Current shunts revisited (and current calibration woes) - Page 1
Forum section: electronics / testgear

author: SharpEars
author_rank: Unknown
timestamp: October 17, 2015, 01:48:13 am
content: As some of you may remember, I wanted to calibrate my power supply using a shunt made out of aluminum foil. Some of you suggested that I should get a manganin based shunt, due to the thermal drift I was experiencing with all of my attempts at home brewed shunts (foil, long wires, etc...).So I bought some manganin shunts (5A, 10A, 15A, 20A) from China, all 75 mV at full scale, to try to calibrate my power supply and I'll be danged, the power supply ammeter is more accurate than the damn shunts!The 5A shunt seemed to be a bit off, because it was just a coiled piece of manganin wire, so I moved to the 10A which was a thin strip that someone filed down in three places for better accuracy, apparently (i.e., some effort was put into it to get it to be accurate).On that one I'm reading a voltage of 7.48 mV at a 1 A reading on the power supply. So this is like what a .267% accurate shunt and it's not accurate enough. I guess I need to find a more accurate set of shunts.

author: TiN
author_rank: Super Contributor
timestamp: October 17, 2015, 02:04:00 am
content: So you sure your current is 1.000A , not like 0.997A? Also voltmeter used is accurate enough, as well as do you use Kelvin sense?Even hi-end commercial shunts usually have trim at elements, it's part of manufacturing adjustment.

author: sleemanj
author_rank: Super Contributor
timestamp: October 17, 2015, 04:52:27 am
content: Quote from: SharpEars on October 17, 2015, 01:48:13 amIf we are to assume that your power supply is accurate, then as you say, it's 99.73% accurate (7.48 realised, 7.5 ideal), just 0.26 percent out,- you don't state what the tolerance of your shunts is, but any cheap shunt I'd say 1% you'd be doing well, especially a cheap shunt from China, so your 0.26% is pretty damn good if you ask me.What tolerance do you want.

author: SharpEars
author_rank: Unknown
timestamp: October 17, 2015, 04:08:28 pm
content: Quote from: sleemanj on October 17, 2015, 04:52:27 amI want 0.02% or less so I can actually use it to calibrate "something!"

author: steve207a
author_rank: Contributor
timestamp: October 30, 2015, 06:48:06 pm
content: Hi I bought some of those digital current meters off ebay that came without a shunt so i experimented with ..... now dont laugh.... with old metal coat hanger wire i started with about 2 inches in length then just played around with different connections across the wire eventually i bent the ends around 2 small bolts and soldered them in as connectors then bent the wire in a loop or 2 depending on the value i needed ive fitted them into 2 power supplies that have a max 5 amps and they calibrated very well across the amp range so i am pleased with the result maybe you could try something similarsteve

author: SharpEars
author_rank: Unknown
timestamp: October 30, 2015, 06:51:02 pm
content: Quote from: steve207a on October 30, 2015, 06:48:06 pmSo, coat hangers = manganin, in your experience? If you go to these extremes, you can just clip a long steel nail to size and weld it to the two bolts or their connectors...One problem with anything iron is oxidation. Also, the white coating on hangers can trap in heat, causing a larger shift in resistance for higher current values.

author: pelule
author_rank: Frequent Contributor
timestamp: October 30, 2015, 06:58:41 pm
content: Yes, in principle your are correct.But keep the nail at a constant temperature and don't use too much current to prevent the selfheating.The reason of using manganing or similar material is not the resistance, but the resistance change via temperature (the TempCo).BRLudger

author: SharpEars
author_rank: Unknown
timestamp: October 30, 2015, 06:59:16 pm
content: Quote from: pelule on October 30, 2015, 06:58:41 pmActually, it's both. Manganin has quite a bit of resistance to it as well. (Copper: 1.68, Iron: 9.71, Manganin: 48.2 (equivalent units))

author: robert_
author_rank: Regular Contributor
timestamp: October 30, 2015, 07:42:39 pm
content: Quote from: SharpEars on October 17, 2015, 04:08:28 pmSo, what were the shunts you bought rated for? 0.02%?If not, why do you expect 0.02%? If you want precicion, you have to buy it and pay for it, its that simple.http://www.burster.de/de/kalibriergeraete/kalibrier-und-praezisionswiderstaende/p/detail/1240/here you get low value with 0.02% initial accuracy and low enough TC to be of actual use. Cost is well in the 4-digit range.

author: Seekonk
author_rank: Super Contributor
timestamp: October 30, 2015, 07:53:20 pm
content: "7.48 mV at a 1 A" Just get out your file.

author: SharpEars
author_rank: Unknown
timestamp: October 30, 2015, 07:53:31 pm
content: Quote from: robert_ on October 30, 2015, 07:42:39 pmWhat were the shunts I bought for 0.02%?Answer:Not Chinese crap!I did have to buy it, but I didn't have topay (through the nose) for it- 100 buxes + ship!

author: ltz2000
author_rank: Regular Contributor
timestamp: October 31, 2015, 02:22:30 pm
content: Quote from: SharpEars on October 30, 2015, 07:53:31 pmI would be very suspicious about the "calibration" service from that seller (and from Ebay surplus sellers generally). He has sold a huge variety of surplus test equipment over the years, everything NIST calibrated. Calibrating all of them to the manufacturer specifications would require so well equipped lab that there are only a few in the world. And even if we forget the massive capital investment required for the lab assets and traceable calibrations, he would still need a very skilled techician working for almost free to make it economically possible.I have actually seen a calibration certificate from that seller. That was interesting, because the stated calibration would be impossible with the assets listed in the certificate. And when asked, the seller refused to answer even the most simple questions of their calibration methods, traceability etc. Just some marketing speech about the best service and satisfied customers.

author: Vgkid
author_rank: Super Contributor
timestamp: October 31, 2015, 03:31:23 pm
content: I have one of those L&N shunt boxes(actually 2, though 1 might be for parts). 2ohms measures 2-ohms, all the way down to .10 ohms. Measured with my OC-compensated 3456A. Stable that way as well.I can make a teardown thread if you like.

author: eevblogfan
author_rank: Frequent Contributor
timestamp: October 31, 2015, 07:31:48 pm
content: HelloI've read that thread , and couldn't understand your needs .Where is the standard you are reffering to know your absolute ? . You claim you need to calibrate your p.s and then claim based on the p.s that the shunt you just bought is out of spec ?Since i've worked half A year in the metrological lab . I can tell you that this bussines is hierarchical , and even the standards have to be tested based on the expected results , you have time depended figures ( 24hrs drift figures , 1 year figure and so on ...)For example , i have 10mohm and 100mohm shunts , they are ex-military (yokogawa branded ), and they stated to 200ppm and 100ppm (for the 10 and 100 mohm respectively) if i remember correctly . The actual performance is better and the results are nulled based on the calibration temp compared to the temp you are *now* working at , so say te tempco is as bad as 10ppm/c . The calibration temp was 20C . And when your shunt experiances 10 amp(when you calibrate your p.s) of current , at 10mohm . That's 1W of power . Say it rose the temperature up by 7 degrees after 1 houre of usage . That 70ppm of inacuracy , though due to the predictable result , you can take that into account and extract that extra 70ppm from the given result , and there you have it , clean "un-effected" measurment , only if the resistive metirial is efected linearly by the temperature . So when asking to see "100.000mv" as mark for 10A , you are actually seeking after the un necessary . You may very well see 107.253mv *but steady* and account for that with ratio you place into the system , instead of seeking for 100.00000mohm , becouse that's obsession and un-necessaryThose are the main titels you should be aware of when beggining to get in to the calibration bussines . There are also many other names you should know such as the thermo-couple effect- temperature coefficient- perscision- accuracy

author: SharpEars
author_rank: Unknown
timestamp: November 01, 2015, 02:30:54 pm
content: Quote from: ltz2000 on October 31, 2015, 02:22:30 pmI'll say this much: I tested all ranges against three 34401As that I own and they were, based on the average readings of the three meters truly within 0.02%. So, I don't have any evidence to say that it was not calibrated or at least tested to be within calibration. For lower currents, it's easy enough to use a precision resistor to test for accuracy, which I may do as well.

author: ltz2000
author_rank: Regular Contributor
timestamp: November 01, 2015, 05:37:01 pm
content: Quote from: SharpEars on November 01, 2015, 02:30:54 pmThose old instruments were very well built and most of them perform like new even decades later. But that has nothing to do with the calibration. The idea of the calibration is to change the believing to knowing (uncertainty still exists though).Calibrating an 8 range current shunt to 0.02%, from tens of millamps to 15 A and from DC to 1kHz according to the manufacturers specifications is not a trivial task. Far from it.And in case of "measurement uncertainty of one quarter of the specification of the unit under test" which the seller writes in his certificates, we are talking about primary / national lab level.I just don't like the idea of making money / boosting sales by writing false calibration certificates. Even though the buyer knows (or should know from the price tag) the name of the game.

author: ocw
author_rank: Regular Contributor
timestamp: November 01, 2015, 07:32:25 pm
content: QuoteI use an accurate voltmeter along with a Vishay Y092610R0000T9L 10 ohm 8 watt 0.01% resistor to verify the accuracy of current measurements up to 0.89 amps. Its current price is $36.76 at Mouser. For more information on this VPR221 series resistor see:http://www.vishaypg.com/docs/63012/vpr22021.pdfLower resistance options are available with somewhat reduced accuracy.Using that resistor it was easy to confirm that the displayed current reading on a 34401A meter is understandably about 5% low when measuring 0.25 to 0.75 amp currents due to the 0.5 ohm shunt resistance of the meter in the current mode.

author: ltz2000
author_rank: Regular Contributor
timestamp: November 01, 2015, 11:18:24 pm
content: Quote from: ocw on November 01, 2015, 07:32:25 pmHow does the internal resistance of the 34401A cause error when measuring current ?

author: ocw
author_rank: Regular Contributor
timestamp: November 02, 2015, 12:09:17 am
content: QuoteI'll pretend that it's a piece of equipment rather than my test situation...You have a 5 volt regulated power supply which is feeding a circuit which at the time acts as a fixed load of 10 ohms. Obviously, the circuit is causing 500 mA (5/10 A)of current. When you insert the 34401A into the current path to measure the current the meter adds about 0.5 ohms of series resistance to the current path. So, the meter will read 5/10.5 A or about 476 mA which is 4.8% below the actual current without the meter added into the circuit. That's assuming that the meter has zero error itself.I read a current of 475.779 mA in my actual test with a 34401A and the mentioned 10 ohm resistor.

author: macboy
author_rank: Super Contributor
timestamp: November 02, 2015, 12:56:46 am
content: Quote from: ocw on November 02, 2015, 12:09:17 amThe instrument can affect the current due to its burden voltage (or to look at it in another equivalent way, it's series resistance).But this does NOT imply that the current displayed is inaccurate by 5%! That is absurd. The meter is reading 475.779 mA, and it is actually (around) that value. The meter measures the current flowing through it, not the current that *would have* flowed had the meter not been inserted into the circuit. Your example is a good one, and you did the math right: it predicts a reading of 476 mA and that is what you got; why do you think that there is 500 mA flowing and the meter is somehow wrong or inaccurate?! You don't seem to understand the fundamentals of current measurement.

author: ocw
author_rank: Regular Contributor
timestamp: November 02, 2015, 01:27:51 am
content: QuoteI was obviously referring to trying measure the current in that situation and incorrectly assuming that inserting the meter would provide the answer. For a load which is equivalent to a fixed resistor it's easy to correct for the meters insertion. However, not all loads are linear. The decreased voltage due to the meter's insertion could cause a logarithmic or other non-linear change in current or cause the circuit to not work at all due to the decreased voltage. So, you may not know if the displayed reading is inaccurate, just that it's different than what it is without the meter inserted.QuoteSince the burden voltage depends on the current being measured, I have all of my meters marked with their shunt resistance when in the current mode marked near the meter's terminals. I'd obviously rather not to have to do that and I'm looking for a meter where that effect is less significant. That's why "errors" caused by the meter's shunt resistance are just as important to me as their error in measuring a current source in reviews.I'll leave the insults to others...

author: ocw
author_rank: Regular Contributor
timestamp: November 02, 2015, 02:19:56 am
content: FYI - I simplified the 475.779 my reading on my 34401A meter. With that high of current and 10 ohms of resistance the effect of lead resistance can be noticeable. I compared the 475.779 mA reading on my 34401A to the voltage measured on the sample terminals of the four terminal VPR221 series 10 ohm resistor divided by 10. Without correcting for the added series resistance of the meter, the displayed reading was 4.916% low. Correcting for the shunt resistance of the meter, the reading was just 0.016% low. Both of those "error" per cent values are shown on my personal accuracy verification report of the meter and both are important to me. The accuracy at 46 other currents was also tested with 92 "error" values listed. That evaluation tells me much more than seeing a single 1 mA verification of a current source.

author: deadlylover
author_rank: Frequent Contributor
timestamp: November 02, 2015, 03:19:29 am
content: Quote from: macboy on November 02, 2015, 12:56:46 amIt kind of just depends on your perspective though, doesn't it? We really only care about the actual current flow in the circuit without the multimeter, so it's not thaaat much of a stretch to say that the current displayed is inaccurate by 5%.

author: ltz2000
author_rank: Regular Contributor
timestamp: November 02, 2015, 08:11:13 am
content: Quote from: ocw on November 02, 2015, 01:27:51 amYour goal seems to be measuring the current with some specified voltage. That is easy to do by simply placing the voltmeteroutsidethe current measuring circuit. Now the voltage drop of the current meter is excluded from the voltage measurement.Power supply - current meter(s) - voltmeter - devide under test. Just adjust the power supply so that the voltmeter reads the desired voltage.No need for your correction tables. Which are problematic, because wire/connection resistances can easily cause an additional error of the same magnitude as the internal resistance of the 34401A.And no need to buy a new meter. Which wouldn't help you to get rid of the internal resistance anyway, because the current measurement in practically all multimeters is based on measuring the voltage drop accross the internal shunt.

author: ocw
author_rank: Regular Contributor
timestamp: November 02, 2015, 02:17:12 pm
content: QuoteMy goal is to be able to measure the current in a piece of equipment without having the measurement process significantly change that current. Knowing what the current is after the voltage is reduced can be meaningless at times. The load might not be linear so a corrected reading might be nearly impossible to determine. I'd rather not have to worry about making a correction anyway. It's like having to make a non-linear correction to a speedometer reading every time that you looked at it.QuoteBut a 0.5 ohm shunt resistance for measuring 1/2 amp of current is too large. It's fine for some of the lower currents that its used for until the 34401A meter changes to about a 5.5 ohm shunt resistance. Having a greater number of automatically selected shunt resistances will minimize the problem. Meter reviews should document those shunt resistances (and a few do) and point out whether they are significant or not.

author: SharpEars
author_rank: Unknown
timestamp: November 02, 2015, 03:13:27 pm
content: Guys... guys...Burden voltage, "shmurden" voltage! When I want an accurate current measurement and therefore put an ammeter in series with a DUT, I use sense wires from my power supply across the DUT to make sure that the voltage across it is what it is what it should be, regardless of the ammeter's burden voltage, the parasitic resistance of the leads, etc... There is no other way, since the burden voltage is load dependent and is different for every single ammeter (even if the model number is the same and even if the serial numbers are 1 apart) and even if the same unit is used at a slightly different temperature or runs a little longer than last time (allowing the shunt to warm up a little).

author: ocw
author_rank: Regular Contributor
timestamp: November 02, 2015, 03:54:46 pm
content: Perhaps another example is better. It's not unusual to find alignment instructions similar to:Adjust the bias control so that the current in this Class C amplifier is 500 mA +/-2%I am required to do something similar (with higher currents) when replacing UHF MOSFETs in a UHF PA. It's hard to compensate for the reduced voltage in that non-linear amplifier if inserting the meter causes a linear load current to decrease by 5%. Rather than finding a trick around the problem, it saves me time to instead use a meter which has an insignificant shunt resistance. The 34401A has an acceptable shunt for some currents, but not at 500 mA. A simple shunt resistance change will prevent having to play games when aligning equipment.

author: SharpEars
author_rank: Unknown
timestamp: November 02, 2015, 04:00:14 pm
content: Quote from: ocw on November 02, 2015, 03:54:46 pmOr just use a 20A, 50A or 100A shunt with negligible resistance... (and less ability to measure current accurately, of course).