Short Term Logging of Mains AC Current - Page 1
Forum section: electronics / projects

author: David_AVD
author_rank: Super Contributor
timestamp: October 13, 2014, 12:41:20 pm
content: Is there an off-the-shelf solution for measuring AC current (mains potential) and logging (graphing) it with reasonable detail over a short period of time?What's required is to measure the inrush current (in the 10A to 50A range at a guess) of conventional transformers and possibly switch-mode power supplies.I would expect the measurement period to only a few seconds for each capture. Perhaps a current transformer and a DSO that can export is all that's required?

author: David Hess
author_rank: Super Contributor
timestamp: October 13, 2014, 01:16:07 pm
content: Quote from: David_AVD on October 13, 2014, 12:41:20 pmThis is how I would do it and have done it in the past. Some multimeters have logging and export capability into the 1000s of measurements per second or higher and would be more suitable.

author: denelec
author_rank: Regular Contributor
timestamp: October 13, 2014, 11:40:20 pm
content: I bought the "Watt'up Pro" a few years ago It works well.Ideal for measuring and logging the consumption of an equipment over a period of time.https://www.wattsupmeters.com/secure/products.php?pn=0&wai=376&spec=4There is a 250V version with Australian, UK and European plugs.The precision seems to be good for a $130 device.You can log voltage, current, power, duty-cycle, power factor, etc.You can export the data in Excel.

author: denelec
author_rank: Regular Contributor
timestamp: October 13, 2014, 11:48:50 pm
content: But if you want to measure inrush current, this won't do.The fastest it can go is 1 sample per second.A work we use Hioki Power Analyzers but they cost thousands of $.I think a Digital Storage Oscilloscope to measure the current and voltage would be the best and simplest solution.

author: David_AVD
author_rank: Super Contributor
timestamp: October 13, 2014, 11:51:50 pm
content: Yeah, I'd need to see the current with much finer resolution than a logger would do. The thread title doesn't convey that well.

author: David_AVD
author_rank: Super Contributor
timestamp: October 14, 2014, 01:54:27 am
content: I'm looking at getting a Rigol MSO-2072A as the logic channels may be useful to capture analog events.It seems to have reasonable reviews, but I couldn't find one from Dave?

author: David Hess
author_rank: Super Contributor
timestamp: October 14, 2014, 02:30:03 am
content: Quote from: David_AVD on October 14, 2014, 01:54:27 amLook for a review of a DSO in the same series. Within one series they are identical except for options and bandwidth.A DSO can certainly do what you want when combined with a current transformer.

author: simingx
author_rank: Contributor
timestamp: October 14, 2014, 04:41:08 am
content: Quote from: David Hess on October 14, 2014, 02:30:03 amIf you don't need a DSO($$) + current clamp($$$), then one of these may be cheaper:http://www.fluke.com/fluke/m3en/clamp-meters/Fluke-374.htm?PID=70402It has an inrush current mode to capture the peaks when they occur.

author: tautech
author_rank: Super Contributor
timestamp: October 14, 2014, 04:48:45 am
content: Quote from: David_AVD on October 13, 2014, 11:51:50 pmHow fine resolution???Inrush to what?Expected average?There are several easy solutions just depends on precisely what you need.

author: David_AVD
author_rank: Super Contributor
timestamp: October 14, 2014, 05:36:42 am
content: I'd like to see the current draw on a cycle by cycle basis over a few (maybe 10 max) seconds.By finer resolution I was meaning not sampling as infrequently as a logger would (not better than 1Hz ?).The steady current (after the inrush) is around 10 Amps max.

author: David_AVD
author_rank: Super Contributor
timestamp: October 14, 2014, 05:39:54 am
content: Quote from: David Hess on October 14, 2014, 02:30:03 amThanks for that. I'll check out the DSO2xxx series for reviews. I don't currently own a DSO.I figured I may as well get the DSO version as the logic capability would be useful for other projects.

author: tautech
author_rank: Super Contributor
timestamp: October 14, 2014, 05:55:12 am
content: I'd use a scope current probe and record with a DSO because I have both.As have been mentioned a CT will also do the job, and often can be acquired from old PCB's in your parts pile.You could also get what you need with a current shunt and a differential probe setup to a DSO.If your present gear serves you well enough maybe just a 2 CH USB scope for this job?

author: David_AVD
author_rank: Super Contributor
timestamp: October 14, 2014, 10:47:16 am
content: Any recommendations on a CT that is suitable for direct connection to the DSO / MSO input ?Something that can measure up to 100A would be nice.

author: RedOctobyr
author_rank: Contributor
timestamp: October 14, 2014, 01:19:30 pm
content: Quote from: tautech on October 14, 2014, 05:55:12 amI'm new at this stuff, so apologies if I'm way off-base. But I recently ordered an inexpensive USB scope, and one of the things I have in mind is measuring some brief startup current spikes for small DC motors.Some Googling brought me to current shunts (shunt resistors), which sound suitable for my needs. From what I found on eBay, they come in a range of current capacities, say 3A to 1000A, and seem to have a max voltage drop of 75mV, regardless of the amperage rating.So you'll need to be able to accurately measure voltage differences which are quite small. My little USB scope may not have enough voltage resolution when looking at signals (or signal differences) which max out at 75mV. From what I gather, you can use 2 normal probes, just with one on either side of the shunt. I paid about $4 for a 20A shunt on eBay. The 100A versions were maybe $15 or something, but still not that expensive.I don't know whether shunts lose accuracy if you briefly push them past their rating. Use a 10A/75mV shunt to measure a 20A startup load, and do you simply (and accurately) measure a 150mV drop, instead of 75mV? Or do things get all non-linear and inaccurate? If you measure 150mV, that could be a crude way to provide a larger signal to measure, for use with less-sensitive instruments.If my shunts could also be used for AC measurements of startup currents, that's good to know.

author: jahonen
author_rank: Super Contributor
timestamp: October 14, 2014, 02:32:23 pm
content: I measured inrush current of an amplifier I built some years ago with Agilent MSO6034A and Fluke 80i-110s current probe:Worked just fine. I suppose that you'll be fine using Rigol DSO for this purpose.Regards,Janne

author: David Hess
author_rank: Super Contributor
timestamp: October 14, 2014, 05:16:20 pm
content: Quote from: David_AVD on October 14, 2014, 05:39:54 amThe DS2000A series is massive overkill for your current logging application but inexpensive even so. A DS1000Z series unit would still be overkill but cost less and they have 4 channels and an optional logic analyzer as well.

author: David_AVD
author_rank: Super Contributor
timestamp: October 14, 2014, 08:34:39 pm
content: Quote from: David Hess on October 14, 2014, 05:16:20 pmYeah, but I will need something with those specs and functionality for other jobs in the pipeline so may as well buy it now and buy it once.I found the Fluke i200S switchable 20A/200A probe which seems like a good fit. Now to find it at a reasonable price.

author: David_AVD
author_rank: Super Contributor
timestamp: October 14, 2014, 09:10:33 pm
content: Finding a current probe at a reasonable price (considering the limited use it will get) is proving difficult.I did see thisCT from Ocean Controlswhich I guess could be adapted to work with the DSO/MSO ?

author: os40la
author_rank: Regular Contributor
timestamp: October 14, 2014, 09:33:49 pm
content: http://www.theenergydetective.com/5000cI bought this one for my house. Works great. 1watt resolution at 1sec. I may have missed it but you didn't say exactly what resolution you were looking for.They even sell the current probes separately for $47 for a set of 2http://www.theenergydetective.com/set-of-current-transformers-cts.html

author: David_AVD
author_rank: Super Contributor
timestamp: October 14, 2014, 10:54:16 pm
content: Resolution required is cycle by cycle.

author: eneuro
author_rank: Super Contributor
timestamp: October 15, 2014, 10:19:53 am
content: Quote from: David_AVD on October 14, 2014, 10:47:16 amHow long those 100A peak pulses are, while it is at 230VAC 23kWIn my spot welder project I used for 230VAC mains monitoring HALL effect based current sensor which at 5 turns gives me +/-25Amax range with sensor response about 100mV/1A @ 5V sensor supply voltage with 0A output at 2.5V.Which is accuracy you want to measure those high current peaks close to 100A +/-0.1A or better ?Cycle by cycle @ 50Hz is 20ms-maybe a few moving average (http://en.wikipedia.org/wiki/Moving_average) I used in my current sensor could do the trick depending on how long are those pulses you want to detect?I decided custom design it with my own source code for small DIP8 Attiny85 for post processing measurement data, while I have full control how incoming data is taken and this can do exactly what I want depending on project requirements with a few changes in C source code between projects, while main code and functions are ready and tested.

author: David_AVD
author_rank: Super Contributor
timestamp: October 15, 2014, 11:25:12 am
content: OK, "cycle by cycle" was bad wording on my part. What I meant was "I want to see the actual current waveform" as opposed to getting some type of slow logging.If I could end up with a measurement rig that allowed for 100A peak reading but resolution down in the 100mA area that would be great.I have no desire to make something to capture / display the data. I'll simply buy the DSO/MSO and get on with it.Does anyone know any reason the $40 CT I linked to wouldn't be suitable?

author: David Hess
author_rank: Super Contributor
timestamp: October 15, 2014, 01:47:10 pm
content: Quote from: David_AVD on October 15, 2014, 11:25:12 amDo you mean measuring peaks to 100A with 100mA resolution or being able to accurately measure peaks to 100A while also delivering useful data at 100mA full scale? The later is much more difficult.100A by 0.1A is 1 part in 1000 which is going to require either an oscilloscope in high resolution mode (most do these days) or a datalogger which would could typically sample 1000+ times per second but still deliver 12+ bit accuracy.QuoteIt is designed to work into a resistive load of less than 10 ohms which sets the burden voltage so the maximum output at 120 amps will be 400 millivolts which is plenty for an oscilloscope and should allow a full scale range down to 1.5 amps or lower.There are also inexpensive current clamps designed to interface directly with an oscilloscope.

author: eliocor
author_rank: Unknown
timestamp: October 15, 2014, 03:14:26 pm
content: I suggest youLEMproducts: they work flawlessly, have a very wide bandwidth and are not too expensive.Very easy to connect to a digital scope.Other than the devices shown at the link, they produce much more current monitoring devices... take a look at their production:http://www.lem.com/hq/en/content/view/13/86/

author: eneuro
author_rank: Super Contributor
timestamp: October 15, 2014, 06:25:54 pm
content: Quote from: eliocor on October 15, 2014, 03:14:26 pmhttp://www.gmw.com/electric_current/LEM/hall-transducers.htmlQuoteUnfortunatelly you can not tweak LEM's internals, while when I used 2$ SS495A Hall sensor and $1.5 16Mhz ATTiny85 with its 10bit ADC it is quite easy do specyfic tasks in software and with ADC set to 250kHz close to 20000 current samples per second, which combined with moving average is fine in many applicationsIt has advantage we know how those integrations are made since we have MPU's C source code and can monitor for example AC mains and triger external events when ADC conversion takes place, so we know exact timings and its internal processing program flow from analysis of assembler instructions generated in AVR binary.For example this 25A LEM for about $22http://www.digikey.com/product-detail/en/LTS%2025-NP/398-1002-ND/409815has25mV/Asensitivity @ 5V power supply, so it means that only 0.625V difference for 25A currenthttp://www.lem.com/docs/products/lts%2025-np.pdfEven with my SS495A and about 100mV/A I have close to +/-2.5V voltage difference at the same 5V power supply in th ecase of 25A current...Nope, I do not like this LEM while yep according to his datasheet linked above:QuoteIt looks like that maybe they ... make the same thing for higher currents while for 100A with 25mV/A we get: 2.5+/-2.5VAnyway, maybe they are really good-I do not need them

author: eliocor
author_rank: Unknown
timestamp: October 15, 2014, 07:28:11 pm
content: But have you tried them or are you just talking about?Almost every good current sensor uses a feedback to measure the real current passing through (to compensate the non linearities of the sensor).Did you characterize your LEM sensor? BTW, what was the model you were using?Otherwise you can use aRogowski coil...BTW, the OP asked for a way to measure inrush current and never stated it was to be acquired in industrial quantities: it seems he needs only one sensor, so the price argument should be not too sensitive.

author: johansen
author_rank: Super Contributor
timestamp: October 15, 2014, 08:13:20 pm
content: Quote from: eneuro on October 15, 2014, 06:25:54 pmyou can indeed.i cut the core in half to make it openable.its just 2000 turns on the toroid.the location of the turns was engineered i'm sure, but i could just as easily replace the 2000 turn iron core with a 100 turn winding on a toroid with a slot cut for the hall effect sensor (i have some 125u metal powder cores)

author: David_AVD
author_rank: Super Contributor
timestamp: October 15, 2014, 08:13:56 pm
content: Quote from: David Hess on October 15, 2014, 01:47:10 pmSorry, I meant 100A FS with an accuracy of 1A resolution for the peak/inrush measurement, or 100mA resolution down at the steady current of around 10A. So, 100:1 in both cases.Quote from: David_AVD on October 15, 2014, 11:25:12 amQuote from: David Hess on October 15, 2014, 01:47:10 pmI might grab the $40 CT for now, add a resistor and BNC tail on it and see how that goes.Quote from: David Hess on October 15, 2014, 01:47:10 pmThere is a huge variation in current clamp pricing. The more expensive ones seem to offer switchable ranges and lower minimum currents, etc.

author: eneuro
author_rank: Super Contributor
timestamp: October 15, 2014, 10:36:42 pm
content: Quote from: johansen on October 15, 2014, 08:13:20 pmNice teardownI mean non destructive way to change its software, but there is no MCU I guess.Quote from: eliocor on October 15, 2014, 07:28:11 pmThis LEM from teardown above looks like have no MCU inside.25mV/A means that when we use AVR with 10bit ADC and 5Vref, than we have about 5mV/LSB,so only 0.2A/LSB, while when I have 100mV/A than I have 50mA/LSB in my custom SS495A Hall sensor basedMade@Homenot in ChinaNot so bad-for precision current measurements probably CTs will be better, but my interests are high power applications, so those Hall sensor based current sensors are fine and quite cheap <<$10.

author: RedOctobyr
author_rank: Contributor
timestamp: October 16, 2014, 12:48:21 am
content: I'm admittedly not quite following all of thisBut I'm really impressed by what you guys have figured out, and actually built, for doing this. That is awesome.I wish I had the knowledge and talent to build and program my own little device for a need like this. Very cool

author: eliocor
author_rank: Unknown
timestamp: October 16, 2014, 10:09:19 am
content: QuoteHave you measured the non linearities of your device?I think your measurement method is an "open loop" one which is not able to compensate for them (and also for temperature changes, ...)There are some reasons why the best sensors use a closed loop approach. But maybe you need only a coarse resolution in your measurements...

author: eneuro
author_rank: Super Contributor
timestamp: October 16, 2014, 05:30:42 pm
content: Quote from: eliocor on October 16, 2014, 10:09:19 amThis Hall effect sensor is available there for $2 if one buy 10+ , so everybody can teardown it at low costhttp://www.digikey.com/product-detail/en/SS495A/480-2005-ND/701360I do not like precision measurements, while I'm fuzzy logic and artificial neural networks fanHowever, this SS495A sensor was used in serious science experiments like this one:A three-dimensional magnetometer for motion sensing of a balloon-carried atmospheric measurement packageftp://ftp.sic.rm.cnr.it/incoming/ifa.rm.cnr.it/Maurizio.Viterbini/Public/Telem/HarrisonRogersHogan07.pdfI will put somexscopewaveforms, while I used it in spot welder mains transformer primary current sensing and monitoring in recent project.

author: eliocor
author_rank: Unknown
timestamp: October 16, 2014, 09:56:41 pm
content: maybe cheap, but you need to cut the toroid by yourself and to calibrate the sensor...Sometime it can be cheaper to spend a little more and get a turnkey (and already calibrated) solution.

author: eneuro
author_rank: Super Contributor
timestamp: October 16, 2014, 10:17:41 pm
content: Quote from: eliocor on October 16, 2014, 09:56:41 pmI use also non toroid versions for high currents-than all one have to do is solder this sensor and well designed PCB tracks do the trickIt costs less than $1 to manufacture such PCB in China, so $3 per Hall current sensor module-not so bad

author: eliocor
author_rank: Unknown
timestamp: October 16, 2014, 10:32:03 pm
content: I know them: also sold by LEM, but too insensitive for my needs. Only useful for high currents.They are also easier influenced by external magnetic fields: in some situations their readings are useless.

author: eneuro
author_rank: Super Contributor
timestamp: October 17, 2014, 05:04:59 pm
content: A few turns of 2.5mm2 wire used in home grid mains and hot air to glue everything together and we have quite nit 57mV/A current sensorEstimate calibration is easy-simply used 12V car battery and at 9A current (two old halogen light bulbs 55W 12V in parallel as dump load) voltage difference from 2.5V (null zero) was about 0.51V, so I have bigger resolution in AVR 10bit ADC than with those LEM 25mV/AAdditionally this current sensor is ready to... put it into 230VAC mains wall socket and monitor loads close to 10kW, while at 40A positive currents will go to about 4.8V @ 5Vref