Cheap Chinese Volt Amp Panel Meter won't zero

RE: Cheap Chinese Volt Amp Panel Meter won't zero

mrh586 — Fri, 14 Oct 2022 11:59:26 GMT

My voltmeter turns on but only shows zero
I reset but it didn't work

RE: Cheap Chinese Volt Amp Panel Meter won't zero

clem57 — Tue, 11 Feb 2020 12:09:38 GMT

Glad [mention:dc11ece8204f479e8dce2a47a33b20d3:e9ed411860ed4f2ba0265705b8793d05] found the data sheet.Big thankful to y'all. But I found a guy testing his way trough the paths and thought I'd share it.

[View:https://www.youtube.com/watch?v=UubXv_w4x5s:740:466]

I found it rather delightful to watch...

Clem

RE: Cheap Chinese Volt Amp Panel Meter won't zero

ralphjy — Mon, 10 Feb 2020 20:34:36 GMT

Hi John,

I've been thinking of trying one of these meters but they don't have much documentation so it's interesting to see your experiences. Have you seen the schematic that was reversed engineered by UpcycleElectronics? https://github.com/Upcycle-Electronics/DSN-VC288

[View:/resized-image/__size/823x664/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-9ec6cac2-405d-4258-93e5-8068abe67b18/5164.contentimage_5F00_206000.png:823:664]

Can't vouch for the accuracy but you could probably verify it. Looks like they are using the A/D on the STM8S103F3 MCU for the measurements.

I also found this reference but don't know if it works: https://www.ittsb.eu/forum/index.php?topic=1351.0

[View:/resized-image/__size/1012x116/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-9ec6cac2-405d-4258-93e5-8068abe67b18/4578.contentimage_5F00_206001.png:1012:116]

[View:/resized-image/__size/620x465/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-9ec6cac2-405d-4258-93e5-8068abe67b18/0876.contentimage_5F00_206002.jpg:620:465]

Hope this helps. I'll probably get one of these to play with.

Ralph

RE: Cheap Chinese Volt Amp Panel Meter won't zero

jw0752 — Mon, 10 Feb 2020 04:09:24 GMT

I began the process by looking at the board and identifying components that might affect the zero of the display. In the past I have had situations where this problem could be corrected by trimming the Null on an Op Amp. There was an LM 358 on the meter board. It is a dual Op Amp so this was my first target. I made a probe with 10K of resistance in series and connected one end to the circuit ground. Each of the Op Amp's active inputs and outputs was probed and the display observed. This had no affect on the display and the non-zero digit remained. Next I moved the probe connection to the Vdd of the circuit board. Once again the active legs of the OpAmp were probed. This time the display zeroed when the probe was put on leg #6 which was the inverting input of one of the Op Amps.

The 10K ohm connection to Vdd was likely an extreme compensation so I built a 100K Trimmer with a 1 Meg Ohm resistor off the sweep. This would allow me to slowly increase the voltage level that was being applied to the pin 6 of the LM358. The trimmer and 1 meg resistor were attached to the meter board and while observing the display the trimmer was slowly turned from the ground side towards the Vdd side. At one point, later determined to be about 1.5 K above ground, the display zeroed. With the display now showing zero with no current passing through the meter I applied current and adjusted the meter calibration so that the meter reading matched the current showing on one of my better bench meters.

Here is a schematic of the trim patch:

[View:/resized-image/__size/620x401/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-9ec6cac2-405d-4258-93e5-8068abe67b18/6560.contentimage_5F00_205996.png:620:401]

Here are pictures of the trimmer patch installed on the meter. The red wire connects to the meter positive power and the black wire to the ground. The 1 Meg resistor is soldered to the wiper of the trim pot and the yellow wire comes from the 1 meg resistor. The legs of the LM358 were small and difficult to get to so I traced the circuit and found a more convenient location to make my connection. In this case it was on one side of a 47K surface mount resistor.

[View:/resized-image/__size/620x465/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-9ec6cac2-405d-4258-93e5-8068abe67b18/0702.contentimage_5F00_205997.jpg:620:465][View:/resized-image/__size/620x465/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-9ec6cac2-405d-4258-93e5-8068abe67b18/7658.contentimage_5F00_205998.jpg:620:465]

[View:/resized-image/__size/620x588/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-9ec6cac2-405d-4258-93e5-8068abe67b18/0876.contentimage_5F00_205999.jpg:620:588]

I experimented with changing the supply voltage to the meter to see if this would affect the patch but over a reasonable range 6 volts to 18 volts it did not disturb the zeroing nor the reading on the meter.

John

RE: Cheap Chinese Volt Amp Panel Meter won't zero

kmikemoo — Sun, 09 Feb 2020 14:41:43 GMT

[mention:f80b53cee57c44bc9d7c577d07d7c791:e9ed411860ed4f2ba0265705b8793d05] I have purchased a few of these and, like [mention:b0bc65b9ecdc4307bd967592f00e340a:e9ed411860ed4f2ba0265705b8793d05], just had low expectations of accuracy. Personally, I'm super impressed that you figured out how to make the adjustment trimmer do what it's supposed to. Awesome job!

I assume that it's a hardware fix. I'm curious. I have a few of these in project boxes that I use as remote meters during training. So... thanks for offering to share - even if there's not enough interest to do the blog. [View:/resized-image/__size/16x16/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-9ec6cac2-405d-4258-93e5-8068abe67b18/contentimage_5F00_11380.png:16:16][View:/resized-image/__size/16x16/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-9ec6cac2-405d-4258-93e5-8068abe67b18/contentimage_5F00_11380.png:16:16]

RE: Cheap Chinese Volt Amp Panel Meter won't zero

shabaz — Sun, 09 Feb 2020 13:50:25 GMT

I've recently purchased some panel meters too, but different models (I'll have to check them now! : ). I wasn't looking for too much accuracy, more just ballpark for the use-case (which was a step-down transformer from 240V, so I just wanted to see if the output was ballpark 120V etc).

RE: Cheap Chinese Volt Amp Panel Meter won't zero

clem57 — Sun, 09 Feb 2020 11:33:30 GMT

Curious if this patch fixed by hardware or software? Thanks [mention:f80b53cee57c44bc9d7c577d07d7c791:e9ed411860ed4f2ba0265705b8793d05]