Mains current measurement with Halleffect sensor? - Page 1
Forum section: electronics / projects

author: Windows9Professional
author_rank: Contributor
timestamp: September 15, 2019, 11:31:40 am
content: Since I own a scope, I kind of always wanted to measure mains current. Today I asked myself, If it would be possible to make a reasonably accurate current sensing extension cord with a BNC connector for my scope.Now I just wanted to ask, If measuring current with a halleffect sensor has any flaws, or is as easy as it sounds likeI already created a list for the components I need:One cheap extension cord (with phillip screws)A halleffect sensorA BNC connectora powersupply for the sensor (probably some 12VDC LED power supply which I will hide inside the extension if possible)(and also a accurate voltage regulator)I already have the coil, but I'm not quite sure if 15 turns of 1mm insolated copper wire around a 1cm (diameter) ferrite core will do it. It generates near to no heat at 5A continuously.So, is this as easy as it sound or are there some flwas in my plans?

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 15, 2019, 11:35:53 am
content: Flaws? Well, the Hall effect sensor requires power and a bog standard current transformer doesn't. You do get better high frequency bandwidth from the H.E. sensor if you are interested in the harmonic distortion of a load, but otherwise the current transformer is overwhelmingly preferred here.EDIT: wait, whut?Quote from: Windows9Professional on September 15, 2019, 11:31:40 amWhat coil are you talking about here?

author: Kleinstein
author_rank: Super Contributor
timestamp: September 15, 2019, 11:41:39 am
content: The system with a hall effect sensor works about as describes. The main downside of Hall effect sensing is that near by iron or other magentic material can effect the reading and there can be an offset, as the hall voltages are quite small. The sensor-circuit man also pic up some other signals.For AC current, there is an even simpler variation, with a current transformer. This does not even need a power supply. Clamp on amp-meters work this way.A 3rd way for measuring is a Rogowski coil - kind of inductive pic-up without a core. However this usually needs amplification signal forming.

author: Windows9Professional
author_rank: Contributor
timestamp: September 15, 2019, 11:42:32 am
content: Thank you for your quick reply.With flaws I meant something like f.e. a HES has non-linear output (which it luckily hasn't)Yes, I am more or less interested in higher resolutionSo, thank you for your reply

author: Windows9Professional
author_rank: Contributor
timestamp: September 15, 2019, 11:48:13 am
content: Quote from: Kleinstein on September 15, 2019, 11:41:39 amLike really low power signals (like RF) or nearby power lines in the wall?A current sensing transformer would only work with AC and I planned to calibrate it with DCI don't think I will go with the 3rd option, just because it is more complicatedThanks for your reply

author: Windows9Professional
author_rank: Contributor
timestamp: September 15, 2019, 11:53:07 am
content: Quote from: MagicSmoker on September 15, 2019, 11:35:53 amSome copper wrapped around a ferrite core (not that one out of a transformer, just a ferrite stick)But I can easily increase the turns on it if the magnetic field wouldnt be strong enought

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 15, 2019, 01:15:37 pm
content: Quote from: Windows9Professional on September 15, 2019, 11:48:13 amThis makes no sense, really. A mains-frequency current transformer won't need calibration since its output will be as precise as the value of the burden resistor and the turns ratio, and will range in price from 3-15 Euros depending on form factor and current range. For example, this part from Triad Magnetics has a 1:500 ratio and max primary current of 30A for $3.37:https://www.digikey.com/product-detail/en/triad-magnetics/CSE187L/237-1103-ND/242546Just wire a burden resistor across the output to convert the secondary current into a voltage and read directly with your scope. For example, a 50 Ohm burden resistor will give 3V out with 30A through the primary. Easy-peasy.

author: Windows9Professional
author_rank: Contributor
timestamp: September 15, 2019, 04:44:59 pm
content: Quote from: MagicSmoker on September 15, 2019, 01:15:37 pm... Oh, you're right.. To be honest, I didn't notice that bevore...Does a CST blur out all detail of the current waveform or is there at least some detail? (f.e. when measuring a switch-mode power supply, there's some switch noise as far as I know)Edit: the example above is bullshit... I mean when measuring f.e. an iron-core transformer

author: schmitt trigger
author_rank: Super Contributor
timestamp: September 15, 2019, 05:00:15 pm
content: The downside of a CT, is that it can’t measure DC, or AC with a DC component. Otherwise they are very sturdy and don’t require external a power source.Cheap CTs will also have some bandwidth limits, but there are good wideband CTs, although they are not cheap.https://www.pearsonelectronics.com/To your idea of hall-cell sensing; Allegro Microsystem and others already have such devices. The beauty of such devices is that it incorporates the features and processing circuitry required to make an accurate IC current sensor.

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 15, 2019, 09:53:25 pm
content: Quote from: Windows9Professional on September 15, 2019, 04:44:59 pmThe cheapest current transformers with plain old laminated silicon steel E cores (like the one I linked to previously) won't have terribly high bandwidth so wouldn't be ideal for observing the waveform into something that draws peaky currents with high harmonic content, like a switcher. If that's what you want to do then a Hall effect sensor intended for mains applications like the ones from Allegro that @schmitt trigger mentioned would be better.