Why can't a multimeter measure current and voltage simultaniously?  - Page 1
Forum section: electronics / testgear

author: staze
author_rank: Frequent Contributor
timestamp: May 06, 2013, 11:06:10 pm
content: Subject says it all. Why can't a dual display multimeter, with 3 probes (shared common), display both voltage, and current. Am I missing something really obvious here?Edit: obviously a clamp multimeter will, but I'm talking more theoretically about standard bench/handheld multimeters.

author: gxti
author_rank: Frequent Contributor
timestamp: May 06, 2013, 11:11:46 pm
content: Some do:http://www.gossenmetrawatt.com/english/produkte/metrahitenergy.htmA simple reason is cost (and those meters aren't cheap). You need two analog pathways, both capable of meeting the specified accuracy. And if you want to measure true power, it's even more complicated.

author: Lightages
author_rank: Unknown
timestamp: May 06, 2013, 11:16:54 pm
content: Well there is one relatively cheap one, the Uni-T UT71E. Not the best quality or safest in the world but it does do this.

author: staze
author_rank: Frequent Contributor
timestamp: May 06, 2013, 11:21:06 pm
content: Oh, okay. *phew* I thought it was something really bloody obvious that I was missing... here, it's just cost. =PYeah, I've kept looking at my Fluke going "huh, it's got mA and A separate, why not just allow another probe, and display V while you're displaying current"... though, thinking about it, I could see where this might end up being a 4 wire operation. But no reason a bench meter couldn't do this. =/

author: c4757p
author_rank: Super Contributor
timestamp: May 06, 2013, 11:27:22 pm
content: Quote from: staze on May 06, 2013, 11:21:06 pmFluke 45 can.

author: staze
author_rank: Frequent Contributor
timestamp: May 06, 2013, 11:32:19 pm
content: Does it?

author: staze
author_rank: Frequent Contributor
timestamp: May 06, 2013, 11:40:17 pm
content: wow, it sure can. Page 4-2 of the manual... damn.We should start a thread arguing with Dave's hatred of bench meters. =P

author: c4757p
author_rank: Super Contributor
timestamp: May 06, 2013, 11:55:30 pm
content: Quote from: staze on May 06, 2013, 11:40:17 pmAs usual, Dave will jump in saying he doesn't hate them in 3... 2...(I agree with him, I don't think he hates them, just finds them inconvenient. It depends on your working style. I'm actually the exact opposite - handheld meters make an ungodly clutter of my bench and I hate them with a burning passion. I have shelves for a reason. Also, batteries suck.)

author: gxti
author_rank: Frequent Contributor
timestamp: May 07, 2013, 12:49:16 am
content: I'm normally the type to complain about batteries, but I've had my 87-V for nearly 3 years now and haven't had to change it yet. I even bought new ones in anticipation of wearing it out quickly, maybe it's staying alive to spite me. Granted, I don't do this full time. I'm sure it's more of an issue for someone who has it powered up for hours a day.

author: staze
author_rank: Frequent Contributor
timestamp: May 07, 2013, 12:55:52 am
content: *nods*If I had a huge workspace, handheld meters are far superior. You can take them with you, put them where you need them, etc. That said, I have a tiny workspace (it's my home office desk currently), which I share with other uses. So having something live on a shelf is much easier... but, it also means it's not at eye level.... but then my scope won't be either.Damned if you do, damned if you don't.All that said, you CAN power the Fluke 45 from a battery. =P

author: jarvis
author_rank: Contributor
timestamp: May 07, 2013, 01:11:08 am
content: You could refer tohttp://fieldpiece.com/clamp-meters/sc53

author: staze
author_rank: Frequent Contributor
timestamp: May 07, 2013, 01:12:06 am
content: I should have indicated that I didn't mean clamp meters, obviously they'll do it just fine. =P

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 07, 2013, 01:23:37 am
content: The input amplifiers for voltage and current are very different (the current channel needs a lower noise floor and offset voltage, while the voltage channel needs a high impedance), so I wouldn't expect too much extra cost there for a quality meter. A second ADC will add a lot of cost, or they can multiplex the ADC the way many DSOs do. It would also require a DSP that is twice as fast, but processing power is really cheap nowadays.

author: Wytnucls
author_rank: Super Contributor
timestamp: May 07, 2013, 01:23:55 am
content: Quote from: Lightages on May 06, 2013, 11:16:54 pmTo be clear, AFAIK, the UT71E doesn't display the current on the screen. It only shows the measured real power W, apparent power WA (RMS) VA and power factor cos Phi (phase angle) on the triple display.It also only works with AC power, with a voltage between 50V and 250V.The current shunt is also limited to 10A for 15 sec (5A continuous), otherwise the shunt will overheat and the meter could be damaged and at best, be out of specs.To obtain the true RMS current (A), one would have to divide the displayed apparent power (VA) by the known true RMS voltage.

author: Electro Fan
author_rank: Super Contributor
timestamp: May 07, 2013, 02:08:48 am
content: Here are the Amprobe ACD-14 Plus and ACD-14 TRMS-Plus:http://www.amprobe.com/Amprobe/usen/Clamp-Meters/Multimeter-Clamps/ACD-14-PLUS.htm?PID=73068The have the ability to display current and voltage simultaneously. One thing that could be better though would be to make the displays larger and more easily readable.Another product that might also be of interest to you in helping with current reading (since almost every DMM reads voltage) is the ELS2A:http://www.amprobe.com/amprobe/usen/accessories/other/els2a.htm?PID=73229http://www.amazon.com/Amprobe-ELS2A-Current-Clamp-Accessory/dp/B001DPR0FE- not very expensive and pretty handy if you have a meter with a current clamp. Some manufacturers sell their version for even less but the Amprobe seems pretty good.

author: skipjackrc4
author_rank: Regular Contributor
timestamp: May 07, 2013, 02:58:20 am
content: Quote from: staze on May 06, 2013, 11:32:19 pmYes, but it uses a relay to constantly switch measurements because it has only one ADC. I don't use it because I don't want to wear out the relay. That's why I have multiple meters. And yes, I prefer bench meters, though at least one good handheld is absolutely necessary.

author: c4757p
author_rank: Super Contributor
timestamp: May 07, 2013, 03:05:14 am
content: Quote from: skipjackrc4 on May 07, 2013, 02:58:20 amAh, yes, thank you - this didn't really occur to me. I've performed such extensive repairs on my 45 that I don't really care if it wears out a relay, I'll just toss in another. That thing is more like my 12-year-old car than like my "nice" meters...Hell, not even my car has an EEPROM bodge-wired in because I didn't have one with the right pinout...

author: iloveelectronics
author_rank: Frequent Contributor
timestamp: May 07, 2013, 04:16:03 am
content: Some DMM's certainly can measure V and A at the same time, such as this Prova 803.

author: staze
author_rank: Frequent Contributor
timestamp: May 07, 2013, 06:25:31 am
content: Quote from: Electro Fan on May 07, 2013, 02:08:48 amI have a craftsman AC current clamp "probe". Obviously won't do DC, but yeah. I guess this was all more of a theoretical question than a prove me wrong. =D

author: staze
author_rank: Frequent Contributor
timestamp: May 07, 2013, 06:26:56 am
content: Quote from: iloveelectronics on May 07, 2013, 04:16:03 amCool!

author: Lightages
author_rank: Unknown
timestamp: May 07, 2013, 06:29:35 am
content: Quote from: Wytnucls on May 07, 2013, 01:23:55 amYou are partially correct. I should have been clearer. It measures voltage and current at the same time but only in AC and not very low in either with any accuracy. It defaults to showing power, a VA and cos Phi, but you can switch it to display Hz on the main display and current and volts on the secondary displays. This really is not of much use but it does do it.

author: BravoV
author_rank: Super Contributor
timestamp: May 07, 2013, 06:33:57 am
content: Quote from: iloveelectronics on May 07, 2013, 04:16:03 amHey Franky, is that yours ? Do a teardown please !

author: iloveelectronics
author_rank: Frequent Contributor
timestamp: May 07, 2013, 07:06:56 am
content: Yes, it's mine, but someone else already did a teardown:https://www.eevblog.com/forum/testgear/prova-903-discussion/msg182116/#msg182116Quote from: BravoV on May 07, 2013, 06:33:57 am

author: BravoV
author_rank: Super Contributor
timestamp: May 07, 2013, 07:19:01 am
content: Thanks, after seeing the board & reading the impressions made there, not very convincing.

author: Wytnucls
author_rank: Super Contributor
timestamp: May 07, 2013, 07:31:09 am
content: Quote from: Lightages on May 07, 2013, 06:29:35 amOK, I wasn't aware of that feature. I think the count is 4,000 on the secondary displays. Still useful I guess, if true RMS figures.

author: iloveelectronics
author_rank: Frequent Contributor
timestamp: May 07, 2013, 07:34:31 am
content: Quote from: BravoV on May 07, 2013, 07:19:01 amThats why I don't talk about that meter muchIt is nonetheless a very unique meter.

author: Wytnucls
author_rank: Super Contributor
timestamp: May 07, 2013, 07:36:37 am
content: Quote from: BravoV on May 07, 2013, 07:19:01 amYes, not impressed either. Kudos for innovation, but the implementation is dodgy at best. The better option is to buy 2 meters, I think.

author: retiredcaps
author_rank: Super Contributor
timestamp: May 07, 2013, 07:41:49 am
content: Quote from: iloveelectronics on May 07, 2013, 07:34:31 amFranky, I think it is time for you to post a photo of all your multimeters in this thread ...https://www.eevblog.com/forum/chat/show-your-multimeter!/

author: jpb
author_rank: Super Contributor
timestamp: May 07, 2013, 08:56:26 am
content: My bench multimeter will take two measurements - the ADCMT 75451A, it has two A2Ds for 4 wire resistance measurements and allows you to also use them separately for V and I with a B input though the voltage is limited to 10V.I've not used this facility and the manual/meter is at home so I'm not sure what it displays in terms of the second reading, it might be more geared for computer controlled measurements via the USB.

author: Neilm
author_rank: Super Contributor
timestamp: May 07, 2013, 08:20:16 pm
content: I'm surprised that that many meters do measure both at the same time - the potential for safety issues are quite high. Imagine accidentally probing your voltage source with your ammeter.Neil

author: staze
author_rank: Frequent Contributor
timestamp: May 07, 2013, 08:28:40 pm
content: Quote from: Neilm on May 07, 2013, 08:20:16 pmMeans you blow a fuse, if the meter is "safe". I did this before I realized what I was doing... used an old Radioshack meter, set it to amps, and measured across a circuit. And what do you know, it no longer measured current (until I replaced the fuse). With a crappy meter, yes, you could have things go BOOM.

author: Unknown
author_rank: Unknown
timestamp: May 07, 2013, 08:39:03 pm
content: Quote from: jpb on May 07, 2013, 08:56:26 amWhy would it need two ADCs for four terminal resistance measurements? Four terminal measurements use the same constant current source + voltmeter approach that regular two terminal measurements use.

author: robrenz
author_rank: Super Contributor
timestamp: May 07, 2013, 08:40:37 pm
content: My Fluke 8846A measures DC current and DC voltage simultaneously.From the manual:"The resistance of the common lead combines with asmall amount of internal resistance in the meter to cause an IR dropthataffects the accuracy of the voltage measurement. Depending onthe circumstance,this effect can be substantial. As an example, 20m? of lead resistance can cause more than 20 mV of additional errorat 1 A."I would suspect that this error probably applies to most of the meters that can do this.

author: Unknown
author_rank: Unknown
timestamp: May 07, 2013, 08:56:26 pm
content: Yes, I don't see how any meter that shares an input for current and voltage measurement would avoid this. 20 mV may very well be negligible, however, depending on the required accuracy and voltage. For mains for example, you can safely ignore it, unless you have one of those rare meters that do 0.01% accuracy on AC (Agilent 3458A maybe?).

author: robrenz
author_rank: Super Contributor
timestamp: May 08, 2013, 01:14:43 am
content: The 8846A only does DC V and I simultaneously. Even if it did AC it is only 0.06% rdg +0.03% rng 1year spec.

author: jpb
author_rank: Super Contributor
timestamp: May 08, 2013, 08:31:30 am
content: Quote from: alm on May 07, 2013, 08:39:03 pmI don't know in detail but I'd guess it was to get an accurate measurement of the nominally constant current. The B channel is used for the sensing (at least the terminals are labelled that way) so presumably the main channel is used to determine the current. The test current ranges from 50nA to 10mA. I would also surmise that it is difficult to make a perfect constant current source and it is easier to make one that is close to constant and measure it to get accurate results.

author: Unknown
author_rank: Unknown
timestamp: May 08, 2013, 05:04:18 pm
content: So how would this be any different for a two terminal resistance measurement?

author: jpb
author_rank: Super Contributor
timestamp: May 08, 2013, 05:38:16 pm
content: Quote from: alm on May 08, 2013, 05:04:18 pmThe current would be measured on different wires to that for which the voltage is measured so there would be negligible voltage drop on the wires used to measure the voltage across the device.A high impedance voltmeter attached to the DUT by one pair of probes. A second pair of probes supplies current via a known internal resistance the voltage across which is used to measure the current.The current is not measured by measuring voltage across the DUT as the resistance of this isn't known anyway.

author: Unknown
author_rank: Unknown
timestamp: May 09, 2013, 11:40:53 am
content: I'm aware of the difference between two and four terminal resistance measurements, but why would connecting the source and sense wires at the DUT require more ADCs than connecting them at the DMM terminals, as is the case for two terminal measurements?

author: jpb
author_rank: Super Contributor
timestamp: May 09, 2013, 12:08:36 pm
content: Quote from: alm on May 09, 2013, 11:40:53 amOK I get your point. I don't know the inner workings of the meter as though I took it apart to replace the electrolytic caps after buying it on e-bay I wasn't trying to reverse engineer it.But what I guess is that when measuring resistance it applies a fixed voltage with a known resistance in series. Measuring the voltage across the known resistance gets an accurate measurement of the current and the voltage across the terminals (being the reference voltage minus the voltage across the resistor). This is all you need for a 2 wire measurement. (It probably measures the voltage across the terminals and subtracts this from the reference voltage to get the voltage across the resistor.)For a 4 wire measurement it has a second meter (which can be limited to 10V) which measures the sense wires to get a more accurate measurement of the voltage across the DUT instead of at the terminals.