A multimeter solely for current measurement, or some other device entirely? - Page 1
Forum section: electronics / testgear

author: derGoldstein
author_rank: Regular Contributor
timestamp: October 28, 2017, 08:12:31 am
content: Sorry if this question has already been asked a bunch of times, I just don't know what search terms to use to find a solution...I do a lot of tests on DC-DC circuits, motor driving, LED driving, battery charging, etc., where I need to measure current at multiple points along the way. Unlike voltage test points where I can just clip and unclip probes, I have to re-route the wires through multimeters in order to measure the current accurately (I've tried hall-effect sensors, so far they're unreliable).Dave often says you need 4 good multimeters -- input voltage/current and output voltage/current. For me it's even worse in that I'll often want to test something from <1mA (standby power or quiescent current) up to ~5A, so I have to switch back and forth between the multimeter's 20A range and 200mA (or 400mA) range, and too often I'll blow a fuse because I forgot to rewire.The result is that I'll spend a stupid amount of time disconnecting and reconnecting multimeters. Sometimes I'll forget to do a particular test in a specific range and I'll catch myself in the middle of rewiring, I'll get distracted by something, and then I'll sit and stare at the mess of wires trying to remember how I was going to reconnect things.There's one device which would be a godsend and solve MANY of these problems, the Gossen Metrahit Energy:But that thing's about $900 and there's no way on earth I can justify that expense.Any tips/suggestions that might help me shave off some rewiring are welcome, but there's one question I could narrow it down to that would solve a lot of it: Is there an accurate ammeter that would let me measure current from 1mA to several amps, which doesn't require me to switch between ranges? There are multimeters with built-in relays to do this switching, do they have a particular name or category?

author: ogden
author_rank: Super Contributor
timestamp: October 28, 2017, 08:36:55 am
content: Quote from: derGoldstein on October 28, 2017, 08:12:31 amYes. Bench multimeters. You can get quite decent 4.5 digit "chinabrand" for around 350$. For ~500$ there's 5.5 digit Rigol DM3058E which is one of the best value/money for hobbyist at the moment.

author: IanB
author_rank: Super Contributor
timestamp: October 28, 2017, 08:41:54 am
content: Why not a clamp meter like the UNI-T UT210E? Then you can measure current in-circuit without any need to disconnect wires. I know you said you don't like hall-effect sensors, but many of us have tried the UNI-T and it works fine.Also, there's no theoretical difference between wiring the Metrahit Energy into a circuit and wiring two multimeters into a circuit. Both solutions will measure current and voltage simultaneously. The Metrahit can calculate power, but you didn't say you needed to do that.

author: David Hess
author_rank: Super Contributor
timestamp: October 28, 2017, 10:13:13 am
content: Some 4 digit multimeters have 1 milliamp resolution on their 10+ amp range; isn't that enough? I almost always use the high current range in circuits where the current is not limited by impedance.Where lower accuracy is acceptable, a clamp on DC ammeter is much easier and safer to use and some of them have 1 milliamp resolution.

author: nctnico
author_rank: Super Contributor
timestamp: October 28, 2017, 10:32:12 am
content: For this reason alone I never buy a DMM with less than 4.5 digits. It means that a 10A or 20A range still has 1mA resolution (but not necessarily accuracy!). If you want to have the accuracy then you'd need a 6.5 digit meter. For example: my Keysight 34461A has a 10A range.

author: Ice-Tea
author_rank: Super Contributor
timestamp: October 28, 2017, 10:34:35 am
content: The TTi Iprober 520 allows current measurements on traces but won't be much good for 1ma and in that mode won't be very accurate. Still an awesome tool, though.

author: HKJ
author_rank: Super Contributor
timestamp: October 28, 2017, 10:43:22 am
content: Gossen and a few other brands only have one current input terminal and can handle from uA to A without moving any wires or breaking the circuit.But you always have to watch out for the burden voltage, it can be fairly bad in the high mA range.Another solution is a power supply with a good current measurement. I use Keithley 2280S-32-6, it can measure below mA to its maximum current at 6A, always maintaining a constant output voltage.

author: WVL_KsZeN
author_rank: Regular Contributor
timestamp: October 28, 2017, 10:45:48 am
content: The older fluke 183 and 185 have a single input for mA and A. Also known as the tektronix tx1 and tx3.

author: billfernandez
author_rank: Regular Contributor
timestamp: October 28, 2017, 01:56:34 pm
content: Take a look at the Mooshimeter: "Measure from -10A to 10A in 2uA steps".$125 at Amazon.com:https://www.amazon.com/Mooshim-DMM-BLE-2x01A-Mooshimeter-Wireless-Multimeter/dp/B011W4L7JA/ref=sr_1_1?ie=UTF8&qid=1509198910&sr=8-1&keywords=mooshimeter

author: derGoldstein
author_rank: Regular Contributor
timestamp: October 28, 2017, 03:21:31 pm
content: Quote from: IanB on October 28, 2017, 08:41:54 amI have the UT210. Maybe I got a lemon, but I'm not getting anywhere near the precision that I've seen other reviewers get, and it's too erratic.Under perfect conditions, with a clean supply and constant load, it's "ok", but too many things interfere with it. Noise on the line gets confused readings, it seems to latch sometimes on a particular value when I know that I'm adjusting the load and therefor should be seeing a change, and the 2A range sometimes just decides to go down to reading 0, mid-test, for no apparent reason. Also by applying very slight pressure on the clamp with my thumb and forefinger I can easily get the reading to fluctuate by 5% (and then I'm left wondering what amount of pressure on the clamp gets me the more accurate measurement).I'll use it when I have no way to splice a connection, but otherwise I don't find it reliable.Quote from: IanB on October 28, 2017, 08:41:54 amStill, it's hard to beat the combination of features -- one device for voltage and current, plus a wattage reading (which is convenient, even if I don't need it all the time), plus automatic range switching between the shunts. It's just an elegant solution for a very high price.Quote from: David Hess on October 28, 2017, 10:13:13 amYeah, that's what I'm using now, but for some applications the convenient range is 0.1mA to 1A. LED driving circuits for example. Also any circuit where I need to see what happens when the load shuts off and there's some mechanism that puts a DC-DC controller into sleep mode (they don't always go to their quiescent current immediately, sometimes it takes a few seconds).I realize this isn't a common requirement, it would just be very convenient to have.Quote from: HKJ on October 28, 2017, 10:43:22 amI do a lot of tests with batteries so I have to interrupt the circuit to test it, but yeah, for everything else I'm going to have to invest in a power supply with precise readings at some point.Quote from: billfernandez on October 28, 2017, 01:56:34 pmIt needs another device as a display, and introduces problems with firmware updates and wireless connectivity issues. I think it might cause more problems that it would solve, but I'll look over reviews and see what others say.Maybe I should have prefaced the question with "I can't spend more than around $200 right now". There are some great solutions, but they're mostly very expensive.I've been thinking of rolling my own with an INA138 and some precise ADC and voltage reference (an LM4040B?). Maybe just start with a test circuit with the INA138 and an arduino so I can get an idea of what I can expect, and gradually add precision parts to see what the added value is.

author: bd139
author_rank: Super Contributor
timestamp: October 28, 2017, 03:42:11 pm
content: Annoyingly my GW bench meter can’t do what is required. You have to manually range it for anything above 5A.

author: ogden
author_rank: Super Contributor
timestamp: October 28, 2017, 05:07:45 pm
content: Quote from: derGoldstein on October 28, 2017, 03:21:31 pmNot good. Bench meters are out of reach and multimeters usually due to various reasons do not autorange between A/mA ranges, not in < 200$ price category.Quote from: derGoldstein on October 28, 2017, 03:21:31 pmIf you plan to measure huge dynamic range, then better take reference from REF50xx series which have lower noise and better tempco compared to LM4040. INA138 is even worse choice. Actually all INA series current sense amplifiers are "narrow range". You shall go to product folder and play with "error calculator" trying to get good precision out of whole planned range, supposedly 1mA to 10A. Good luck!Better just look at proper instrumentation amplifiers. If you can make (battery-powered) isolated current meter then just place shunt in midrange of some chopped zero-drift opamp (OPA333). Further reading:http://www.ti.com/tool/TIDA-01012

author: David Hess
author_rank: Super Contributor
timestamp: October 28, 2017, 08:02:07 pm
content: Quote from: derGoldstein on October 28, 2017, 08:12:31 amThis gets to the root of the problem. Automatic ranging is problematical if it alters the shunt resistance so good meters avoid it.One trick I have used in the past is to place two current meters in series with one on the 10+ amp range and the other on a lower current range but with back to back diodes across the higher resistance current shunt. So high currents get directed around the low current meter and the higher burden voltage of the lower current meter is limited.

author: HKJ
author_rank: Super Contributor
timestamp: October 28, 2017, 08:08:04 pm
content: Quote from: David Hess on October 28, 2017, 08:02:07 pmI wonder if you can find any meter that avoids changing shut resistance. Usual you have two ranges on one resistors, then you have to change resistors. The Gossen meters uses fet transistors to change range, this means no break, but the resistance will change.

author: David Hess
author_rank: Super Contributor
timestamp: October 28, 2017, 08:28:48 pm
content: Quote from: HKJ on October 28, 2017, 08:08:04 pmNone of my meters change shunt resistance automatically. My manual ranging meters have a separate shunt resistance for each range and my automatic ranging meters use one shunt for each user selected range like amps, milliamps, and microamps. So for instance the high current jack has one fixed shunt but the shunt resistance of the low current jack is selected by the function switch position.

author: HKJ
author_rank: Super Contributor
timestamp: October 28, 2017, 08:43:49 pm
content: Quote from: David Hess on October 28, 2017, 08:28:48 pmExactly, they are very limited in dynamic range. The Gossen can go from uA, through mA to A on auto range, because it can change the shunt with fet transistors. I have seen a few other meters that can do the same, but I do not remember the brands.

author: jeroen79
author_rank: Frequent Contributor
timestamp: October 28, 2017, 09:12:13 pm
content: Instead of using the multimeter in current mode you can also use it in voltage mode to measure the voltage drop across a known resistor that already is part of the circuit.I = U / RYou could also get some resistors of various powers of 10 (.01, .1, 1, etc) and build small shunt modules.You then put these in your circuit where ever you need to measure the current and then measure the voltage drop across them.You could even add an amplifier to them to keep the burdenvoltage low while still making the best use of your meter's resolution.(like the uCurrent does)

author: kasone
author_rank: Contributor
timestamp: October 30, 2017, 12:33:22 pm
content: derGoldstein: I use a CenTech 98025 DMM received free from the local Harbor Freight for testing the charging circuit of batteries in an industrial remote control transmitter. They have current ranges of 200 and 2000 micro amps, 20 and 200 milliamps, and an un-fused 10 amp current range. Mine has been very reliable and accurate enough for my needs.If you can get these meters in your country, I would get several of these, set them on the expected current range for each point of the circuit, and use one at each point in the circuit that you are testing. I modified the standard probes with good 18 Ga. wire and alligator clips. If I have a blown fuse or a meter that does not work, I will just plug in a new meter and repair or replace the bad one later.