Voltcraft VC-850 Teardown

[Hagis2k]

Hello!

Well i decided to buy a newer multimeter for my electronics works and so will use this mainly around up to 50V AC/DC:

Got it delivered yesterday from conrad:

http://www.conrad.com/ce/en/product/124602/VOLTCRAFT-VC850-Digital-Multimeter-with-Software-included-6000-counts-CAT-IV-600V-CAT-III-1000V?ref=list

It is on sale here right now in sweden and is on sale for like 50€ so i know it might not be the best quality like fluke, appa and gossen and so on was a bit curious
on how the inside of this meter looked like so i took it apart and took some photos of it.

The feeling of the multimeter otherwise in quality feels pretty solid it got 4 screws holding it together and the input jacks are pretty solid compared to uni-t ut61x series
where there is a flimsy little metal tube pushed down in the front and held down by the back cover it also warn you of cables in the wrong terminals too, me i find it pretty
ok even if i know well enough not to use the wrong inputs and so.

I have looked at the construction and i like it pretty much seems to have both ptc and movs and a bunch of serial resistors on the input 3 x 33m? and 4 x 2.5m? before the input of the ic
seems pretty ok and after i saw the other side of the pcb i am pretty happy with this unit.







So updated the post with more images of both sides of the pcb, and i changed the thread name as it became a teardown lol

[Wytnucls]

Looks well built and, at first glance, has decent input protection. Would have been nice to see the other side of the board for further comments. You did good, at 50 Euros, it is a bargain. Enjoy it!

[Forser]

Is this a better choice then say Uni-T UT61E? pretty much around the same price range right now with the discount.

[Wytnucls]

Lower count, no peak (has Max/Min) and no bar graph, but safer, better built, low Z, temperature measurement and it has a backlight. You decide.
The USB interface cable costs extra though.

[rollatorwieltje]

I'm pretty sure it's made by Uni-T. Same IR port, same tilting bail, UT61 front case... It looks like a special though, the board and input terminals look different.

[Hagis2k]

The ir cable that fits uni-t shall fit i think i found one for like 12-14$ on ebay i shall buy one and then try it

[Hagis2k]

And this i shall order too:

http://www.ebay.com/itm/UNI-T-UT-D04-USB-INFRARED-INTERFACE-CABLE-UT71-UT81B-UT61-/360647971220?pt=LH_DefaultDomain_0&hash=item53f84b5194

As conrads "voltcraft" version is like 3 times the price lol and maybe i buy a better pair of test leads too:

http://www.ebay.com/itm/171162377470?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649

Or maybe fluke tl71...

[Retep]

I bought this meter about 2 years ago, also for €49,99 (special offer). I was planning to post a teardown of it here (already made the photos), but never got around to it and Hagis2k beat me to it.

FWIW my impressions of this DMM:

Pro:

• Decent build quality; it feels quite solid, not quite the quality feel of a Fluke but I'm sure it would survive a drop of a meter or two;
• Appears to be quite safe, I'm not sure if really lives up to its CAT-III (1000V)/ CAT-IV (600V) rating, but it does have: proper fuses, blast protection, wrong lead detection, high voltage warning, locked battery/fuse compartment when leads are attached;
• Easy to read display;
• Functionality: Volts, Capacitance, Resistance, Continuity, Diode, Frequency, Duty Cycle, A, mA, uA, Temperature(C/F)...what more can one expect?
• True RMS;
• Low impedance measurement option.

Con:

• Slow update rate (update rate of bargraph is OK);
• Slow auto ranging;
• Very slow continuity beeper (latched); close to useless (if they can't put in a fast latched beeper in it I'd rather have a unlatched one);
• Difficult to operate with one hand, the hard plastic case has no grip so turning the selector dial with one hand tends to rotate the meter instead of the dial;
• Weird arrangement of functions on the selector dial; most frequently used functions are not on the first positions, and some frequently used functions are made secondary (e.g. continuity beeper);
• The (very nice looking) backlight automatically turns off after a few seconds, rendering it pretty much useless;
• Frequency measurements are unreliable when the selector dial is in the volts position (but are fine in the Hz position);
• Capacitance measurement is almost 3nF off and drifting on my unit (this can be REL'ed out once it stabilizes);
• Though the diode measurement supplies just enough volts (3.1V) to light up a blue LED, the measured voltage drop is way off. (regular silicon diodes are no problem for this meter);
• No min/max function; the Fortune EF9922-DMM4 chip (http://www.ic-fortune.com/upload/Download/FS9922-DMM4-DS-11_EN.pdf) on which this meter is based does support Min/Max but no button is attached to this pin (i.e. if you care you could hack it in);
• Accuracy specs aren't great (given those specs I wonder what those 6000 counts are good for), though in real life it is only a few counts out compared to my Fluke 179 (which isn't that accurate either, but is the best I have);
• Input protection (?) It certainly has some, but I'm not too sure how well this DMM would survive serious abuse.

Though the con list is longer than the pro list, I do believe it is a pretty good buy for €50,- , at least for hobbyist use. In my country Conrad nowadays sells this meter for €125,- which is in my opinion way too much. If you are a hobbyist on a tight budget (or are just a cheapskate like me) in need of a DMM and can get this one for less than ~€70,- I'd say go for it. Otherwise I'd look further.

[Hagis2k]

They also state in the data that the ic shall have sleep mode guess it's been disabled can it be pin 94? (txen operation = low and sleep = high) so if i am not mistaking pulling this pin high with like a 10k resistor will that give the ic the sleep option then?

[retiredcaps]

It is on sale here right now in sweden and is on sale for like 50€ so i know it might not be the best quality like fluke, appa and gossen and so on was a bit curious
on how the inside of this meter looked like so i took it apart and took some photos of it.

Again, thanks for the teardown photos.

[Wytnucls]

Stated voltage accuracy is dreadful for a 6,000 count meter. A Gossen 12S with half the count (3,100) has a far better performance, at 0.25%+1 digit, instead of 0.6%+8 digits.
I couldn't find a MOV on that board, which is strange, considering the trouble they went through to make an average meter as safe as possible.
400Hz is also pretty limiting for a true RMS meter.

[Hagis2k]

Really wonder what RV1 and RV2 could they be varistors? but they are such odd shape then too makes me wonder...

[Wytnucls]

They could be MOVs, similar to this Littelfuse package:

http://www.littelfuse.com/~/media/electronics/datasheets/varistors/littelfuse_varistor_ra_datasheet.pdf.pdf

[ModemHead]

Really wonder what RV1 and RV2 could they be varistors? but they are such odd shape then too makes me wonder...

They look like Taiyo Yuden spark gaps to me.  Filling essentially the same role as a varistor.

[Hagis2k]

They do have the T with a circle around makes a bit of sense now



http://www.ebay.com/itm/50-Taiyo-Yuden-15PC-Spark-Gaps-1500V-/200683676892?pt=LH_DefaultDomain_0&hash=item2eb9ade4dc

If that is the case then it should be a bit more safe then others from uni-t like ut61x series

[Hagis2k]

Is there anyone who knows where i can obtain some information in like calibration of this device tried both uni-t and conrad i assume the last to do is to evaluate the design and compare it to the standard design of a multimeter and mark the potentiometers up and try it the hard way?

Think i shall give it another try on uni-t as they manufacture the device

[Retep]

Considering the rather poor accuracy specs I doubt that UNI-T went to much trouble calibrating/adjusting this meter. Unless you get any response from Conrad or UNI-T (I wouldn't hold my breath until that happens), probably your best bet would be the datasheet I linked here before and a bit of reverse engineering to figure out what the pots in this meter do. Before you start playing with these you better have access to some kind of reference that is significantly more accurate than the stated accuracy of your meter.

Anyway I wonder why you care. The VC-850 is simply not a precision instrument and it is not going to be no matter what you do with it. For many purposes its accuracy is good enough, but if you need really accurate measurements this is the wrong meter. Compared to my Fluke 179  (stated DC volts accuracy: 0.09%+2 digits, which is average at best but still a lot better than the VC-850), I noticed that the divergence is not uniform across the voltage range. At some voltages the indication on the VC-850 is a few counts higher than on the Fluke 179, at other voltages it is a few counts lower, and at some voltages they pretty much agree. I.e. a simple adjustment won't make those all go away.

[Hagis2k]

That is true probably need to revere engineer it i assume no luck with conrad wonder if they have any aka service department and ask for some info mailed uni-t a few days ago wouldn't keep to high hopes on that lol