New UT139C weird behaviour - Page 1
Forum section: electronics / testgear

author: dimoniet
author_rank: Newbie
timestamp: December 23, 2015, 06:26:50 pm
content: I'm new here despite of being an old dave's videos follower, so hi all!I'm a half hobbiest half professional embedded developer and I've acquired recently an UT139C as my main DMM to complement my beloved plus very old 90s one.I've purchased it through eBay and after 40 days of waiting it arrived home, sweet home.The DMM feels good and after some testing it behaves ok with some weird exceptions.The capacitance meter never gets to 0, with no cap or leads attached.It always starts displaying a 0.254nF value which ramps up to 0.270nF slowly, about 0.001nF a second with some changes up and down (e.g.: 0.254,0.255,0.254,0.255,0.256,0.255, etc.).When I test any cap this previous value is added to the displayed value (e.g.: 10.5nF real value, gets displayed as 10.754nF value).It's Really annoying.I know I can rel the meter but I don't feel confident as It's a new unit and it's supposed to come factory calibrated.Correct me if I'm wrong.The other strange behaviour comes on the mV scale, where the initial value is never 0 also, and it starts to decrement fast, forever.Assume 5mV is displayed, then it counts down 4.xx, 3.xx.. until 0 and then gets counting up negative values up to -200mV and beyond.Finally the backlight is barely visible, one can't tell if it's on with any other light on, only in total darkness.Anyone who has this meter experienced such an odd behaviour?Is it normal? Or do I got a defective unit?Excuse my poor English as it's not my native language.Thank you all in advance!

author: Monkeh
author_rank: Super Contributor
timestamp: December 23, 2015, 06:50:36 pm
content: Quote from: dimoniet on December 23, 2015, 06:26:50 pmQuite normal, get used to that 'rel' button. You can't calibrate out something which is not constant (change leads, move leads, move hands..).QuoteQuite normal, very high input impedance.

author: ProBang2
author_rank: Frequent Contributor
timestamp: December 23, 2015, 06:58:48 pm
content: Hello.Nothing to complain. Totally normal Behaviour.Simply every DMM has a residential capacitance. That´s the prize for being so sensible...Therefor is the REL- function.(Same with the residential resistance of the leads.)It is a little different with the mV- scale.Again: Totally normal. Your probes working as an antenna for surrounding electric fields.But this does not affect your readings. Short the probes. The display immediatly changes to zero.Which is the enviroment for activation of backlight?When other lights are on? Or in total darkness? And... do you often work in dark rooms...Serious: Nothing to worry about. Really.

author: dimoniet
author_rank: Newbie
timestamp: December 23, 2015, 08:18:07 pm
content: Quote from: ProBang2 on December 23, 2015, 06:58:48 pmYeah backlight is almost useless to me but it felt weird not noticing any difference between on and off state.My other DDM is 20 years old and cost about 20€, paid on currency before euro xDSo mV and pF were science fiction to me until now.Not bad DDM for the price and time tough, I can post some photos if you are curious about my ancient DDM brick-meter.Thank you both for the accurate answer, now I feel more confident ^^

author: PChi
author_rank: Frequent Contributor
timestamp: December 23, 2015, 08:28:01 pm
content: I have had problems with lack of 50 Hz rejection with a Tenma Multimeter, could that be the issue with the mV range?

author: dimoniet
author_rank: Newbie
timestamp: December 23, 2015, 09:07:43 pm
content: Quote from: PChi on December 23, 2015, 08:28:01 pmI've tested it with no probes, so I don't think so.Or are you talking about some EMI radiation that causes interferences?

author: SeanB
author_rank: Super Contributor
timestamp: December 24, 2015, 06:18:28 am
content: mV range issue is that in mV the input impedance is that of the ADC converter only, no voltage divider resistors are connected to ground to handle the very small bias current of the ADC chip. Thus it will float around until it autoranges and clamps back to zero, till autorange decides to switch it back to high impedance. Tip is just to connect a 1M resistor across the input and you will see it settle down to close to zero. Tried using another mater on ohms range, and when you manually switch ranges to other than mV it shows 11M input impedance, but flip down to there and it is over 20M ( overrange on the other meter, I do not have one that has a gigohm range with low voltage) and shows the open circuit measurement voltage of the other meter as value.

author: jitter
author_rank: Frequent Contributor
timestamp: December 24, 2015, 08:46:29 am
content: Quote from: dimoniet on December 23, 2015, 06:26:50 pmWelcome to the forum!QuoteNormal. Everything has capacitance, whether it's intended (capacitor, obviously) or not (e.g. traces or wires). When it's not intended, it's called a "paracitic capacitance". Your meter will indeed add this to the capacitance of the device under test (DUT).You can do some experiments to see the effect. Connect the leads with no capacitor connected and hold them parallel and close to eachother. Now move them apart and see the value drop. This effect is something to keep an eye on when measuring low value caps. You can indeed null this capacitance, but then moving the leads means you need to null it again.QuoteHigh input impedance and high sensitivity will do this, so it's again normal. Only shorting the input terminals together will give you a stable 0. Ever seen a 6.5 digit bench meter with nothing connected to it in the mV range? It's going all over the place until a connection to a DUT is made.QuoteWell, darkness is usually the only situation you will need the backlight. And in that situation, you don't want it too bright to prevent it being hard on your eyes. Besides, very bright backlight would most likely seriously shorten the battery life.QuoteQuite normal.QuoteIt's good, actually.

author: PChi
author_rank: Frequent Contributor
timestamp: December 24, 2015, 12:22:35 pm
content: Quote from: dimoniet on December 23, 2015, 09:07:43 pmI am talking about direct capacitive pick up of the 50 Hz plus harmonics when measuring a high impedance circuit. I had a few mV offset whem measuring a few 100 kohm source impedance circuit even when measuring a battery powered circuit with no connection to the mains supply. Fluke and others quote common mode rejection of a 50 and 60 Hz signal.

author: dimoniet
author_rank: Newbie
timestamp: December 24, 2015, 12:42:15 pm
content: Thank you for all the details and kind answers.As you all should figured by now I'm not an electronics engineer but a networking one, trying to set a little home lab and expand my electronics background by tinkering with some MCUs and simple electronic projects.Back at my college on the early 2000 we had rather limited equipment, the only bench equipment I used was a DSO and a function generator.The DDMs were all cheap unbranded ones with less sensitivity than any of the actual 2$ chinese ones.One question that has come to mind talking about this matter..Is it worth to get a voltage reference to check DMM on a regular basis for consistency on the values or it's better to get another unit of the same brand/model to cross check them both.I've seen this voltage reference on some videos, but it feels little expensive to me to check a 40$ DMM.http://www.ebay.com/itm/271791808061I can create a new thread if I'm mixing different things on this one.

author: PChi
author_rank: Frequent Contributor
timestamp: December 24, 2015, 12:57:54 pm
content: I have an ancient Hung Chang Multimeter that uses the 7106 Integrated Cicruit and it quickly settles to reading 00.0 on the 200 mV range with no probes attached.I bought a Hioki DT4282 recently and on the DC Volt range with no probes attached it reads a few mV. The reading does seem to change when I wave my hand around it. Hioki claims high rejection of 50 Hz commmon mode signals so I guess that it's picking up it's own internally generated noise.Interesting. I will try the test with a few other multimeters when I get the opportunity.

author: SeanB
author_rank: Super Contributor
timestamp: December 24, 2015, 01:54:15 pm
content: DMMCheck is good, solid and reliable for checking basic low range of a meter. Not really needed, you generally have either working or not, and using just cheap new non rechargeable batteries you can easily check the low voltage ranges.

author: nrxnrx
author_rank: Unknown
timestamp: December 24, 2015, 02:11:42 pm
content: Quote from: dimoniet on December 23, 2015, 06:26:50 pmOn mine (a Tenma rebrand), the backlight is nice, though it turns off too quickly.It's difficult to tell from a photo, but if you don't find the backlight useful, you should probably get a replacement unit.During the day:

author: retiredcaps
author_rank: Super Contributor
timestamp: December 24, 2015, 10:33:49 pm
content: Quote from: dimoniet on December 24, 2015, 12:42:15 pmThere are references in the $10 USD range that have been discussed here on eevblog that will likely be good enough. Having said that and probably seeing/using around 50 meters, I have only seen one meter (old 25 year) out of spec.

author: dimoniet
author_rank: Newbie
timestamp: December 25, 2015, 12:35:57 am
content: Quote from: retiredcaps on December 24, 2015, 10:33:49 pmI'll revise the forums then to look for cheaper one in case of getting paranoid about specs. I don't have much trust on chinese ebay meters but I do trust your expertise though.Quote from: nrxnrx on December 24, 2015, 02:11:42 pmAbout the backlight it's possible my leds are kinda rubbish because yours look a lot more brighter than mine, but I think I can live without this feature.My only concern was about some more top level failure that could cause all the weird (now I know it's normal) behaviour described on the first post.See attachment to check how it looks off (left) and on (right).Barely visible if not in total darkness.

author: sreeb
author_rank: Contributor
timestamp: December 25, 2015, 02:33:05 am
content: Quote from: PChi on December 24, 2015, 12:57:54 pmBetter meters will have higher input impedance and will therefore couple more noise when unconnected. If you want to know how your meter is zeroed, jumper the input to gnd with a short banana plug cable.

author: Maxlor
author_rank: Frequent Contributor
timestamp: December 25, 2015, 02:34:35 am
content: Quote from: dimoniet on December 24, 2015, 12:42:15 pmI don't think it is worth it, unless you just want to know the accuracy, or have a project where you really need 4 digits of precision. It's very rare for a multimeter to drift out of its spec (UT61E excepted, heh), and you can just compare it to another multimeter to notice that something's amiss. The money is much better spent getting a second multimeter. Don't get the same model either (unless you *really* like it), try to find something that complements the ones you already have, so you end up with more measurement capabilities.