DT 838 Digital Multimeter - Just Curious - Page 1
Forum section: electronics / testgear

author: Mkrtich
author_rank: Contributor
timestamp: May 25, 2016, 10:37:15 pm
content: I purchased a DT838 Digital Multimeter from an eBay source based in Hong Kong recently for curiosity sake - I have seen non complimentary teardowns of the DT830 series multimeters on You Tube. The previous versions of this series I had seen were very rudimentary productions that were hand soldered, some patching a mini PCB extension board for the input jacks to the main PCB and others with a one PCB that had an elbow to accommodate the input jacks. For $8.00 delivered to Australia, I thought what the heck and ordered a DT 838 which has the additional feature of inbuilt temperature gauge which I thought was neat (turn to Temp position and the temperature in the room is displayed) and pseudo micro K-lead set.My research on the OEM manufacturer lead me to Sinometer in China who make many well known name brands of multimeters ranging from economy to medium serious qualities e.g UNI-T, Mastech and others.http://www.sinometer.com/?Digital-Multimeters-DMMs-category171-------12-sort-asc--4.html. See also technical specifications PDF attached.Much to my surprise it is the size of an Apple iPhone 5 and when I opened it up, the PCB has considerably shrunk to maybe three quarters of it original DT 830 series size and it now uses surface mounted components with only a few hand soldered points. There is no glass fuse inside for input protection on the 200mA jack and none on the 10A jack which I never intend to use. I thought I might keep the multimeter in the glove box of my car in case of any future battery test situations.The back of the case housing indicates a safety notice that only a 0.5A/250V Fuse should be used but there is no traditional fuse housing for any fuse - perhaps the case is from older models which did have one glass fuse for the V/Ohms/mA jack. I understand it is possible to have surface mounted fuses on PCBs or to use an Metal Oxide Varistor for some level of protection and I am wondering if that is the orange coloured component near the buzzer is a MOV. I can not see any modern looking micro surface mounted fuse but there is a square solder covered component on the board. I have not looked on the underside of the PCB as generally there is not much room there and I would not expect a user to have to dismantle the multimeter to replace the fuse and the function dial mechanism dominates that area.Does anyone know if there is any protection at all on this multimeter? Just curious, as I don't ever intend to use the multimeter on 240 V.I found installing the battery a challenge as you can see from the photo, it has slightly bent the COM input jack as the contact sleeve cover for the battery rubs right into it; turning the battery around the other way involves metal to metal contact which I was not keen on and it did not sit well.Otherwise, for the price of two cups of coffee, I am happy with the purchase - the display is clear , 1999 count and steady, no wondering.thanks

author: retiredcaps
author_rank: Super Contributor
timestamp: May 25, 2016, 10:47:40 pm
content: QuoteIt is NOT a MOV. It is a capacitor probably used for reference or timing.QuoteFor the price, there is likely no input protection.

author: mos6502
author_rank: Frequent Contributor
timestamp: May 25, 2016, 11:10:36 pm
content: Quote from: Mkrtich on May 25, 2016, 10:37:15 pmThat could bite you in the ass. These meters have no low battery warning and start to measure wrong as the battery gets low.Say your car doesn't start. No crank. You jump start the car and measure the battery voltage with the engine running. Your DT838 shows 12.4V. This means your alternator is bad, so you buy a new alternator, problem fixed. Right?Well, no. The meter's battery was low and thus it measured wrong. In reality, the voltage was 14.4V. The alternator was fine and you just wasted $300 on a new alternator.If you just want to get a glove box meter, get a Fluke 101. That only costs 40 bucks and you can actually rely on the measurements.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 25, 2016, 11:17:15 pm
content: The transistor socket may be a clue. No fuse for the shunt. Did you see the leads I destroyed last weekend? Imagine a non- fused cheap lead plugged into the current inputs then trying to measure your battery voltage. Clearances, what clearances?!

author: Mkrtich
author_rank: Contributor
timestamp: May 26, 2016, 02:11:07 am
content: Thanks for your feedback and your advice is appreciated . Yes, I agree with all the above - the meter does not meet worthwhile design and engineering standards - handy for 1.5 and 9.0 Volt battery testing only and to keep to show others what not to use for anything serious and reasons for so. BTW, according to the specification sheet, it does feature a Low Battery Indication (not personally tested - same sheet that said it had a physical fuse) and I agree , a Fluke is a better tool; I have a Fluke 77 Series 1 , circa 1984 as reliable benchmark which I will use instead.I suspect that the 830 Series Multimeters are made by a variety of factories in China and the one I have actually purchased may not be an Sinometer M838 but a copy of it and rebranded as at DT838. I have found schematics of the M838 on the web that shows a physical 250V fuse, so there has been a change in the PCB design and update to SMT sometime in the one I have.

author: Mkrtich
author_rank: Contributor
timestamp: June 01, 2016, 03:57:51 am
content: Just some additional follow up. I emailed the seller and suggested they advise the manufacturer to take note of their apparent misrepresentation of the fused function in the user manual and on the back of the case branding and to have a warning on the eBay site specifically stating the DMM DT838 being sold is not fused on the 10A or 200mA inputs. Also another suggestion to the manufacturer was to alter the positioning and spacing of input connectors inside the DMM so that there is no rubbing between the battery and the metal COM connector and that people would be prepared to pay additional dollars for enhanced safety features even if it was a single fuse on the mA input. The eBay seller, "vollter" based in Shen Zhen, communicated back to me that they would pass that information on to the manufacturer - one can be an optimist at times.I did manage to track down what look like circuit schematics [GIF format] for the DMMs from the Web Images of Google and have listed them here if anyone has an interest.(i) M838 which resembles my DT838 - no fuses at all - not sure if more recent production or superseded by the alternative version below - IC7106 used.(ii) MAS 838 (one fuse for 200mA) DMMs IC 7106Y.

author: Mkrtich
author_rank: Contributor
timestamp: June 22, 2016, 10:37:02 am
content: In case anyone is interested in comparing their DT 83X Series PCB, my recently purchased DT838 B unit appears to have the followingThe PCB is branded with a Circle Ke logo with markings XT-838 20130809 - August 2013 production.1. COB - unknown due to blob cover - likely to be IC 7106 family - I have seen AME 7106Y on another schematic2. Single LM324 Four Channel Operational Amplifier coded XJSYOMLH8 - used in single supply or four channels split over multiple voltages. I have seen 2 x LM358H Dual Channel Op Amps in other versions.3. M7 PNP Transistor - other models have a Q1 9103 or Q1 91044. Single Trimmer Potentiometer marked 202.5. Usual 200mA/250V glass fuse on mA input missing in action. The piezo buzzer, the size of a 10 cents coin, sits on top of most of the area where the fuse is usually mounted.6. I can't see anything that looks like a temperature sensor - the DT838 tells you the room temperature without any leads connected and also has the K-Type Temp. Cable Leads kit included, so how that function works will remain a mystery.With a stable linear external power supply set to 10 volts DC, I tried to calibrate the DT838 with adjustments to the T-Pot, but it made absolutely no difference to the voltage shown on the DT838 display. Suspecting that there may have been a fault with the T-Pot, I dabbled some solder on one of the legs which looked slim on solder and this made no difference either. I then measured the T-Pot resistance on full clockwise which read 1.54 k Ohms and then full anti-clockwise which read 0.003 Ohms thinking it must be fully operational. I set the T-Pot to its original middle position of approximately 750 Ohms concluding that the DT-838 may not be designed for calibration and that the T-Pot may be for functions other than DC volts or just for decoration.I then came to realise that the DT838 is indeed made to the stated specifications for DC volts accuracy being +/- 1% +/- 5 digits. The meter read 10.07 volts DC for the 10 V setting - this would place it into a 9.9 volts to 10.1 volts range without the 5 digit variation. So maybe the COB controls the calibration on this model.Even with its lack of safe design for anything other than non serious DC volts only measurements, one can not help but admire how all this magic that is surface mounted for under AUD $8.00 delivered from Hong Kong somehow works.I have come away with the impression that most people are aware of the DT 83X Series at one time or another in their journey into electronics, albeit with all the non-complimentary in nature YouTube Reviews and Tear Downs, when compared to modern mainstream more competent DMM designs. Perhaps in twenty years time, it will become a vintage collectors item. May all your experiences be safe ones.