100uR 1+W current sense resistor - Page 1
Forum section: electronics / beginners

author: Simon
author_rank: Unknown
timestamp: July 09, 2015, 08:56:18 am
content: I'm looking to do some high amperage current measurements (up 100A) with a shunt resistor, any ideas where I can get one. The ones i find don't have great specs often 5% and well over 100ppm. I can easily use a higher wattage value to try and keep in spec.Note: i have tried a current clamp and I'm not happy so don't tell me to use one.

author: German_EE
author_rank: Super Contributor
timestamp: July 09, 2015, 09:01:58 am
content: I faced this problem a few months ago. Pull apart an old clock and extract the main spring. Three 5cm lengths of this in parallel clamped at both ends provided about 1,2 mV per amp, you then add a series resistor in your meter lead for calibration to bring this down to a 1 mV per amp ratio.

author: Simon
author_rank: Unknown
timestamp: July 09, 2015, 09:05:33 am
content: I'm considering using wire, but I can't calibrate it 5AWG cable has 0.001028R per metre so just under 100mm of that will do it and I can even do a twisted pair to get rid of inductive effects, or maybe a larger guage. But I don't know how accurate AWG wire cross sections are and it may depend on wire coatings etc and of course more heat more resistance as this is at 25C,

author: krish2487
author_rank: Frequent Contributor
timestamp: July 09, 2015, 09:08:12 am
content: Try a proper current shunt.There are shunts available from ohmite.0.5% tolerance and 30ppm tempco.Look at the S and SH Series.

author: rs20
author_rank: Super Contributor
timestamp: July 09, 2015, 09:11:02 am
content: You shouldn't care about the tolerance at all, you only care about the tempco. You can calibrate any tolerance in the error out trivially (either in software if this is going via software, or with the voltage divider if you absolutely must have the correct analog voltage in-circuit, which I very much doubt you actually need). Dave regularly mentions this in multimeter reviews.

author: Simon
author_rank: Unknown
timestamp: July 09, 2015, 09:13:35 am
content: The problem is I have nothing to calibrate with, I have nothing to measure 100A with I can add up both fluctuating current readings from 2 power supplies that I hardly trust.I need ideally to buy something that come warranted to work as mentioned above

author: TMM
author_rank: Frequent Contributor
timestamp: July 09, 2015, 09:16:14 am
content: Quote from: Simon on July 09, 2015, 09:13:35 amYou don't need to calibrate it at 100A if you can ignore the self heating effects at 100A.edit: might have been unclear - you can calibrate the resistance of the shunt at a lower current.

author: Simon
author_rank: Unknown
timestamp: July 09, 2015, 09:34:20 am
content: Yes i could flow 10A and use my meter as a reference

author: Pillager
author_rank: Unknown
timestamp: July 09, 2015, 10:15:38 am
content: Some of these sound quite promising:http://www.isabellenhuette.de/en/precision-and-power-resistors/products/And Mouser has a nice one:http://www.mouser.at/ProductDetail/Vishay/WBPK600L0A00010001/?qs=kcL3wPdn6%2f2VZ5DGC96cKQ%3d%3d

author: Simon
author_rank: Unknown
timestamp: July 09, 2015, 10:37:39 am
content: hm not for 87 quid, I was hoping some something chuncky rather than surface mount although I suppose i can solder it to copper bar with a hole in for screw terminals.

author: Zero999
author_rank: Super Contributor
timestamp: July 09, 2015, 10:41:34 am
content: Why are you even considering a resistor?A Hall effect transducer would probably be much more suitable.

author: Simon
author_rank: Unknown
timestamp: July 09, 2015, 10:42:29 am
content: hm thats true. I think i have a couple of those from Allegro

author: rs20
author_rank: Super Contributor
timestamp: July 09, 2015, 12:08:56 pm
content: Quote from: Hero999 on July 09, 2015, 10:41:34 amBeware of Hall effect transducers; one that I got is very good at detecting its orientation w.r.t. the Earth's magnetic field (and yes, this is a current-measurement hall effect chip with pins for the current). I don't recommend that part, I was really confused while debugging it until I realised that tilting the device caused massive offsets in the measurement. You *might* get away with using it for AC though, but it's dodgy in my books -- I mean, if it picks up the Earth's magnetic field so well, surely some nearby 100A AC cables will gives if some pretty decent competing fields?My v2 for the same circuit is ditching the hall effect transducer for a uCurrent-style arrangement instead -- ultra small resistance current shunt, with gain after that (I never needed the isolation in the hall effect transducer in the first place).

author: HighVoltage
author_rank: Super Contributor
timestamp: July 11, 2015, 06:55:48 am
content: May I ask, why you don't want to use a current clamp?The Tektronix A622 (same as Agilent, AEMC and more, rebranded) goes to 100A and is pretty linear and reliable.Otherwise, buy one of these cheap but large resistors from ebay:http://www.ebay.de/itm/Nebenwiderstand-Shunt-100-A-60-mV-Mess-Shunt-zur-Strommessung-/121698634939?pt=LH_DefaultDomain_77&var=&hash=item1c55cdccbbThey are pretty accurate as well and can be re-calibrated at 10A

author: Simon
author_rank: Unknown
timestamp: July 11, 2015, 06:57:04 am
content: I tried a current clamp and was 5A out on 45A. I just don't trust them.

author: HighVoltage
author_rank: Super Contributor
timestamp: July 11, 2015, 07:04:53 am
content: Quote from: Simon on July 11, 2015, 06:57:04 amWhich model of clamp did you use?The resistor in the ebay link I sent earlier will for sure work.I have the same one and used it up to 100A for precision measurements.

author: Simon
author_rank: Unknown
timestamp: July 11, 2015, 07:09:40 am
content: cant remember, some no name thing, didn't even have a display you have to plug it into a multimeter in mV mode.Mind you it was a motor driver I was measuring so plenty of emf floating around as spikes of 100's of amps get shoved in and out so might upset measurements. partly why i want a resistor and then I'll low pass filter it.

author: tszaboo
author_rank: Super Contributor
timestamp: July 11, 2015, 07:46:05 am
content: If you want that much resistance, without extra size and price, the best is to parallel smaller resistors. Now, I'm not talking about just paralleling 10 1mOhm resistor. You should use an amplifier next to each (current sense amlifier with fixed gain), and average the outputs. Use 4 wire resistors, a nice amp like INA199, 4-5 shunt each 1mOhm. It is not 100uR, but you dont really need that low, as all the cable and connection losses will be a lot already.

author: Zero999
author_rank: Super Contributor
timestamp: July 11, 2015, 07:50:59 am
content: Quote from: Simon on July 11, 2015, 06:57:04 amDid you zero it first?And this may seem silly but many current clams are current transformers and only measure AC: are you sure it was a Hall effect sensor, designed for DC use?