Got my GDM-8251 from eBay today...one question - Page 1
Forum section: electronics / testgear

author: alank2
author_rank: Super Contributor
timestamp: January 17, 2017, 08:00:20 pm
content: It had some scratches in the display bezel which I'm not thrilled about, but I guess it is what it is.Now for my question:I prepared a 50Hz 5V RMS signal from my SDG2082X with 2V DC offset and decided to try AC on the first display and DC on the second display. In this case, it displays 30mV!!! My Fluke 8845A has no problem showing 2V. It seems to be very sensitive to the frequency. If I change it to 1kHz, it goes to 58mV. At 10k it is fine.Now if I switch it back to 1K, it is fine, back to 50 Hz, it is fine. It is an auto ranging type of problem it seems. Same thing happens in DC mode.Firmware is V2.30 - what is the latest?

author: xrunner
author_rank: Super Contributor
timestamp: January 18, 2017, 04:49:33 pm
content: I'm going to use your thread to post some things abut this meter, as I'm trying to get the firmware and do that upgrade (still waiting on the file from GWinstek).Quote2.3Quote from: alank2 on January 17, 2017, 08:00:20 pmOK, I tried it as you did, and got the same results, but check this - after you use 10 kHz, go right back down to 50 Hz. On my unit it will then readout the correct DCV number after that.It does read a freq. of 50 Hz just fine also.Also, starting from the beginning, in combined ACV + DCV, it will readout the correct value 5.38Vsqrt (AC2+ DC2)

author: alank2
author_rank: Super Contributor
timestamp: January 18, 2017, 04:53:24 pm
content: Quote from: xrunner on January 18, 2017, 04:49:33 pmYes, because it is in the right range at that point. The problem is that with the slower frequencies, it doesn't select the correct range initially. If you up the frequency and then it does, when you lower the frequency, it will stay.

author: xrunner
author_rank: Super Contributor
timestamp: January 18, 2017, 05:23:26 pm
content: Quote from: alank2 on January 18, 2017, 04:53:24 pmHmm, but that can't be the reason either, because if you use DC offsets in the mV range in the beginning, it doesn't display the correct value either, at least on mine.

author: xrunner
author_rank: Super Contributor
timestamp: January 18, 2017, 07:11:59 pm
content: Well, having played with this now for an hour, it's pretty obvious that it works OK in auto range (displays the correct DCV) with an initial freq. of at least 2 kHz or higher. It also displays the correct DC or AC voltage reading if not in a dual-display mode, and it also correctly displays the calculated value of a single combined reading of ACV + DCV.And, since you at least, have the latest firmware ...Since it's obviously gone through several firmware revisions, and since, even though we are pretty smart people here, it's doubtful we are the only ones who have noticed this since it's been out.The answer then is that it's acting within it's design parameters. The issue we observe - a dual-display ACV and DCV reading issue of the DCV being wrong (below 2 kHz initially) -->in auto-ranging mode only, is a specification/limitation that is not explicitly listed in the manual. Many pieces of test gear have limitations and bugs, this is a limitation for this meter.But, as you realize, and which works, is to simply acknowledge the issue and turn off auto ranging for the DCV second display. All you have to do is up-range it to a higher range and you get the correct reading.

author: alank2
author_rank: Super Contributor
timestamp: January 19, 2017, 06:18:02 pm
content: Response from GW Instek:>The issue is caused by dynamic range. For GDM-8255A/8251A, customer need to manual set the range for it.>The newer model like GDM-834x or GDM-8351 has already solve this issue.

author: xrunner
author_rank: Super Contributor
timestamp: January 21, 2017, 07:24:43 pm
content: Quote from: alank2 on January 19, 2017, 06:18:02 pmYep.I ordered the GDM-01 cal key from Tequipment, but then found out that they couldn't ship it until March 28, so I had to cancel that order. I found the key for sale on Ebay so it should be here in a few days. I also got the firmware file from GW Instek, so I'm ready to try the upgrade as soon as I get the cal key, via the RS-232 serial connection.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 26, 2017, 04:05:59 am
content: @xrunnerCould you check when the usb cal key arrive, if you have a short between pin 1 and 4 with an ohm meter ... and report the results.thks

author: Housedad
author_rank: Frequent Contributor
timestamp: January 26, 2017, 08:24:18 am
content: is it the 8251 or the 8251A you have?

author: coromonadalix
author_rank: Super Contributor
timestamp: January 26, 2017, 11:54:18 am
content: GDM-8251A the cal key GDM-01 should be the same between these two I've been toldthksOn a few web sites they sold it for 8 to 10$ but the minimum shipping fees are 23$ for a mere few grams ... sent email to gw instek support

author: xrunner
author_rank: Super Contributor
timestamp: January 26, 2017, 01:23:24 pm
content: Quote from: coromonadalix on January 26, 2017, 04:05:59 amOK I just got up, stand by ...Quote from: coromonadalix on January 26, 2017, 11:54:18 amI got one off Ebay for $6.88 + $10 shipping. I agree the shipping is stupid as it could be sent in a small envelope.GDM-01I did complete the firmware update yesterday and all went according to the document, now I'm at v2.3.

author: xrunner
author_rank: Super Contributor
timestamp: January 26, 2017, 02:02:48 pm
content: OK I checked pins 1 & 4 (the outside pins) of the cal key - they are essentially shorted together. I also plugged in a USB test connector I have into the back of the DMM into the cal key USB slot, and I measured 4.88V between pins 1 & 4 (the outside pins). So the cal key is connecting 5V back to a sense terminal of some component inside?I'll be interested in what you all do with this info.

author: alank2
author_rank: Super Contributor
timestamp: January 26, 2017, 02:59:57 pm
content: Wait, let me get this straight, they decided on a design decision to short the + and - lines in a USB A connector? Is that right?

author: xrunner
author_rank: Super Contributor
timestamp: January 26, 2017, 03:06:18 pm
content: Quote from: alank2 on January 26, 2017, 02:59:57 pmApparently - yes they did.

author: edpalmer42
author_rank: Super Contributor
timestamp: January 26, 2017, 03:53:26 pm
content: I was looking at the specs for the 8251A and there seems to be some confusion on the input resistance. The GW website shows 10M for DC and 1.1 M for AC.The manual doesn't say anything about input resistance. But there's a datasheet floating around that reverses the two numbers!For a 5.5 digit meter, 10M is marginal. 1.1 M is depressing.Can someone measure their 8251A and report the results?

author: xrunner
author_rank: Super Contributor
timestamp: January 26, 2017, 07:25:19 pm
content: Quote from: edpalmer42 on January 26, 2017, 03:53:26 pmJust measured it using an external 1M R and 10.0V. The input R is 10M.

author: sequoia
author_rank: Unknown
timestamp: January 26, 2017, 07:58:28 pm
content: Quote from: xrunner on January 26, 2017, 03:06:18 pmThis is probably why there is this note in the firmware update procedure document:"Check to ensure that the CAL Key is inserted into the CAL Key port and NOT the USB port. Failure to do so may cause failure of USB function."Seriously, if a device has decent USB port implementation it should survive plugging this "CAL Key" into it, but I wouldn't be surprised if there is lot of computers and other devices nowadays that don't have any protections on USB ports....

author: xrunner
author_rank: Super Contributor
timestamp: January 27, 2017, 12:47:12 am
content: Quote from: sequoia on January 26, 2017, 07:58:28 pmYea kind of a dumb design. You make the design with twoidenticalports next to each other with the knowledge that the electrical design with the cal key (a simple short) stuck in the "real" USB port "may cause failure of USB function".People get in a hurry, it's on the back and might not be well lit, and you think just stick a blurb in the manual and call it a day. C'mon Gw Instek - why not just use the B type USB port, or some other type of port?

author: coromonadalix
author_rank: Super Contributor
timestamp: January 27, 2017, 12:59:43 am
content: thks for the tips, saved me money and i will be able to calibrate it.

author: cdev
author_rank: Super Contributor
timestamp: January 27, 2017, 01:03:16 am
content: Does this meter measure inductance in any way? Also, how precise is the capacitance measurement - does anybody know what frequency its done at, and does it measure ESR?

author: xrunner
author_rank: Super Contributor
timestamp: January 27, 2017, 01:21:25 am
content: Quote from: cdev on January 27, 2017, 01:03:16 amIt don't do none a' that stuff ...But it does do a lot of other nice measurements -http://www.gwinstek.com/en-global/root/Discontinued_Products/Discontinued_Meters_LCR/GDM-8251A

author: xrunner
author_rank: Super Contributor
timestamp: January 27, 2017, 01:24:54 am
content: Quote from: coromonadalix on January 27, 2017, 12:59:43 amCool beans, wish I'd have known about the trick. Ah well, it's all good.

author: bitseeker
author_rank: Super Contributor
timestamp: January 28, 2017, 05:17:29 am
content: Kudos to xrunner for taking one for the team and sharing the solution to the cal key.Plus, we get to have a good laugh at Instek for a very poorly conceived key "solution".

author: SeanB
author_rank: Super Contributor
timestamp: January 28, 2017, 05:03:16 pm
content: Could they not have put a 1k or so resistor in series with the "key" lines. would solve that problem, and I doubt the cal input on the meter is drawing any current other than that for a pull down resistor keeping the line low. They could have put a red LED on the cal key that lights up to show it is in Cal mode, wank gimmick, but useful, and free as you already have the USB socket needing a small PCB, just add the 2 resistors and put the LED out the rear of the overmould where you normally have that cable, saves needing a non standard injection mould in production.

author: bitseeker
author_rank: Super Contributor
timestamp: January 28, 2017, 11:16:56 pm
content: They could've put a resistor between pins 1 and 4 to prevent a complete short circuit when plugged into a real USB port. In one of the related threads, someone used a 100 Ohm resistor in their DIY cal key, IIRC.

author: xrunner
author_rank: Super Contributor
timestamp: January 29, 2017, 12:39:57 am
content: Quote from: bitseeker on January 28, 2017, 11:16:56 pmBut that would have cost an extra few cents per unit.

author: bitseeker
author_rank: Super Contributor
timestamp: January 29, 2017, 12:43:49 am
content: Quote from: xrunner on January 29, 2017, 12:39:57 amYeah, I was going to say that, but for the price they're charging, they can afford it.

author: sequoia
author_rank: Unknown
timestamp: February 05, 2017, 08:01:40 pm
content: Quote from: xrunner on January 21, 2017, 07:24:43 pmDid it take long to get response from GW Instek? I requested firmware via their "contact us" web form atwww.gwinstek.commaybe a week ago but so far no reply...

author: xrunner
author_rank: Super Contributor
timestamp: February 05, 2017, 08:19:15 pm
content: Quote from: sequoia on February 05, 2017, 08:01:40 pmOnly took 24 hours. Here's the email address I wrote to -services@goodwill.com.tw

author: sequoia
author_rank: Unknown
timestamp: February 07, 2017, 06:14:48 am
content: Quote from: xrunner on February 05, 2017, 08:19:15 pmThanks, that email address worked. Still waiting to receive some USB connectors I ordered from eBay, so I can make my own "CAL key" with small resistor soldered between pins 1 and 4 inside the connector....If firmware upgrade is successful, then need to figure out if there would be some place in Southern California to get this thing checked/calibrated at reasonable cost.... (

author: xrunner
author_rank: Super Contributor
timestamp: February 08, 2017, 12:00:52 am
content: Quote from: sequoia on February 07, 2017, 06:14:48 amCool!QuoteIf ... if? Of course it will be successful!Been using mine quite a bit. As a matter of fact was just using it to monitor the temporary RAM voltage I applied to my HP 3457A while I replaced the backup battery. That's something I did not want to make a mistake on because the ram-(no pun intended)-ifications would have been unpleasant..

author: sequoia
author_rank: Unknown
timestamp: February 09, 2017, 08:27:46 am
content: Got some USB (type-A) connectors from ebay (10 for $2.35 with free shipping) to make my own "cal key".First, I soldered 100 ohm resistor across pins 1 and 4, but unit didn't enter calibration mode on boot (simply booted up normally). Next, I tried shorting pins 1 and 4 and that worked!Firmware upgrade was successful with the home made "cal key"...

author: xrunner
author_rank: Super Contributor
timestamp: February 10, 2017, 12:00:12 am
content: Quote from: sequoia on February 09, 2017, 08:27:46 am10 for $2.35 ... Considering the ridiculous price they are being sold for, you could make a killing selling home made cal keys.Quote

author: BloodyCactus
author_rank: Frequent Contributor
timestamp: February 10, 2017, 01:56:12 pm
content: Quote from: xrunner on February 10, 2017, 12:00:12 amsomeone must be sitting on a metric tonne of original cal keys because one comes with each unit when its purchased. if all these are hitting ebay with no cal key. someone is hoarding them lol!

author: bitseeker
author_rank: Super Contributor
timestamp: February 10, 2017, 07:18:40 pm
content: Quote from: BloodyCactus on February 10, 2017, 01:56:12 pmSince the bulk of these units are from the ITT liquidations, someone might have them. But, more likely, they were tossed as extraneous accessories.

author: frog7227
author_rank: Newbie
timestamp: February 13, 2017, 12:22:54 am
content: A little bit of a thread hijack here , but here it goes.After lots of pain with the original GW-Instek DMM Viewer...I wrote program to communicate with a PC in Java.At the moment it needs the RXTX library installed system wide.I've only tried it on Windows, I'll be trying it on Linux later.I'll offer a .jar version once I've compiled and compressed everything (Including RXTX).Link:https://github.com/frog7227/SerialInterfaceIf you have any issues or proposals for features post them on my Github, I'll see them much faster that way.

author: xrunner
author_rank: Super Contributor
timestamp: February 13, 2017, 12:25:44 am
content: Quote from: frog7227 on February 13, 2017, 12:22:54 amCool - we like that kind of hijacking.

author: CustomEngineerer
author_rank: Frequent Contributor
timestamp: March 03, 2017, 05:06:06 am
content: Quote from: sequoia on February 09, 2017, 08:27:46 amShorting pins 1 and 4 also worked for me. Had an extra usb connector laying around so copied sequoia's key. Thanks for showing pics.