Measuring 100A DC current - Page 1
Forum section: electronics / testgear

author: gaminn
author_rank: Frequent Contributor
timestamp: June 06, 2024, 08:34:54 pm
content: Hi,occasionally I need to measure current up to 100A DC, which is obviously too much for any handheld multimeter. I found e.g. this shunthttps://www.aliexpress.com/item/1005005721758936.html. It promises +- 0.1% accuracy. I would use digital multimeter to read voltage drop accross the shunt (75 mV @ 100A) and then calculate the current. It would be nice if the shunt was 100 mV, then no calculation would be needed....Is there something better for max 100 USD that can measure 100A +- 200 mA?

author: Martin72
author_rank: Super Contributor
timestamp: June 06, 2024, 08:43:49 pm
content: Hi,It should not be more than 100mV.At 100A, this is already 10W, which heats up well and influences the measurement result, which has a corresponding effect on the accuracy.Here is a shunt that has 0.25% tolerance and 1mV/A:https://de.rs-online.com/web/p/shunts/8233589

author: BeBuLamar
author_rank: Super Contributor
timestamp: June 06, 2024, 09:55:33 pm
content: For such a high current I would use a clamp meter which should have sufficient accuracy and much easier to use.

author: Martin72
author_rank: Super Contributor
timestamp: June 06, 2024, 10:03:40 pm
content: But that is precisely the problem.Finding a current clamp that is that accurate.If you're lucky, you'll find one with 1%, which is already very accurate for a current clamp, but costs much more than a current shunt, which is even more accurate.

author: Stray Electron
author_rank: Super Contributor
timestamp: June 06, 2024, 11:00:12 pm
content: Even thebestclamp on DC amp meters (Fluke, AEMC, etc) are only rated for 3% accuracy. Forget the cheap made Chinese meters. Clamp on amp meters are pretty trivial but DC current meters are very tricky and expensive.I would use a good quality American made current shunt and meter and keep your measurement time short in order to avoid heating up the shunt. American made current shunts are almost always designed to output 50 mA at the fully rated current so a 100 Amp shunt will need to dissipate 5 Watts of power (0.050 x 100) so power dissipation and heating of the shunt shouldn't be hard to deal with.Good quality current shunts and the corresponding 50mA meters can be found cheaply on Ebay. Here is just one example, this is a 400 Amp shunt that outputs 50mA.https://www.ebay.com/itm/404932876476?itmmeta=01HZQWBXFRHYMQP7DY3KJFZ527&hash=item5e47e158bc%3Ag%3AW70AAOSwB1JkLYUd&itmprp=enc%3AAQAJAAAAwIFn1Fck%2FY3gpneHseC0olXbRZuhefV3YEwPVSqu3eK714H%2BIDsGa%2FtWRJD67lBpuoJDAtWuYwJb1eMsIigKuPFBza5ZcxyyOUx3UR7Zk3L2KoTiieJOX%2FkXvAVv%2FBVoYe0hDPwBDjJdqj7J2p6ymW13DGTtMekzmSiJNCmJ9udmJKTsGW3ZRlp5%2FxcKsQbZTx9kTueS0W2QDtJUg5Oo9giGoiyZoFe9ZwkKEreNHj%2Fc2GJfjfSsFO37aUyUv3hUYw%3D%3D%7Ctkp%3ABk9SR4TYr_z9Yw&LH_ItemCondition=4

author: Stray Electron
author_rank: Super Contributor
timestamp: June 06, 2024, 11:06:45 pm
content: Here is a 150 Amp rated 50mA shunt made by Weston (the people that used to make good quality meters) for $19.95https://www.ebay.com/itm/276027384654?itmmeta=01HZQWKWGV8E21VKAVNKFQJRAT&hash=item404483a34e%3Ag%3ATRUAAOSwTlBk8z8M&itmprp=enc%3AAQAJAAAAwL6GTXNiQgU11FXOLXudpRlvuScMCN%2FCA%2BUqNyNLt%2B1IWEiEPASvKY6sP1jIw3QSIKks6ZB3u%2FNJgxQlO%2FKKr9Y94tPs73AlTfA%2FQI0jkY6ATJ%2B43Tg3sVRUMQssyz2H8A9tQatR4C9zD2%2F9ahhAG9TD8LlR%2FGEaC0tvHh6KFZ1cW%2FWLS3V5r2BYtF3pMsobB2x74RXONa860ITg%2FnpZ2mXBgO2oV37eDahGy6FvbW0Cn29Z%2FDgxRE80Xl%2BTCFbhow%3D%3D%7Ctkp%3ABk9SR8bIz_z9Yw&LH_ItemCondition=4

author: johansen
author_rank: Super Contributor
timestamp: June 06, 2024, 11:47:31 pm
content: look for closed loop LEM hall effect sensors on ebay.this is a random datasheet i foundhttps://www.lem.com/sites/default/files/products_datasheets/lah_100-p.pdf-1db out to 200KHZ and the accuracy is .25%

author: Fungus
author_rank: Super Contributor
timestamp: June 07, 2024, 02:20:30 am
content: Quote from: gaminn on June 06, 2024, 08:34:54 pmIs there a bettermethod? Not really.Is that shunt good? I'd vote "probably".It has a free 90-day return policy so what have you got to lose?If it were me I'd spend $7 more and go for the 300A version. It'll heat up three times less and 25mV is an easier number to work with in your head than 75mV.(multiply by 4 instead of multiplying by 1.3333)https://www.aliexpress.com/item/1005005748456772.html

author: gaminn
author_rank: Frequent Contributor
timestamp: June 07, 2024, 07:58:37 am
content: Thanks, I will buy that Murata 100A 100mV shunthttps://www.digikey.com/en/products/detail/murata-power-solutions-inc/3020-01108-0/3438619,it is quite cheap on Digikey. Hopefully I will be able to screw in some banana sockets (and they will not melt...) instead of that 8-32 screws to make my measurements easier.

author: trobbins
author_rank: Frequent Contributor
timestamp: June 07, 2024, 08:26:57 am
content: Perhaps best to indicate what your voltmeter is, and its accuracy, and resolution if you want to use the shunt for say a 5A 1% level measurement, as you will be down below 10mV and sort of need 100uV sensitivity at least.

author: gaminn
author_rank: Frequent Contributor
timestamp: June 07, 2024, 08:32:51 am
content: I have Keithley 2000 or Agilent 34411A. I'm not sure what is their accuracy, also they haven't been calibrated for a long time, but I hope it will be an order of magnitude better than the shunt.

author: tooki
author_rank: Super Contributor
timestamp: June 07, 2024, 08:40:16 am
content: Quote from: gaminn on June 06, 2024, 08:34:54 pmIf you really care about accuracy, do you really think you should be buying from AliExpress?

author: gaminn
author_rank: Frequent Contributor
timestamp: June 07, 2024, 09:08:59 am
content: Quote from: tooki on June 07, 2024, 08:40:16 amThat's why I expect 0.2% accuracy from 0.1% Chinese shunt.

author: nctnico
author_rank: Super Contributor
timestamp: June 07, 2024, 09:11:39 am
content: Quote from: gaminn on June 07, 2024, 07:58:37 amPersonally I'd buy a bigger shunt (like one rated for 200A) if a current of 100A is flowing for longer than a couple of minutes. Otherwise you'll operating it at its limits and it will get quite hot. Also the temperature coefficient may add to much error when a shunt is operated near its power limit.

author: tooki
author_rank: Super Contributor
timestamp: June 07, 2024, 09:58:44 am
content: Quote from: gaminn on June 07, 2024, 09:08:59 amI like your optimism! :p

author: coromonadalix
author_rank: Super Contributor
timestamp: June 07, 2024, 10:09:41 am
content: even with 34410a 34411a or k2000 even uncalibrated you get around 0.003% precision but your shunt will never be as precise lolland yes take an 200 amp minimum to avoid driftand please be realistic of your needs and measurement capacity

author: ArdWar
author_rank: Frequent Contributor
timestamp: June 07, 2024, 10:17:51 am
content: I don't think even the best reputed shunt manufacturers offer 0.2% accuracy, much less 0.1%.It's current shunt, if you need accuracy you're expected to calibrate out for gain errors. The sought after qualities here is usually the temperature coefficient instead.

author: gaminn
author_rank: Frequent Contributor
timestamp: June 07, 2024, 10:30:20 am
content: I'm probably OK with that 100A Murata shunt, most often I will be in 50 - 60 A range. If I need it I will buy 200A 50mV version later.I didn't know max. shunt accuracy offered by reputable manufacturers is 0.2%. Then the 0.1% chinese accuracy is bullshit. However I still like their shunt design with banana sockets.

author: gaminn
author_rank: Frequent Contributor
timestamp: June 07, 2024, 10:31:31 am
content: Quote from: coromonadalix on June 07, 2024, 10:09:41 amYes, that's what I need - no more error added by the multimeter.

author: mariush
author_rank: Super Contributor
timestamp: June 07, 2024, 11:21:29 am
content: Why not just parallel a bunch of resistors to get a low resistance ...ex parallel 10 of these 10mOhm 4W resistors with 20 ppm/C to get a 1mOhm current shunt :https://www.digikey.com/en/products/detail/isabellenhuette-usa/VMS-R010-1-0-U/16836578

author: Martin72
author_rank: Super Contributor
timestamp: June 07, 2024, 11:27:13 am
content: Quote from: gaminn on June 07, 2024, 07:58:37 amBanana sockets are not designed for such high currents, which is why most power supplies that deliver 20A and more also have regular terminals.At higher currents, contact resistances can no longer be neglected.I really hope that if you “drive” currents over 10A, you no longer do this with normal banana cables...

author: Psi
author_rank: Super Contributor
timestamp: June 07, 2024, 11:41:28 am
content: A little 10x/50x/100x gain current sense amp IC and a good quality precision current-sense resistor, or array of them, is how i would do this.You need some gain to keep the resistor power loss manageable at 100AThe current sense amps output 0-5V. So with some careful selection of the right resistors you can make the output match current.1V = 100A for example.

author: coppice
author_rank: Super Contributor
timestamp: June 07, 2024, 11:59:37 am
content: Quote from: gaminn on June 07, 2024, 09:08:59 amThe problem isn't with China, its with AliExpress. It could be E-Bay or any other unregulated source, and your chances of good results are just as bad. Get things through a well regulated supply chain from a Chinese maker and they will most likely perform to spec.

author: coppice
author_rank: Super Contributor
timestamp: June 07, 2024, 12:01:39 pm
content: Quote from: ArdWar on June 07, 2024, 10:17:51 amFor 99% of uses a high accuracy shunt makes no sense, as its cheaper to get a shunt with a great temperature coefficient, and calibrate out its errors. However, there are certainly 0.1% accuracy shunts on the market.

author: trobbins
author_rank: Frequent Contributor
timestamp: June 07, 2024, 12:19:29 pm
content: Both those meters have the same 1yr 100mV range accuracy, and only have a 3Adc current range (the Keithley seems much better spec). You could compare both those meters at say 50-100mV to get some confidence that they haven't strayed far from their original accuracy, but be mindful that unless you keep the meters on full-time and keep them close to 23C ambient, the meter may have a significant tolerance itself, and especially for readings well below 100mV FS from the shunt.Even with the +/-0.25% shunt, your uncertainty could still exceed 1% when used with your meters if you don't take some effort to cross-check your meters, and importantly use a fan on the shunt to keep its temp rise as low as practical (even at 50-60A where the accuracy of the meter gets worse). But I think that is the best and easiest path to take.Martin72, I think the comment about banana plugs was for the voltage sensing connection to the voltmeter - perhaps look at the link to the 0.1% shunt, and to the Murata 0.25% shunt for context.

author: Njk
author_rank: Frequent Contributor
timestamp: June 07, 2024, 02:46:00 pm
content: Consider a welding machine Hall sensors (available as spare parts). Something like that

author: geggi1
author_rank: Frequent Contributor
timestamp: June 07, 2024, 07:01:43 pm
content: If you want to measure DC very accurate look into fluxgate current sensors.As far as i know it is only Lem and Danisense that sell these.https://danisense.com/flux-gate/https://www.lem.com/en/fluxgate-current-sensorsThey are expensive but you can get precision down to hundreds of percentage.

author: Hydron
author_rank: Super Contributor
timestamp: June 07, 2024, 09:45:12 pm
content: Old ultrastab sensors (i.e. what was suggested above) go for pretty cheap on eBay. They have a current output of a specified ratio to the measured current, and you can always just use a bench meter in current mode (even with autoranging) to measure the output if you don't have some precision resistors better than the internal meter shunts. If you have a decent meter with a scale function then you can even get direct readings.

author: Martin72
author_rank: Super Contributor
timestamp: June 07, 2024, 10:05:56 pm
content: I find the idea of using Danisense or Ultrastab quite charming.We have both for our ZES LMG power analyzers, they are accurate.And we had an LEM converter built into a case plus supply, plus huge terminals - as a current transformer for currents up to 2500Adc.....

author: Stray Electron
author_rank: Super Contributor
timestamp: June 07, 2024, 10:25:33 pm
content: Quote from: Martin72 on June 07, 2024, 11:27:13 amX2. Adding additional wires and sockets to the shunt is going to wreck the accuracy of the circuit! The resistance of a 100 Amp 50 mV shunt is only 0.0005 Ohms. And the OP going to add wires to it! What is the resistance of your wires? And the resistance of the sockets? No where near as low as that of the shunt, I can guarantee.The math: OMG! With a 50 Mv 100Amp shunt; every millivolt represents 2 amps of current! So millivolts x2 equals the current in amps. How much simpler could the math be? If you use a 150 amp 50 mV shunt then millivolts x3 equals the current in amps.And if you (the OP) takes a look at the various brands of shunts being sold you'll see that the better ones have four terminal connections, two large ones for the high current circuit and two smaller terminals for the voltage metering circuit. It forms a four wire Kelvin connection and the theory behind it is exactly the same as that for very accurate power supplies and very accurate meters and very accurate electronic Standards.The OP is very naive is he thinks that a 200 Amp shunt from Alibaba is going to be anywhere near as accurate or the size or will dissipate as much heat as a good quality 100 Amp American shunt! Chinese products simply aren't made that way.If you look at the shunts on Ebay, you can easily find shunts that are rated for 0.25% accuracy and that are made by companies like Weston, Fluke, and Simpson, that actually build meters and understand how measurements are supposed to be taken. Specs for many of the US and Canadian made ones are online and you can look up their tolerance and have some degree of faith that they will actually meet their spec, unlike anything made in China today.

author: trobbins
author_rank: Frequent Contributor
timestamp: June 07, 2024, 10:54:45 pm
content: StrayElectron, the OP is proposing to use a 'standard' current shunt - that is a 4-point measurement. That is also quite an acceptable method for 1% uncertainty if appropriately managed, and the cheapest and most convenient, given the two meters available.Also you just provide a stereotyped response to anything made in China. A large number of top rated companies set up shop in China - their decision, and have sold worldwide.

author: Fungus
author_rank: Super Contributor
timestamp: June 07, 2024, 10:57:00 pm
content: Quote from: Stray Electron on June 07, 2024, 10:25:33 pmComplete rubbish.The Chinese are perfectly capable of making quality products if you're prepared to pay for them.The $10 shunts are probably not much use but a $75 solid copper one? What exactly could go wrong? A chunk of metal is a chunk of metal no matter where it's made.Shunt calibration isn't difficult - manufacture them to be a bit on the low side then use a file to increase the resistance to spec. The shunts inside every multimeter you own will have a few little nips in them if you look.I've bought a lot of really nice stuff on Aliexpress. The only things I definitely wouldn't buy are power MOSFETS, 16850 batteries, and multimeter fuses. They're almost guaranteed to be fake.Everything else? I'd give them a chance. Aliexpress returns are easy peasy, lemon squeezy - just drop the thing off at a collection point (usually the same places as take Amazon returns take Aliexpress returns too).

author: BillyO
author_rank: Super Contributor
timestamp: June 07, 2024, 11:11:29 pm
content: Quote from: Fungus on June 07, 2024, 10:57:00 pmI agree completely. I get sooo tired of all the China bashing that goes on.If your willing to pay $27,000 for a scope, the Chinese will make you a$27,000 scope that is probably better than any $27,000 US made scope. However, if you only have $40 to spend on a scope the Chinese can help you out and I guarantee it will be better than every $40 US made scope.There is no reason to believe those AliExpress shunts are garbage. None at all.

author: Martin72
author_rank: Super Contributor
timestamp: June 07, 2024, 11:21:57 pm
content: QuoteAbsolutely.But it's also true that you get what you were prepared to pay for.Because even the Chinese can't do magic.If you pay €10 for a shunt, you get a shunt for €10, but don't expect the properties of a €100 shunt (which may have a different material property so as not to drift too far away at higher temperatures).If you are aware of this, everything is fine.If you ignore the circumstances that make it so cheap in the first place, all the better.

author: Stray Electron
author_rank: Super Contributor
timestamp: June 07, 2024, 11:46:36 pm
content: Quote from: BillyO on June 07, 2024, 11:11:29 pmEveryone here seems to overlook the fact that the OP said that he wanted somethingcheap. He's not going to buy any top of the line product, Chinese or otherwise. It's not a question of whether the Chinese can build a good product but more a question of what kind of quality when you buy from an unknown seller on Alibaba at the cheapest price that you can find!FWIW I have no reason to bash the Chinese products, there are plenty of postings on this forum and many others, complaining about the quality of every Chinese electronic product that you can name! Fake transistors, counterfeit ICs, solder that won't stick to anything, "copper" wire that can't be soldered to; tiny capacitors inside of large cans with much higher ratings, batteries with physically impossible amp hour ratings, a water heater that uses the outer metal body as a current carrying conductor; the list goes on and on! I know from experience that even when Chinese products to work, most of them are rated right on the edge of destruction and there is no margin for error. And that's why with everyone here worried about the heat dissipation of the OP's shunt, I would not recommend buying one from China, regardless of the price.

author: johansen
author_rank: Super Contributor
timestamp: June 08, 2024, 12:08:29 am
content: Quote from: Stray Electron on June 07, 2024, 11:46:36 pmYep that why i said look on ebay for LEM closed loop sensors.I bought 3, 100 amp units for maybe 30$ last year.

author: pdenisowski
author_rank: Super Contributor
timestamp: June 08, 2024, 12:26:38 am
content: Quote from: Martin72 on June 06, 2024, 10:03:40 pmHow about a currentprobe? We (R&S) make current probes that measure over 100 Amps DC with 1% accuracy.https://www.rohde-schwarz.com/us/products/test-and-measurement/oscilloscope-probes/current-probes-for-oscilloscopes_63493-73797.html

author: Fungus
author_rank: Super Contributor
timestamp: June 08, 2024, 01:31:35 am
content: Quote from: Stray Electron on June 07, 2024, 11:46:36 pmIn other news: People don't read seller ratings, buy things at prices that are too good to be true, and are predisposed to hate on "China".If it doesn't work, send it back.PS: I can buy Fluke multimeters on Aliexpress, Rigol/Siglent oscilloscopes, etc.

author: coromonadalix
author_rank: Super Contributor
timestamp: June 08, 2024, 03:23:21 am
content: the problem is : when you say "cheap" and put 6.5 digit meters on them, what do you expect ?i have 6.5 digit dmm's and 1 amp of resolution is enough for 200 amp shunts

author: Calvin
author_rank: Regular Contributor
timestamp: June 08, 2024, 07:14:16 am
content: Hi,similar as #6 and #28 I´d throw Hall- or Fluxgate sensors in the ringSee for example the N4646 series from Vacuumschmelze sourceable from Mouser (say the T6044-N4646-X412) for less than 17€/pcs.I have KiCad PCB-designs for the X400, X410 and X412 and the voltage-output 15A type X662.regardsCalvin

author: AVGresponding
author_rank: Super Contributor
timestamp: June 08, 2024, 07:57:55 am
content: If "cheap" is the primary operand, I'd buy a couple of hundred of something liketheseand bin them to get a hundred with the correct values to make a 500W 10mΩ 1% shunt. Also you'd have a hundred spare resistors for your stock afterwards.

author: nctnico
author_rank: Super Contributor
timestamp: June 08, 2024, 09:27:57 am
content: Quote from: AVGresponding on June 08, 2024, 07:57:55 amThe self inductance will be too high when using wirewound resistors. Especially if the current is switched off quickly you can get quite high voltage swings. This is already a problem with low inductance, planar resistors at 15A.Another advantage of using a current shunt over a hall-based current probe is having a high pulse handling ability. So you can measure short pulses to hundreds of amps without needing a current probe which is rated for hundreds of amps (which may also have a higher offset / noise floor).

author: AVGresponding
author_rank: Super Contributor
timestamp: June 08, 2024, 09:52:22 am
content: So use a snubber diode or TVS

author: trobbins
author_rank: Frequent Contributor
timestamp: June 08, 2024, 10:09:43 am
content: AVGresponding, how much is the OP going to pay himself to sort those resistors, and then to construct end terminals and then fit the resistors, then test the one off special ??A shunt manufacturer has the facilities to test. The OP doesn't.

author: nctnico
author_rank: Super Contributor
timestamp: June 08, 2024, 10:18:08 am
content: Quote from: AVGresponding on June 08, 2024, 09:52:22 amThat is not the point. The added inductance can influence the circuit you are measuring and/or affect the measurement results.Quote from: trobbins on June 08, 2024, 10:09:43 amIndeed. And 1 Ohm is low enough for the leads to also have a significant contribution to the resistance. Not to mention the leads won't be made from a low tempco. material.That is the reason why current shunts have sense terminals as close as possible to the resistive material; to make sure the leads / connections to the load have a minimum impact on the accuracy of a shunt.

author: HighVoltage
author_rank: Super Contributor
timestamp: June 08, 2024, 10:21:04 am
content: We discussed this before:https://www.eevblog.com/forum/metrology/calibrating-a-100a-dc-shunt/Not in your price range but this is what I have used very successfully for 100A DC:The RUG-Z-R001-0.1-TK1https://www.distrelec.de/de/leistungswiderstand-1mohm-250w-isabellenhuette-rug-r001-tk1/p/16057580?queryFromSuggest=true&itemList=suggested_search

author: coppice
author_rank: Super Contributor
timestamp: June 08, 2024, 12:04:21 pm
content: Quote from: nctnico on June 08, 2024, 09:27:57 amToo high for what? Has there been any indication of how much inductance the OP's application can tolerate?

author: johansen
author_rank: Super Contributor
timestamp: June 08, 2024, 07:16:00 pm
content: Quote from: nctnico on June 08, 2024, 09:27:57 amYou seem weirdly confident that 100 resistors in parallel will have too much inductance.Does a 3 inch long shunt have too much inductance for you?

author: BillyO
author_rank: Super Contributor
timestamp: June 08, 2024, 07:34:37 pm
content: Quote from: johansen on June 08, 2024, 07:16:00 pm100 of those resistors in parallel would not have very much inductance, depending on the frequencies being applied.However, the final device would be kind of large and unwieldy. The resistors are about 1cm square and if you put 1cm between each for cooling then put them in a 10 x 10 matrix it would be square 19cm on a side!

author: johansen
author_rank: Super Contributor
timestamp: June 08, 2024, 07:50:27 pm
content: Quote from: BillyO on June 08, 2024, 07:34:37 pmdidn't look at them till now. yeah would not use those.6$ on amazon - 100 1/2 watt resistors.https://www.amazon.com/EDGELEC-Resistor-Tolerance-Multiple-Resistance/dp/B07XLBY5RF

author: tooki
author_rank: Super Contributor
timestamp: June 08, 2024, 08:28:14 pm
content: Quote from: johansen on June 08, 2024, 07:50:27 pmI truly don’t get it: why would anyone order electronic components on Amazon? Aside from it being a scummy company, it’s not even a good price! For the same price, you can get name-brand resistors from Digikey for example.

author: Martin72
author_rank: Super Contributor
timestamp: June 08, 2024, 08:48:11 pm
content: You're not seriously thinking about using umpteen resistors in parallel here.....

author: mzzj
author_rank: Super Contributor
timestamp: June 08, 2024, 08:57:44 pm
content: Quote from: Martin72 on June 06, 2024, 10:03:40 pmFinding <1% accurate current clamps is not difficult, it is just that finding accurate AND cheap wont happen.Hioki CT6846A 1000A current clamp is 0.2% accurate but about 3000 usd/eur

author: Martin72
author_rank: Super Contributor
timestamp: June 08, 2024, 10:33:00 pm
content: That's clear...There is one more question, which is:How exactly does it have to be and why?

author: BillyO
author_rank: Super Contributor
timestamp: June 08, 2024, 10:45:37 pm
content: Yeah, we know practically nothing about the use case.

author: pdenisowski
author_rank: Super Contributor
timestamp: June 09, 2024, 01:19:26 am
content: Quote from: tooki on June 08, 2024, 08:28:14 pmBecause if you live in a medium- to large-sized city in the United States, you can get many "basic" (and some not-so-basic) electronic components the same day or overnight, with free shipping if you're a Prime member. If you need something quickly with low to zero shipping cost, Amazon is sometimes a very logical choice.But yes, I use Digikey, Mouser, etc. for things that I don't need right away or when I do a "bulk" buy of components, cables, etc.And I'm still mourning the loss of Radio Shack (I worked there for a short time) ... also not the best prices or selection, but you could find one in almost any town of more than 25,000 people back in the day.

author: Psi
author_rank: Super Contributor
timestamp: June 09, 2024, 01:23:04 am
content: It's very rare that I need a part today/tomorrow. I usually know I will need parts in 2 weeks when the PCB turns up.But then, I do have a good stock of generic parts already. I guess anyone starting out would have that issue.The first thing anyone starting out should do though is spend $100 on a resistor SMT/TH book, capacitor SMT/TH book, box of electrolytics and a collection of generic jellybean ICs. Build up some basic parts inventory.

author: egonotto
author_rank: Super Contributor
timestamp: June 09, 2024, 01:55:18 am
content: Quote from: johansen on June 08, 2024, 07:50:27 pmHello,1A per resistor gives 1 W per resistor, which has 0.5 W. And 1 V voltage drop is perhaps too much.Best regardsegonotto

author: johansen
author_rank: Super Contributor
timestamp: June 09, 2024, 04:17:00 am
content: Quote from: egonotto on June 09, 2024, 01:55:18 amSo spend 50 more cents for 0.1ohm resistors

author: nctnico
author_rank: Super Contributor
timestamp: June 09, 2024, 09:31:14 am
content: Quote from: johansen on June 09, 2024, 04:17:00 amWhich then increases the influence of the leads + solder joints even more. Bottom line: you can't construct an accurate, high current shunt from regular resistors in parallel because in the end you can't make the Kelvin connection. Getting the Kelvin connection done right is the pinnacle of constructing a current shunt.

author: Njk
author_rank: Frequent Contributor
timestamp: June 09, 2024, 11:04:53 am
content: Quote from: pdenisowski on June 09, 2024, 01:19:26 amBTW there were also a bigger stories like Fry's. I recalled they'd offered affordable Windows OEM DVDs (about $40, IIRC). For distribution only with a computer system but actually it was enough to buy any HW item, like a napkin for monitor. Almost all my Windows licenses were bought in that way

author: 2N3055
author_rank: Super Contributor
timestamp: June 09, 2024, 12:28:25 pm
content: Quote from: nctnico on June 09, 2024, 09:31:14 amOf course you can make a decent shunt from parallel resistors. It has to be carefully designed, geometrically symmetric and you can achieve good enough Kelvin connection, but you have to think in 3D. It can even be quite low inductance.But before designing it, accuracy targets need to be clearly specified.

author: Martin72
author_rank: Super Contributor
timestamp: June 09, 2024, 07:56:45 pm
content: In my opinion, it's only worth the effort if you can't find anything suitable on the market.Or if you need a shunt on a circuit board, for example we use 5 R50/3W SMD shunts in parallel on a power supply unit for current feedback.Speaking of shunts, I actually still had 2 lying around, I didn't even remember...A 60A/60mV and a 20A/60mV shunt, class 0.5, rather “robust” manufacture.You can see clear traces of machining on the 20A shunt to achieve the resistance value.

author: cv007
author_rank: Frequent Contributor
timestamp: June 09, 2024, 11:05:24 pm
content: QuoteAnyone have experience with these, something like-https://www.digikey.com/en/products/detail/vacuumschmelze/T60404N4646X101/12530732Its not really clear to me what these are doing, and why they are relatively inexpensive (<$20 for the above part). Obviously not the same as a flux gate current sensor of considerably more cost, but they seem to be using the same idea as they describe it as a closed loop with a zero field detector. Are these essentially similar minus the precision? I can see where precision takes a lot more effort/cost, but if that much precision is not required then maybe these are a good option?They list accuracy as max 0.5% with typical 0.1% for the above part (at max rated current, normal temperature), but not sure where you end up considering the rest of the variables in play. A typical 100A/100mV shunt is rated at 0.25% error, so it seems these are in the same ballpark. It looks like the downside is you have to come up with a +/- 12-15V power source, the upside is you can eliminate dealing with shunt heat and connections.Maybe I am missing something obvious, but these look like something that I would at least like to check out since the cost is reasonable. Did not know these existed.