Small, light DC current sense. - Page 1
Forum section: electronics / projects

author: Dubbie
author_rank: Unknown
timestamp: February 08, 2016, 06:25:30 am
content: Hi all,I'm working on a design for a board that needs to incorporate current sensing. The catch is, I need to measure current up to about 200 amps at 48V. There are a lot of Hall effect sensors but they are all pretty large and heavy. I found an ideal sensor from ALPS here:http://www.alps.com/e/news_release/2013/0711_01.htmlThe problem is, it only is rated up to 50AThis is to go on a UAV, so weight is the primary consideration. Accuracy only needs to be about 0.25%.Can anyone suggest a product or even an alternate measurement method?Thanks!Sent from my phone using Tapatalk

author: mmagin
author_rank: Frequent Contributor
timestamp: February 13, 2016, 09:19:09 pm
content: I guess the interesting question is what do you want to do with this measurement.What's your average current?Are you okay with non-isolated sensing using a resistor and amplifier? There are 1 milliohm 4 terminal (Kelvin connection) current shunt resistors readily available.http://www.vishay.com/docs/31805/mrs.pdfThese only go up to 80A, but I suspect as long as the average current stays under that, you're fine. I'm trying to wrap my head around those kind of currents on a lightweight PC board in a UAV.(Edit, also is 4.63 grams too much? The Allegro ACS759 goes to 200 amps.http://www.allegromicro.com/en/Products/Current-Sensor-ICs/Fifty-To-Two-Hundred-Amp-Integrated-Conductor-Sensor-ICs/~/media/Files/Datasheets/ACS759-Datasheet.pdf)

author: uncle_bob
author_rank: Unknown
timestamp: February 13, 2016, 09:29:01 pm
content: HiYou are talking about roughly 10KW of power. You are after much better than 1% accuracy. You will need some fairly good stuff to do that. Are you sure you need accuracy and not just resolution?Is this 200A peak (and don't burn out), but 50A (and be accurate) as an application? That would make a big difference in what you could use.No matter how you do it, a reasonable power source (and motors) for that sort of energy will weight a *lot* more than the sensor if there is anything close to full current being used most of the time. 10 KW is a lot of power.Bob

author: rob77
author_rank: Super Contributor
timestamp: February 13, 2016, 09:51:56 pm
content: have a look at this appnote by intersil:http://www.intersil.com/content/dam/Intersil/documents/an18/an1827.pdffigure 4 in that appnote: using 100microhm shunt (basically a piece of low tempco wire - e.g. manganine ) - i think it's well suitable for your 200Amp. the 48V rail is not a problem because the opamp "floats" on top of the rail but the output is ground referenced. replace R7 and/or R9 with a trim pot and you can calibrate the circuit.i posted that circuit somewhere here in the past with a link to a different document from intersil (with the same circuit) , but that link is broken since then.

author: uncle_bob
author_rank: Unknown
timestamp: February 13, 2016, 10:15:19 pm
content: Quote from: rob77 on February 13, 2016, 09:51:56 pmHiIf I punched the numbers into Google correctly (who knows) your shunt at 0.0001 ohms would be dissipating 4W at full current (I^2 = 40000). That's quite a bit for a "small piece of wire".....Every time I've tried something like that it's been thermal runaway time. Wire glows brighter and brighter. Eventually it stops glowing. Yes, that's empirical data speaking.Bob

author: Marco
author_rank: Super Contributor
timestamp: February 13, 2016, 10:22:00 pm
content: Quote from: rob77 on February 13, 2016, 09:51:56 pmPresumably he can just spend the cash to get a LTC6102 instead. Saving a buck doesn't seem worth it here. Auto-zero high side current sense amplifier, works up to 60V. 100 micro-Ohm shunt should be fine.As for heat, you're generating 10 kW of airflow ... divert a bit of it.Alumina PCBs help too.It's not like your switches won't be burning some power as well.

author: rob77
author_rank: Super Contributor
timestamp: February 13, 2016, 10:25:39 pm
content: Quote from: uncle_bob on February 13, 2016, 10:15:19 pmi wrote "piece of wire" not a "small poiece of wire" . a 100micro ohm will be a fairly thick manganie wire (in the milimeters range) otherwise it would be stupidly shortand you can always heatsink it (but why would you do that if you don't have to heatsink the supply wires right ? the supply wires have to carry the same 200A peak and they'll have more than 100 micro ohmsif you don't mind the tempco you could even use a section of the supply wire as a shunt...

author: uncle_bob
author_rank: Unknown
timestamp: February 13, 2016, 11:13:53 pm
content: Quote from: rob77 on February 13, 2016, 10:25:39 pmHiIt's the 0.25% accuracy spec that suggests (at least to me) the power leads may not be a real good idea for a shunt. A 1C change on a copper wire would blow your whole budget.http://www.cirris.com/learning-center/general-testing/special-topics/177-temperature-coefficient-of-copperIf you actually need a real accuracy at that level for the whole thing, you will need a pretty good shunt. If it's a full scale accuracy, you may not have quite as much pain as if you also want 0.25% on 50A (which is not the same thing at all ... seen people ask for it that way ...).Assuming half of the full scale budget goes to the shunt, you have a ~ 0.001% /C tempco for Manganin. If you don't get into strain effects, your shunt can change by 100C. Assuming that the ambient temperature (-20 ?? to +50) takes up part of that, your shunt can't heat by an awful lot. If the system runs a bit hot, the tempco is likely 4X this, that's even less margin.You need a pretty good shunt !One might question "what else is in the error budget?". We could dig into that, but I don't think that allocating the whole budget to the shunt is justified. This is all hand waving numbers so it's not that crazy to allocate things like this.Bob

author: rob77
author_rank: Super Contributor
timestamp: February 13, 2016, 11:37:35 pm
content: Quote from: uncle_bob on February 13, 2016, 11:13:53 pmagree with you, but i think the 0.25% was just pulled from a hatin such a applications (i assume it's a drone) 1-2% is more than enough, so the shunt is not that extremely critical (of course the section of the supply wire is a bit too much - but still usable to some extent with compensation in software).

author: uncle_bob
author_rank: Unknown
timestamp: February 14, 2016, 12:22:08 am
content: Quote from: rob77 on February 13, 2016, 11:37:35 pmHiI can't dispute the *need* for this or that. The drop in the *leads* will make things nutty at the "several percent" level. All we have to go on is the OP's request. Since we have not heard much from him ... not much to really go on.One could also question the 200A into a single load versus (last time this came up) ... surprise ... it's 40A into eight loads. Automobiles are not *that* sensitive to weight. They go higher voltage for this stuff to cut down on copper weight issues. Again, with no input, this is what we have to go on.Bob

author: Dubbie
author_rank: Unknown
timestamp: May 10, 2016, 10:19:38 pm
content: Oh man, somehow I wasn't alerted to all the posts on this thread. Last I checked it had none.Thanks a ton for all your input.R

author: Dubbie
author_rank: Unknown
timestamp: May 10, 2016, 10:24:08 pm
content: QuoteExactly, Most of the measuring range will be much lower current, I'm happy with reduced accuracy at higher currents.I specified 0.25% as an ideal figure, 1% would still work for my application.This is is for low volume, so the price of the components isn't too much of a worry. I am basically looking at collecting very detailed data on power consumption of the UAV while it is flying in various different situations.You are also correct in that absolute accuracy is less of an issue than repeatable precision, as data after modifications will be compared to pre-modification data.QuoteOf course you are right here.Thanks uncle_bob for your input, it's very much appreciated.