 2018, 01:23:44 am
content: Quote from: 13 on July 23, 2018, 11:34:14 pmThe slit prevents the shield from acting as a shorted turn. The EMI sniffer probe in Linear Technology application note 118 has the same feature as shown below.

author: CopperCone
author_rank: Super Contributor
timestamp: July 24, 2018, 02:36:03 am
content: i want to build the rogawski coil PCB. Have any of you done this before? I was disapoined by the one I wound. I wanted a bandwidth of like 200MHz to capture accurately fast pulses say 70MHz.Since its perfectly printed by a machine, do you still need to put the winding that loops back between the coil?

author: 13
author_rank: Contributor
timestamp: July 24, 2018, 11:29:11 pm
content: Quote from: CopperCone on July 24, 2018, 02:36:03 amI did come across a few PCB style Rogowski coils when I was researching I'll link them in.hereandherealsohereNot so sure about the winding...

author: CopperCone
author_rank: Super Contributor
timestamp: July 24, 2018, 11:39:42 pm
content: that feedback winding drives me insane. its paralyzing.

author: coppice
author_rank: Super Contributor
timestamp: July 25, 2018, 12:39:05 am
content: Quote from: CopperCone on July 24, 2018, 02:36:03 amWho offers this perfection you speak of?A lot of PCB based Rogowski coils are used, but they can be highly problematic. Most people trying to use them for precision measurement eventually give up. Those trying to use them for rough and ready measurements, like protection systems, are generally more successful. There are a couple of companies whose main thing is getting PCB based Rogowski coils to behave well.

author: CopperCone
author_rank: Super Contributor
timestamp: July 25, 2018, 01:01:20 am
content: Won't it generally work better then something hand wound? By perfection I meant a printer/etch.. its really accurate compared to hand assembly. I thought about trying to get a indexer and use a file to make something but I don't know. Laser printer/photoexposure PCB, etched conventionally.I can't figure out how to get good windings without a 3d printer to make a trenched torroid. I thought making a PCB based one would be generally better? I wanted to make VIA's using a copperset kit.

author: 13
author_rank: Contributor
timestamp: July 25, 2018, 03:06:07 am
content: From my experience, precision windings aren’t really necessary for a Rogowski coil. Unless you don’t want to spend time “characterizing” the coil and instead rely on your math.

author: coppice
author_rank: Super Contributor
timestamp: July 25, 2018, 12:19:58 pm
content: Quote from: 13 on July 25, 2018, 03:06:07 amIt depends on the accuracy and tolerance of external fields you are looking for. If you are trying to measure to a fraction of a percent, thermal expansion can be enough to spoil measurements over a cold to hot day range.

author: CopperCone
author_rank: Super Contributor
timestamp: July 25, 2018, 12:42:49 pm
content: how about that feedback loop back winding thats a pain in my ass?

author: 13
author_rank: Contributor
timestamp: July 27, 2018, 12:10:23 am
content: Update:I did a few low current discharges to compare a Rogowski coil with a slit shield and a coil with a closed shield. The first photograph is the voltage from the coil with the slit shield and the second photograph is how I slit the copper tubing. The third photograph is from the shielded coil. Note that the Rogowski coil with the slit is not as sensitive as the shielded coil which resulted in different amplitudes. From what I can tell the slit has minimal effects on pulsed current applications or maybe the slit location matters? However, I will slit the coil in the final design as long as the noise is acceptable. I plan on winding a coil around a much smaller diameter wire to decrease capacitance and increase the frequency range. For the integrator I have ordered the parts and will spin up a PCB while I wait for them.

author: 13
author_rank: Contributor
timestamp: July 27, 2018, 12:13:01 am
content: Here is the picture of the shielded coil output! It was too large for one post

author: PartialDischarge
author_rank: Super Contributor
timestamp: July 27, 2018, 05:51:59 am
content: Quote from: 13 on July 27, 2018, 12:13:01 amIs that the output from a typical RC discharge? If so something is quite wrong, except for the sinusoidal case the current going through a rogowski and its output look nothing alike

author: floobydust
author_rank: Super Contributor
timestamp: July 27, 2018, 06:19:45 am
content: I thought it is strictly an electrostatic shield. So it needs a slit to not interfere with the B-field of interest. It could be copper/aluminium foil with an insulating layer- anything that does not make a conducting loop overtop the coil. Otherwise I wonder if it acts like a shorted-turn.Rogowski coil methods patentsee pics regarding shielding and why the inventor went differential dual-Rogowski.Making Rogowski coils from multilayer PCBs attached to a base board- interesting pics.

author: 13
author_rank: Contributor
timestamp: July 27, 2018, 06:53:04 am
content: Quote from: MasterTech on July 27, 2018, 05:51:59 amNo, I forgot to say I have an inductor in series with the cap! I was messing around with inductors from my junk bin to see which one produced the longest pulse.