Building a current shunt? - Page 1
Forum section: electronics / projects

author: TERRA Operative
author_rank: Super Contributor
timestamp: December 03, 2019, 04:08:31 pm
content: I want to whip up a basic current shunt (well as basic as they are already...), and I'm looking for a few pointers to make sure I get things right without making any basic mistakes as I tend to do...The HP service manual for my 6644A is asking for a Guildline 9230/15 or equivalent for performance testing, which is a 0.1 Ohm, 15A shunt with 0.04% accuracy.Problem is that these things are just a tiny bit too expensive for my taste, so I want to build one and I'm not too fussed with it being such a tight tolerance, just somewhat reasonably accurate.Has anyone built something like this? I was thinking of just using a bunch of power resistors in parallel, or maybe stringing up some nichrome or kynar wire?What's the general go-to for building a semi professional looking and performing current shunt?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 03, 2019, 04:30:57 pm
content: Actually building a shunt is difficult, as the hard part is getting the right material. For low resistance shunt the usual choice is Manganin alloys. AFIAK this alloy is suitable for soft soldering.Soldering a bunch of resistors in parallel is possible, but the connecting wire part can be an important part and this part tends to be pure copper and thus high TC.So I would more like look for a cheaper shunt. Quite often there are relatively cheap ones on Ebay - meant to be used with Panel meters. The DIY part could measuring the actual value if one has a good meter available.

author: TERRA Operative
author_rank: Super Contributor
timestamp: December 03, 2019, 04:37:01 pm
content: I have a 34461A benchtop meter, so I can get 4-wire measurements with an average level of accuracy.It seems most ebay shunts are rated at 75mV at their rated current, whereas what I need seems to be 1.5V at 15A, ir 75mV at 7.5A, something which doesn't seem to exist in the ebay ones.I guess I can get one that's close and trim it to spec with a hand file?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 03, 2019, 05:04:22 pm
content: The burden-voltage is a compromise between noise and thermal EMF problem at low voltage and self heating at higher voltage. So higher current shunts tend to us a smaller burden voltage. For panel-meters 75 mV burden voltage is kind of a standard, at least for higher currents.1.5 V at the shunt is rather high and causes quite some heating. So the resistor would need to be rated for really high power. 1.5 V and 15 A are some 22.5 W. For use as a precision shunt the rule of thumb is using more like 1/10 the normal power rating to limit the self heating.So the 0.1 Ohms shunt for 15 A would indeed be quite large power rating and unusual. If really needed this way I would consider resistors possible several that can be mounted to a heat sink and a fan. The actual needs may be lower and the quoted part just a relatively common high end shunt found in cal labs.One may get away with directly using the DMM to measure the current (use the internal shunt) or using a lower burden voltage shunt, as the 34461 is reasonable good in measuring small voltages.

author: ArthurDent
author_rank: Super Contributor
timestamp: December 04, 2019, 02:16:55 am
content: I have a Leeds&Northrup model 4360 .1 ohm 15A precision shunt that I use and it is big enough and has so much surface area that heat dissipation isn't a problem. It's probably over 60 years old but still works like new. I believe I paid less than $25 for it many years ago. It seems kind of odd to use something so old to check modern equipment but it works!

author: TERRA Operative
author_rank: Super Contributor
timestamp: December 04, 2019, 02:24:49 am
content: Yeah, it seems to be some sort of convention to use this sort of shunt among the larger various brands.For the Guildline shunt, they actually sell a fan module and also specify performance when immersed in oil.Thanks for the pic ArthurDent! I wouldn't mind getting my hands on an old one if I can find one even. Shunts are pretty solid and I wouldn't expect they drift or break too easily...I want to try to get my setup close to what's specified in the manual so I can be sure my measurements correspond with the values stated for the performance tests.I don't want to use a different setup then not be sure if my measurements are accurate or way off.Is it reasonable to assume I can rig up a bunch of parallel lengths of resistance wire for power handling and trim them to the required value to get my 0.1ohm?

author: beanflying
author_rank: Super Contributor
timestamp: December 04, 2019, 02:27:32 am
content: I have just brought three Murata 0.25% shunts to add to my kithttps://au.element14.com/murata-power-solutions/3020-01108-0/shunt-dc-0-1v-100a/dp/1869951(I got a 10A, 100A, 500A)Making your own from scratch isn't difficult. Two bits of copper bar with holes drilled in it and Kanthal (use multiple bits for higher currents) or similar wire between them and grubscrewed from the side on one end for adjustment and crimped in the holes on the other.

author: BravoV
author_rank: Super Contributor
timestamp: December 04, 2019, 02:32:34 am
content: Last year scored few used Japanese PCN brand high powered resistors, all have sensing connections. (Datasheet)10 mOhm , 0.02% ("Q" Grade) , 50 ppm/K (at 0 up to 100 C) , 50 Watt with heat sink / 20 Watt on free airShot with TO-220 chip as reference size, and this one has solid sense terminals instead of wires like above.10 mOhm, 0.1% (B grade) , 50 ppm/K (at 0 up to 100 C) , 50 Watt with heat sink / 20 Watt on free airAnd ended many othersas they're relatively cheap, as new can cost > $200 a pop, worth searching imo.

author: ArthurDent
author_rank: Super Contributor
timestamp: December 04, 2019, 03:27:42 am
content: I guess hoarding has its advantages. Years ago I parted out a huge RFL model 829 AC/DC calibrator and saved the DC current shunt and the precision AC current transformer with the same ranges. I've attached a couple of photos of the DC shunts in their 5" x 7" box. Below is a link to a photo of a twin of the beast I dismantled. If I recall it weighed about 100 pounds and the Wheatstone bridge, AC/DC dividers, and power transformer with taps from 1 to 1000 VAC were the only parts worth saving.https://www.govdeals.com/index.cfm?fa=main.item&rd&itemID=574&acctID=1017&CFID=11770289&CFTOKEN=39794411

author: coppice
author_rank: Super Contributor
timestamp: December 04, 2019, 03:38:21 am
content: Quote from: TERRA Operative on December 03, 2019, 04:08:31 pmWhen considering designs that put resistors in parallel, beware the horrible temperature coefficient of copper. At 0.4%/C even a small amount of copper in the sensed path can ruin the accuracy of the sensor, unless it operates at an extremely predictable temperature, with next to zero self heating.

author: joeqsmith
author_rank: Super Contributor
timestamp: December 04, 2019, 04:15:25 am
content: I attempted to make a shunt for a cheap meter. Played around a bit before ending up with the one shown installed. Turned down the ends in the lathe and silver soldered them to the wire. I wasn't looking for your 0.04% and 0.01 ohms meant it could be fairly small.Some of the shunts I have are indeed filed, so I see no reason you couldn't do this assuming the tolerance of the one you bought was low.

author: tszaboo
author_rank: Super Contributor
timestamp: December 04, 2019, 09:25:47 am
content: Your specification doesnt make it easy. At 15A, your shunt will dissipate 22.5W. No matter what you use, the shunt will heat up. Say, it heats up by 100 degrees, then your tempco has to be less than 4ppm. There are only a handful of metals that would even come close to this, and you need a lot of this material, because you need to dissipate 22W.This is easyer to buy than to build. Here is a resistor:https://www.digikey.com/product-detail/en/riedon/FHR4V-0R1F1/696-1809-5-ND/7867339Bolt it on a heatsink, and calibrate it.

author: maginnovision
author_rank: Super Contributor
timestamp: December 04, 2019, 09:38:44 am
content: https://www.digikey.com/product-detail/en/riedon/RSN-100-100B/696-1764-ND/6819384This is what I use for my "precision" shunt. Nice thing is it has a large range meaning you can use it more than once, bad thing is the precision isn't quite as precise as I'd like.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 04, 2019, 05:02:34 pm
content: I don't think one would really need exactly that value of shunt. The main point would be to get an accurate current reading for a large current (e.g. some 5 A or 10 A depending on the exact supply type). If you happen to have a suitable meter to directly measure that current with sufficient accuracy it is also OK. Not many meter are very accurate with more than some 2 A and thus the external shunt. However modern meters could well work with a smaller shunt resistance. Another option might be a precision DC current transformer - these are also a good modern alternative for high currents, though usually not cheap.

author: jbb
author_rank: Super Contributor
timestamp: December 04, 2019, 08:53:05 pm
content: Possibly threadjacking, but people have been mentioning cooling a shunt...I've got a task at work which needs a pretty good shunt, and I was wondering if anyone here has any experience or comments about adding extra cooling to reduce the temperature rise. The shunts I have on hand are round cylindrical (encapsulated) types with 4 wire legs. Would blowing a fan across them be suitable, or would I risk inducing big temperature gradients and increasing the tempo and/or thermocouple effects?The resistors I've got are Ohmite 17FPR100E (4 wire though hole, 0.1R, 7W, 50ppm, 2uV/C) and my expected currents are 3.4A, 2A and 0A.In fact, the FHR4V-0R1F1 mentioned by NANDBlog probably would have been a better choice.

author: TERRA Operative
author_rank: Super Contributor
timestamp: December 05, 2019, 01:26:40 am
content: Thanks for the info all!I've been looking around at resistance wires, and I think I have an idea...Looks like Kanthal wire has decent specs, 1.45uohm/mm resistivity (roughly equivalent to nichrome wire), but half the temperature coefficient at 49ppm/oC from what I can find.I roughly calculated that 8 strips of flat wire 4mm x 0.3mm at 630mm long each will give me about 0.1ohm, which I can then trim to spec.Kanthal looks relatively cheap, so I was thinking of getting some to have a play with. I'll maybe try clamping the 8 parallel wires between some brass blocks at each end and then running them back and forth to compact it a bit, kind of like the one in the photo posted by ArthurDent.Anyone know the temperature coefficient of brass by chance?

author: beanflying
author_rank: Super Contributor
timestamp: December 05, 2019, 01:35:59 am
content: Generally if you use a decent sized lump or Brass or better Copper you can ignore it's T/C relative to the wire or strip.Last time I got some Kanthal wire I used this evilbay seller. Postage was about two weeks depending on what your local availability ishttps://www.ebay.com.au/usr/resistance_wire?_trksid=p2047675.l2559

author: TERRA Operative
author_rank: Super Contributor
timestamp: December 05, 2019, 03:53:01 pm
content: Cool, I plan to use a reasonably sized lump of brass.Also, I found that seller, I already had my Kanthal wire selected from them.

author: pwlps
author_rank: Frequent Contributor
timestamp: December 06, 2019, 10:32:28 am
content: For high currents and high accuracy a DCCT might actually be cheaper than a big high quality shunt. I found some on ebay for less than $100.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 06, 2019, 12:34:46 pm
content: Kanthal is a difficult to solder material. It is suitable material for high temperature heaters, not so much resistors. The more obvious choice is Manganin, as it is lower resistance, low thermal EMF, low TC (often < 10 ppm/K) and it is relatively easy to solder. There is a reason that most of the low ohms shunt are made from this alloy. A possibly source for the material could be lower resistance shunts. These tend to be made from several layers, that could be separated and possibly put in series.

author: beanflying
author_rank: Super Contributor
timestamp: December 06, 2019, 01:57:03 pm
content: Depending on the use of flat works fine crimped or drilled and locked down with bolts for heavy applications. Most of the ones I have done were 4-800A at 12V nominal and if they were under or over adding or taking away a strip was the easy option for adjustment. You could always under resist them then file for trim if you wanted but we were not after that level of accuracy.Way back when I was playing with Electric and Solar vehicles at school (30 + years ago) Wire crimped on one or both ends into copper bar worked well. Generally a close fitting hole then a series of centre punches from either side gets a good crimp. For the few trimable ones I have used a copper bar with holes drilled and tapped in from the side with brass screws to allow adjustment. May not come up to metrology grade but was more than capable for data logging use.

author: TERRA Operative
author_rank: Super Contributor
timestamp: December 09, 2019, 12:41:20 pm
content: My plan is to use two blocks of brass and drill and tap them so I can bolt them to each other with the flat resistance wire clamped between.I'll take a look at sourcing some Magnanin, I'll see what I can find in the 1-2mm diameter range to help prevent self heating by providing a decent amount of surface area.Does anyone know where to get Magnanin as a flat wire or ribbon?

author: coppice
author_rank: Super Contributor
timestamp: December 09, 2019, 04:31:31 pm
content: Quote from: TERRA Operative on December 09, 2019, 12:41:20 pmNow you will have some of the current flowing through the brass, and its temperature coefficient is awful. The coefficient varies somewhat with the type of brass, but its not a whole lot better than copper. You really want to electrically isolate the wire from the brass, even though that will spoil the heat conduction.

author: TERRA Operative
author_rank: Super Contributor
timestamp: December 10, 2019, 01:15:22 pm
content: How would you suggest I electrically connect multiple strands of Kanthal to binding posts without using brass blocks?Is there another metal I should look at?

author: perdrix
author_rank: Frequent Contributor
timestamp: December 10, 2019, 02:18:12 pm
content: Quote from: pwlps on December 06, 2019, 10:32:28 amOK ignorance showing - what's a DCCT?David

author: coppice
author_rank: Super Contributor
timestamp: December 10, 2019, 03:04:36 pm
content: Quote from: perdrix on December 10, 2019, 02:18:12 pmIts what some people call a flux gate.

author: TERRA Operative
author_rank: Super Contributor
timestamp: December 11, 2019, 09:34:37 am
content: That's basically how a DC clamp meter works right?I'll stick with a traditional current shunt for now as that is how HP specify the test is to be done, and I just want to make one too.Next current measuring project might end up as a DCCT even.