5 dollar voltmeter with 5 digit (0.1mv) resolution

[janaf]

Be aware that the input of the MCP3421 is quite low, 2.25M at unity gain.

So here's the question... can we upgrade the 0-33.000v meter to perform like the one that costs twice as much by replacing the 20k input R with higher value?

[paulie]

True. In fact the stock meter is much worse with 330k and 20k to ground. This 0-33.000v to 0-3.3000v mod helps a little. Even ignoring the 10x resolution boost, the new shunt brings input impedance closer to a meg.

What really makes a difference is using an op amp like in my 16 channel multiplexer which can get you gigaohms. This was necessary in my case to mask minimum resistance of the CD4051 analog switches. Not that important playing around with the stock 5 digit resolution but big help when boosting this cheapie to 6 digits.

ps. Note the 2.25m is mostly due to a few pf in the sigma delta modulator so capacitive only and not resistive. Not really all that big a deal specially if you tack on a small input cap.

[Vince Piotet]

Couple developments. The references from LabSpokane arrived back and I want to thank him again for all the hard work. It's unfortunate confusion over the circuits resulted in more effort than necessary. He was nice to include the same resistor used in his tests so maybe I can build a jig with that to garner useful info from the graphs.

In other news I am now the owner of one of those Ebay AD584 voltage standards. A friend of mine gave it to me because it did not come with precision readings so not of much use for his purpose. I did determine that the Ebay meter is at least 7ms accurate based on the AD584 initial voltage spec. We probably already guessed that from reviews published earlier in this thread. Probably a lot better in fact based on those. Of course I can't be sure unless I can get some reference readings from somebody who owns a precision meter. My buddy is considering getting his calibrated and got quotes from some standards labs but that may be some time off.

Meanwhile I'm logging a set of the DIY references using the Ebay meter with my 16 channel input expander. Hopefully once it's calibrated I can adjust the data and get useful info. Even if not at least a chance to check out my logger and oven hardware which hasn't been operational since last year.

Amazingly the high resolution version of the meter I ordered showed up in one week. Not a record but pretty good for China. So a set of cheap DIY voltage references were thrown together and surprisingly the readings were very close to what might be expected. This is a good sign. To really check this out I need to get more accurate readings for the references from a friend who has access to a lab grade voltmeter. Stay tuned.

May I chime in with the cheer ?  Amazing accuracy !

I bought 2  five dollar voltmeters and one 4.3V/30V eight dollar voltmeter.
Out of the box they gave the same reading to the last few millivolts.

So I hooked up a 10K trim pot in series and made all readings exactly the same.
Then I was amazed by the result of the following experiments:

1. When I supplied different voltages anywhere  from 5V to 30V.
the 3 readings were identical  down to the last digit +-1 !!!

https://www.eevblog.com/forum/Smileys/default/scared.gif

2. The cheap DC-DC converter I used as variable voltage supply happens to be very stable as shown by the next experiment:

Yesterday at 18:40, I connected this DC-DCconverter to these 3 voltmeters plus another 5 digit +-200 MV large size voltmeter(which costs about $15 on Ebay)  and they have now been hooked up for the last ten hours

The reading on the 3 $5 voltmeters and the larger size $15 voltmeter were:

18:40    13.004 13.005 13.005 13.005V
18:48             4 5 5 3  (meaning millivolts over 13V)
19:45             4 5 5 13.000
20:55             5 5 6 0
2:54 AM         7 7 7 12.998
4:10 AM         7 7 7 12.996



I will use them to build my own private voltage standard.

There are cheap .1% resistors available on Ebay (nikkohm)

[paulie]

Hey, welcome to the forum. Lotta fun to be had here.

Even without thousand dollar references or lab meters I'm coming to the same conclusion: impressive accuracy and stability for very few dollars. All my meters agree within a couple counts too and with an AD584 which is closest thing to a precision standard ATM. Quite a good sign. Looks like the biggest benefit to tech advances and chinese ingenuity is making things that used to cost big bucks available to the poorest hobbyists.

Fortunately a while back another Eevblog member agreed to help me by taking readings of my voltage references again. Overseas this time so it's taking a while but they should be showing up here any day now. I already have his readings which are almost dead on with ones taken by me before they were sent out. When they arrive back I'll take another set to see how these fare traveling half way around the planet then post results. This is fun and must admit I learned a lot about voltages, standards, electronics, and physics in general thanks to these cheapo toys.

[janaf]

Apologies if this has been posted, but is it simple / complicated to move the decimal point?

I'd want to use these for a small bench supp, 0-33V and 0-330mA would be perfect for me. It would then of course be neat   to display XXX.xx mA and XX.xxx V

I would change the voltage divider resistor and use a separate current shunt with a matching op-amp gain, for example 0-3.3V for 0-330mA.

[paulie]

Apologies if this has been posted, but is it simple / complicated to move the decimal point?
...
I would change the voltage divider resistor and use a separate current shunt with a matching op-amp gain, for example 0-3.3V for 0-330mA.

Actually one of the benefits of using this meter  for current monitor in a PSU is no op amp required. Only a 0.1 or 1.0 ohm resistor in the ground leg to get one amp to one tenth ma resolution. A simple DPST switch allows changing from voltage to current using the same display and virtually zero burden voltage drop.

ps. To change the decimal point you would have to reflash the STM. I have blacked out all my decimal points to avoid confusion.

[paulie]

Reading your recent message regarding availability of meter code there may be some misunderstanding. Similar inquiry from another so maybe best to respond here. I don't have an image or any STM8 firmware. I meant one would have to write a program then flash it. Sorry I wasn't more clear. I may give it a shot some day and you will be first on my list to share.

Thanks again for the parts you sent and for the favor. If there's anything else I can do to repay just let me know.

[janaf]

...I have blacked out all my decimal points to avoid confusion.

I interpreted this as you had done it in firmware. Of course, I can simply do it with black & white ink on the panel itself, that will surely do. I have no intention of switching the decimal point anyway.

[ym58]

A while back I purchased one of these:

http://www.ebay.com/itm/310601671625?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

However the new 0-4.3000v-33.000 version has a 0.1mv spec when used with isolated supply. A fellow amateur radio club member swears he gets close to that and I'm wondering if anybody here also has direct experience with one.

http://www.ebay.com/itm/370966721056?_trksid=p2059210.m2749.l2649&var=640165395235&ssPageName=STRK%3AMEBIDX%3AIT

Hi Paulie,
Have you ever measured or got any clue regarding the internal resistance of the ammeter/voltmeter that you mentioned above ?
Otherwise, our discussion continues here ... you're welcome to join us back  !

[Kleinstein]

From the circuit diagramm on the russian page, the input impedance should be about 350 K Ohms from the divider at the input. Other ranges / Versions might have higher or lower values. The ADC itself is not especially high impedance ( 2 M / Gain), so don't expect a really high impedance. Still this is very well suitabe for power supply.

[ym58]

Hallo Kleinstein,
actually, I was more concerned with the impedance of the ammeter than the impedance of the voltmeter.
You may find the ammeter that I am talking about under this link.

[ez24]

A little off subject but since Paulie started this:

 "Where is Paulie?"  There is no "paulie" in the member list.  I am there as is "ym58".   I see "paulhm81" and the next name is
"Pauline"

The reason that this caught my eye is, I think a few weeks ago "paulie" got criticized pretty bad on picking a scope by a member (I could be wrong).  It was kinda funny (that the member would put so much energy in the post - it was long) but I think Paulie must have gotten hurt.  Well anyway there seems to be no paulie in the member list.

Paulie if you are still here - answer up  (FYI you were the one that got me started on my PSU - so I miss you).  If you were the one that got ripped, just ignore it and come back.

Hope I am all wrong about this and I am just having a brain fart.

[eas]

Just click his name on his posts in this thread. It appears his account still exists, but he hasn't visited since the end of May.

[ez24]

My guess is if you cancel your membership your profile will remain

[ym58]

I think a few weeks ago "paulie" got criticized pretty bad on picking a scope by a member (I could be wrong).

I'm trying to figure out what you meant by  "picking a scope" ?
As we are on EEV, I first understood it the literal way i.e. "picking an oscilloscope", but I guess there must be another meaning ...
(being criticized for "picking an oscilloscope" sounds a bit surrealistic to me)

[ym58]

My guess is if you cancel your membership your profile will remain

Just click his name on his posts in this thread. It appears his account still exists, but he hasn't visited since the end of May.

I tried to PM him last Friday but the system replied : "User 'paulie' can not receive personal messages"

[edavid]

I think a few weeks ago "paulie" got criticized pretty bad on picking a scope by a member (I could be wrong).

I'm trying to figure out what you meant by  "picking a scope" ?
As we are on EEV, I first understood it the literal way i.e. "picking an oscilloscope", but I guess there must be another meaning ...
(being criticized for "picking an oscilloscope" sounds a bit surrealistic to me)

I'm sure the problem had to do with this thread: https://www.eevblog.com/forum/testgear/do-cehap-meters-explode-when-you-abuse-them/

[ez24]

From Dave:

And that's how you get yourself banned on a forum 
BTW, heads up, Paulie uses many different IP addresses to post so he could sneak back in under another name.

so that is what happened, I never saw that posting - too bad there is not a note in his profile that he was banned (under the name Paulie)

tks

[ym58]

I'm sure the problem had to do with this thread: https://www.eevblog.com/forum/testgear/do-cehap-meters-explode-when-you-abuse-them/

so that is what happened, I never saw that posting - too bad there is not a note in his profile that he was banned (under the name Paulie)
tks

I've just read the whole thread, the initial intentions were legitimate but it became real fist fighting !
Maybe the mod should have simply closed the thread to calm everyone down ...
 
But anyway, soldier Paulie seems to be lost somewhere in the wilderness and I still don't know what the eBay cheap ammeters impedance is  ;-) !!

[sorin]

In my opinion paulie dont do nothin wrong.
It's injustice that Dave banned him. This forum is not a democratic place.

[Len]

This forum is not a democratic place.

Thank goodness for that, if "democracy" means "freedom to be a dick."

[edavid]

I still don't know what the eBay cheap ammeters impedance is  ;-) !!

This AliExpress seller claims to be a manufacturer, so they should be able to tell you: http://www.aliexpress.com/item/Home-automation-module-LED-display-Color-0-36-Digital-Ammeter-DC-0-3-0000A-Four/32275906238.html

[Rasz]

From Dave:

And that's how you get yourself banned on a forum 
BTW, heads up, Paulie uses many different IP addresses to post so he could sneak back in under another name.

so that is what happened, I never saw that posting - too bad there is not a note in his profile that he was banned (under the name Paulie)

you missed :

Quote from: paulie on May 31, 2015, 12:15:11 AM ---Are you kidding me? I LOVE hearing this stuff. Getting guys like you going is one of the main reasons to keep coming back here. Hooking borderline personalities is every trolls dream. Actually it's better than just trolling for me... more like shooting the fish in a barrel.... no tackle or lines required. LOL

[ez24]

This AliExpress seller claims to be a manufacturer, so they should be able to tell you: http://www.aliexpress.com/item/Home-automation-module-LED-display-Color-0-36-Digital-Ammeter-DC-0-3-0000A-Four/32275906238.html

NICE FIND

This seller puts up wiring diagrams 

[ym58]

This AliExpress seller claims to be a manufacturer, so they should be able to tell you: http://www.aliexpress.com/item/Home-automation-module-LED-display-Color-0-36-Digital-Ammeter-DC-0-3-0000A-Four/32275906238.html

Thanks eDave,
Actually, starting from your link, I made a few researches on AliExpress and it turns than there must be two different types of 0-3.0000A ammeters out there in China :

#1 the one you mentioned here, coming in different LED colors :



#2 another type, red LED only, "apparently" a bit more sophisticated (and also more expensive) with a stated 10mR impedance and lab bench extra pics :



Still hesitating ...