Decent quality panel meter? - Page 1
Forum section: electronics / projects

author: Martinn
author_rank: Frequent Contributor
timestamp: April 05, 2023, 01:59:58 pm
content: Hi all,for an electronic load test circuit I added a cheap Chinese panel meterhttps://www.aliexpress.com/item/4000098338261.htmldisplaying voltage and current. You can't beat value for money, still quality is depressing. Dim, uneven lit digits, generally so-so build quality and I have not even measured its accuracy. I would not mind paying 10x as much for something decent, but this slot seems vacant.More from Ali:https://www.aliexpress.com/w/wholesale-voltmeter.htmlIf you look on Mouser&Co you find high quality ones for $250 upwards, which is not helpful either.What I am looking for are better quality ones with configurable decimal points (for the electronic load, I need a 1 kV display range). Any suggestion?Of course you could build one yourself - but this has the potential to get out of hand quickly (what resolution, what accuracy, how many channels, what display? Maybe OLED, high side/differential current sensing, calibration) and I have too many of these projects already.- Martin

author: TimFox
author_rank: Super Contributor
timestamp: April 05, 2023, 03:25:34 pm
content: In the US, I have had good results from Circuit Specialists:https://www.circuitspecialists.com/search?searchquery=panel%20meterReasonably-priced Chinese units.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: April 05, 2023, 03:50:48 pm
content: Datel, as long as you can get them (ILC-DDC will not continue this product line, it seems)Murata power solutions via Arrow or TTI.SchwillePeakTech and TDE Instruments via Distrelec should work too.

author: rdl
author_rank: Super Contributor
timestamp: April 05, 2023, 05:49:43 pm
content: I've bought several DPMs from ColdfusionX on eBay over the years. The UP8135 is nice. All have been very good quality. I've bought some other models from different eBay sellers also, even 99 cent voltmeters, and all were fine. How is Ali even getting poor quality?

author: TimNJ
author_rank: Super Contributor
timestamp: April 05, 2023, 06:04:46 pm
content: I think Murata bought Datel. The now Murata branded meters are pretty good, and not too ridiculously expensive.

author: jonpaul
author_rank: Super Contributor
timestamp: April 05, 2023, 06:08:27 pm
content: old analog panel meters from USA or Yokogawa, Ham fleas, old power supliesJon

author: james_s
author_rank: Super Contributor
timestamp: April 05, 2023, 06:17:15 pm
content: You could look on ebay for a used industrial unit, a lot of sellers think their stuff is made of gold but you get lucky now and then.I've had several random Chinese digital panel meters that had nice clear bright digits though, but the nature of such things is such that it wouldn't do you any good for me to look up where I bought them from now, the same identical looking meter could be totally different inside now than what I bought then.

author: Terry Bites
author_rank: Super Contributor
timestamp: April 05, 2023, 11:21:23 pm
content: Sifam are the dog's bollxxx.www.sifam.com

author: Martinn
author_rank: Frequent Contributor
timestamp: April 07, 2023, 08:50:18 am
content: Thanks all, yes I almost forgot the standard ones that have been around forever.Including the nasty requriement of having a floating 9V supply (and don't connect the grounds!). How do you deal with this? OK isolated DC/DC modules are not that expensive, but noisy. I remember darkly in a 1kW lab supply build, I put three extra windings on the main toroid transformer for the panel meters.It seems that mankind has made no progress here in the last 20 (30?) years (except for adding poorer quality and cheaper Chinese variants). See what I found in a box (the three small ones are from said lab supply carcass that eventually went into the junk).I know I said I don't want to build my own one. But if I would...For display I'd choose (very long story short) an STM32F469 discovery boardhttps://www.st.com/en/evaluation-tools/32f469idiscovery.html#overviewIt has a really nice 4" high resolution display with capacitive touch. It is supported via TouchGFX and has an extension connector on the back for connecting some more circuitry.TouchGFX has quite a learning curve, but not worse than any other display solution (except 7 segment). Displaying "1.234 mV" is not too difficult, but you could of course make a full fledged GUI with dials, numeric keyboard, graphs and more. So I'd consider this a solid basis. If you don't like the display, TouchGFX supports a range of some smaller (IMO not worth it) and from Riverdi even 7" and 10" models.As ADC I'd add an AD7124https://www.analog.com/en/products/ad7124-4.htmlsee also online eval toolhttp://beta-tools.analog.com/virtualeval/which seems to be a quite modern multiplexed sigma delta converter with differential inputs and PGA up to 256x (and much more).Accuracy wise its internal 10 ppm/K reference should be good for a 4 digit display (over a 20K range like 10-30°C, this would be 200 ppm or 0.0002). Of course one could add a better reference, but I'd not go too far into voltnut territory.Input wise I'd add some preconfigured dividers/shunts to be placed depending on application. No input protection (divider should be sufficient).Supply: Isolated DC/DC module (needs to be a bit more beefy powering the display, 3-5 W probably), LDO 3.3 V for the ADC.What do you think?

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: April 07, 2023, 04:26:37 pm
content: Quote from: Martinn on April 07, 2023, 08:50:18 amExactly like this.

author: james_s
author_rank: Super Contributor
timestamp: April 07, 2023, 08:42:43 pm
content: I use the isolated DC-DC converters, they're cheap and they don't have to be noisy, especially with a bit of external filtering. Or in a bench supply if you're making a custom transformer you can add extra windings for this purpose.

author: xavier60
author_rank: Super Contributor
timestamp: April 08, 2023, 07:38:21 am
content: Beware of this style of meters.The refresh rate is usually much slower than advertised, 1 or 2 seconds instead the usually advertised 300mS.Some also have a minimum current below which the display will read zero, like 50mA.

author: Martinn
author_rank: Frequent Contributor
timestamp: April 09, 2023, 08:28:53 am
content: Quote from: xavier60 on April 08, 2023, 07:38:21 amYes, as I wrote the application is an electronic load where I'd like to see voltage, current and (if possible) power. The module I chose is "this style" I guess, with weird quantization and some blanking towards zero. You can see it in the image above.I couldn't help it and designed a PCB - AD7124 with some generic circuitry, nonisolated power (should be fine for my application) matching the Arduino connector on the backside of the display module. As I have three voltages (Voltage monitor after buffered 1 kV divider, current sense 2.5 V and current setpoint 2.5 V from 10T pot) I can show all three on the display.Plus I can put a toggle button for enable and maybe even range switching on the GUI. Currently I have 2.5, 25 and 250 mA ranges switched by a jumper (actually it's a simple test circuit with a 6L6GC tube as pass element), but I should be able to do the range switching with MOSFETs so I could also control these via GUI.The PCB was half a day of effort (it was raining anyway), but as I wrote the real time sink will be TouchGFX. At least there is a video by Karl EEdoing exactly that.With little effort, a gauge and maybe rolling graph should be feasible as well if I ever get this working.

author: Martinn
author_rank: Frequent Contributor
timestamp: April 09, 2023, 04:28:16 pm
content: I was thinking of something like this... let's see if I can make this work.

author: Martinn
author_rank: Frequent Contributor
timestamp: April 18, 2023, 06:23:20 pm
content: Here we go. After some hiccups with a messed up extension connector pinout, I got this thing working.The AD7124 is a beast! Tons of functions and a 93 page datasheet to read through. But with the online simulator provided by AD and their STM32 example I got it working pretty quick.TouchGFX also needs some document reading*... and the videos by Karl EE helped a great deal. (* heavy understatement)The stability of the result values is just plain amazing. I chose a sampling rate with maximum 50 Hz suppression and the readings are rock solid - the 1.2345 V is a real voltage it measures, with absolutely zero displayed noise on it. For the set configuration, the simulator states 330 nV RMS output noise (2.1 uV peak) and 20.2 bit Peak-Peak resolution. In the simulated histogram, the samples lie between 1.23449 and 1.234501 V (for 1.2345 V input, 1.25 V full scale), so at least from the ADC noise, this constellation would be good for a five (six actually?) digit display. INL is specified at +-2 ppm typ +15 ppm max FS. Of course the remaining components (reference, input divider) would need to have sufficiently low drift, which I currently don't have so I am not pushing the display resolution.The current range selector and on/off switch (which I'd have to enlarge somehow) are not connected to any outputs, but that will be easy enough.To me this performance is absolutely jaw-dropping and I am very happy about how this turned out. Of course there's still work to do. I think I will add full isolation - first use is in my max. 1 kV electronic load and if I want to measure negative voltages (CRT bias), the system ground will be at -1 kV. Probably not a good idea to have a touch screen at -1 kV potential. But as the interface is SPI, that is easily done with an isolator. Range switching and enable will be another isolator (the AD7124 even has two GPIOs one could use for that). Maybe I also add a precision reference, but pushing the whole system to 5 ppm/°K might get very expensive.So although this was just a for-fun add-on for an electronic load test setup, I'm glad I did it. This display is just scratching on the surface of what TouchGFX can do and the AD7124 is incredible. Definitely a worthwile upgrade over the crap Chinese panelmeter I had used before and a solid basis for future projects.

author: Martinn
author_rank: Frequent Contributor
timestamp: April 26, 2023, 06:48:21 pm
content: Panel meter is now built into a HV electronic load prototype (seehttps://www.eevblog.com/forum/projects/high-voltage-electronic-load-(500v-500ma)/msg4833872/#msg4833872)I have configured the ADC to measure three channels: Voltage (from 1.5 kV input divider), set and actual current. The range buttons (2.5/25/250 mA) just change the display, as I don't have electronic range switching built into the load (currently it's just a jumper selecting the shunt). Particularly the 250 mA current measurement drifts a lot, next time I put a 5 ppm shunt in there.The enable/disable toggle button actually controls the electronic load via a GPIO. I still have no galvanic isolation, so when testing the 1 kV range I need to be careful.As I already wrote TouchGFX is not trivial to set up. On the other hand, if you are through the first hours of reading documentation and watching tutorial videos, it's a pretty decent piece of software, looks great and is fun to work with. Compared to LVGL for example it obviously helps a lot if there's proper manufacturer support behind it (as he wants to earn $$$ with it).With all my projects, I try to widen my portfolio in a way it most likely helps me with future personal or commercial projects. One might argue that this type of display (and ADC) is a monstrous overkill for measuring a few voltages, but this design certainly will be reused in the future!