Making panel meters for my psu. Any suggestions ?

[gabri.peic]

Hi. I just finished making the main pcbs for my bench power supply based on elektor 1982 precision power supply. It seems to work as it should, But now i need to think of the panel meters. As much as i like the vintage look of analog meters i still prefer a digital display for everyday use. So is there anything i can buy or build for this purpose ?
I would like to have 4 digit 7 segment led displays although 3 digit would be ok too.
I am open to using microcontroller, I am not good at programming but i am willing to learn.
Also it is important to note that i will have two completely isolated supplies in the same enclosure so i need to keep two sets of panel meters completely isolated.

[LateLesley]

Well, a quick search on eBay show some candidates. It's hard to choose, because we don't know the Max voltage or current, but I thought I'd link the search to give you a starting point to peruse.

https://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=p2334524.m570.l1313&_nkw=4digit+voltmeter&_sacat=0&LH_TitleDesc=0&_odkw=4+digit+voltmeter&_osacat=0

[RoGeorge]

There are many models of 7 segments panel meters on Aliexpress, cheaper than one can build.  Various colors and segment sizes, most of them with 4 digits.  Some are single indicator, some are grouped in 2-3 lines for V, A, W.  They usually have a trimmer to calibrate them.

I bought a batch of ten voltmeters some years ago, and they were OK.  2 wires for power and a 3rd to measure the voltage.  Pretty accurate after calibrating the trimmer.  What they couldn't do was to be accurate in both sub 1V and over 1 volt at the same time.

[themadhippy]

As much as i like the vintage look of analog meters i still prefer a digital display for everyday use.

combine the 2,the beauty of analogue with the convenience of digital

[xavier60]

Those cheap LED Volt/Amp meters can be a lucky dip. The 4 digit ones tend to work better because they have auto-zeroing for the Amp section.
The biggest annoyance is the trend for slowing the refresh period which can be as long as 2000ms despite it being advertised as 300ms usually.

[gabri.peic]

Those cheap LED Volt/Amp meters can be a lucky dip. The 4 digit ones tend to work better because they have auto-zeroing for the Amp section.
The biggest annoyance is the trend for slowing the refresh period which can be as long as 2000ms despite it being advertised as 300ms usually.

The slow refresh is exactly what i was afraid of with the cheap ones from ebay...
Maybe i will look around and give some a try.
Are there any dedicated meter ic chips out there 🤔?

[RoGeorge]

Are there any dedicated meter ic chips out there 🤔?

ICL7106 or ICL7107, 3 1/2 digits.

I don't remember mine being 2 seconds slow, that would be unusable slow.  Found the description page I've saved back then, at the checkout payment ($10.36 total for a pack of 10 voltmeters).  The description says something about 0.3s, and that's about how I remember them, not blazing fast but OK.

Wholesale 10Pcs/lot Mini Red LED Panel Meter 3-Digital Voltmeter For DC 0-32V Display Voltmeter Motorcycle Free Shipping

Price:
    US $10.62 / lot



   Item Description:

Features:
Ultra-precision
Low start-up voltage, the red minimum 2.5V, green minimum 2.7V, blue minimum 3.0V. Supply voltage is lower than 3.5V, the only decrease in brightness, but does not affect measurement accuracy.
Has reverse polarity for protection, reverse does not burn.

Specification:
Measuring range: For DC 0V--32V
Input range: For DC 2.5V-32V(Max Input: For DC 32V. The device can be damaged if input is over 32V)
Red For DC2.50-32.0V, Green For DC2.70-32.0V, Blue For DC3.00-30.0V
Display: Three 0.36 "digital tube
Measurement accuracy: 0.1%
Refresh rate: about 300mS / times
Input impedance: About 100K
Operating Current: Red <23mA, Green <18mA, Blue <13mA
Dimensions:33mm*15mm*10mm
pitch of holes:28mm
aperture size :2.8mm
Operation Temp: -10℃-+65℃
Display Color: Red
Lead Length: 15cm
Wiring:
Red:power supply +
Black:power supply -, measure -
white:measure +

Package Include:
100% Brand New
10x Digital Voltmeter

Found a picture, they were 3 digits, not 4!  Sorry.   

[strawberry]

ICL7135 4.5 digits
slow display refresh rate (weak CPU)
slow ADC sample time (may hide REF or input noise problem/immunity)

[RoGeorge]

Or buy 4 pcs of cheap DMMs (some can count up to 6000 for about $10) and fix them behind the front panel of the power supply. 

If not, some ARM devbords have many channels of 16bits ADC for $15, like this FRDM-KL25Z:
https://www.nxp.com/design/development-boards/freedom-development-boards/mcu-boards/freedom-development-platform-for-kinetis-kl14-kl15-kl24-kl25-mcus:FRDM-KL25Z

[ledtester]

The INA226 and INA219 are popular with the Arduino crowd. They can report current, voltage and power back to the Arduino which then can send the values to whatever display you want.

Here's a good video on how these kind of sensors work in your circuit:

#321 7 Sensors tested: Measuring Current with Microcontrollers (Arduino, ESP32, ESP8266)
https://youtu.becG8moaufmQs

[TimFox]

There are lots of decent digital panel meters out there.
Be careful, though (assuming you don't use battery power as in a DMM):
Many (if not most) of them have a common connection between Input Low and Power Supply Common (-), so small extra power supplies may be required.

[Dr.Krieger]

I am impressed by some cheap eBay meters, although some are worst than others.
One of them had smaller LEDs and was losing a digit when measuring less than 10V, but then again i wouldn't trust the last digits of those meters. The good thing is that you get a fairly accurate X.XX reading (ignore the rest) and you don;t get that with the cheaper $1 3-digit voltmeters.

I am not sure about the actual listings and sellers because its been 4-5 years since i bought mine, but here are the ones i find today:

0.36" that loses one digit <10V:
https://www.ebay.com/itm/132474589055

0.56" that is fairly accurate:
https://www.ebay.com/itm/274758652402

Some of those eBay meters have (or had at least) 18bit ADC;s in them according to this post:
https://www.eevblog.com/forum/beginners/43-000-count-voltmeter-from-10-bit-adc-how/

[gabri.peic]

Thanks. I will probably go with one of the ebay ones.
Only one problem.
The ones suggested above are all just volt meters, So are the same meters available but with current insted of voltage ?
In that case i would put 4 meters on the front panel, Two per channel for current and voltage.
Obviously i would supply meters for each channel with isolated power supply to keep the isolation between the channels.

[Mortymore]

There are cheap panel meters with both Volts and Amps

[TimFox]

A normal digital panel meter is merely a voltmeter across a "shunt" resistor.
Be careful again:  you don't want to lose too much voltage across the shunt, 100 mV should be available in a panel meter.
Of course, you are limited by the accuracy of the shunt resistance, and probably need a four-terminal connection, and you need to wire the voltage meter after the current meter.

[Kleinstein]

With a lab supply one may be able to use the same shunt as used for the current regulation also for the current measurement.

In the best case, especially with a lab supply that uses a separate supply for the regulator part (e.g. the Harris / HP design), one may use the auxiliary supply also for the meters.

[Benta]

I've never bothered making panel meters myself. Even if you find the right bezels etc, in the end it still looks amateurish.

My personal preference is Red Lion panel meters. Not cheap, but very, very good. The biggest difficulty using them is making the rectangular front panet cut-out.
https://www.redlion.net/family/panel-meters-0

EDIT: scratch that. I just saw the latest prices. Ignore this post, please.

[Dr.Krieger]

I have seen 4-digit "red-blue" V/I LED Panel Meters but i can't seem to find any on eBay atm... All i can find are the cheap 3-digit ones like in Mortymore's post, but those are NOT accurate at all, for example you can have a 30-70mV error and that is after you calibrate it using the tiny pots on the PCB. The 5-digit Voltmeters i showed are "pretty accurate" (don't have a good voltmeter to compare them i'm afraid), you can still get a small error like 10-20mV on the lower part of the range (like under 1 or 0.5V) but i think you can live with that.

There are also 5-digit Ammeters but as you said you will have to use 4 for a dual PSU...
Also note that there are 5/50mA meters that you can configure using the correct resistors, like in the circuit i show below. So a 5mA (reads 0.0000-5.0000) can be made to show 3.0000 at 3A with fairly good accuracy.
https://www.ebay.com/itm/325088488516
https://www.ebay.com/itm/284289515988

Also pay attention to what TimFox and Kleinstein said above, the Voltmeter has a very high input resistance and causes no problems (besides the supply it needs) but an Ammeter measures a voltage drop on a "shunt" resistor and if you don't connect it correctly the shunt's voltage drop will be an error on your output, so if for example you have 50mV drop at 1A on your shunt and Vout is 10V (your accurate meters measure 1.0000A and 10.000000V) , then you will only have 9.95V on the load, so there goes your "load regulated" voltage... And those Elektor guys even gave you a 4-wire output (remote sense) so you can have exactly 10.00V on your DUT. 

If i am right about the circuit you use, then you kinda have to use a different Shunt to measure the Amps, because the one the OpAmp uses to measure current is on the "high-side" (on the Positive side of the output voltage) and most if not all eBay meters measure from Ground to a positive voltage, so your options are either use a ground-referenced meter and use the output of the big 33V/4A transformer to make a aux supply for the meter (i have used a 5-part Zener Regulator before to power such meters from a +48V supply and it works fine with a TO-220 NPN like a BDX53C or similar an a tiny heatsink), or use an isolated aux psu with either a small 50Hz transformer with Mains input or a small isolated DC/DC supply using one of your existing rails.
You could try and use the already existing high-side 0.22Ω resistor (R22), put a divider on it and use an "accurate" meter like the ones we talked about before (or a ICL7107 meter or something as long as the input range goes low enough).
You can even try to mess with the circuits 0.22Ω/3W shunt (that should really be 5W+) and change it to a smaller value true shunt (cheap 75mV@5A Chinese example https://www.ebay.com/itm/282285363253) if you know what you 're doing, maybe for better accuracy vs temperature because 3A thought a 0.22Ω is 1.98W and that is a ton of heat that will definitely affect the value of the resistance. I think the cheap white 5W resistors have ~500ppm/C (these ROYAL OHM ones are +-400ppm/C https://www.tme.eu/Document/46f95e5fd8d12d8d5e15683d74cddc76/ax5w_axial_royal.pdf) while a "proper" shunt can do at least 20 times better (20ppm for this one https://www.tme.eu/Document/9a11044aeb9919912b385f857afa61b7/oar.pdf).
Just don't go too low with the shunt value because those 741 are not exactly "cutting edge" stuff and you may lose accuracy because voltage drop is too small for the Op Amp...

So i don't know if you can find accurate V/I "combo" meters, but you can for sure use 5-digit meters for Voltage and Current without much fuss, you only have to make a small Zener Regulator (maybe with a 78L05 on the output?) to lower (and regulate because it will have lots of ripple under load) the main transformer's ("33V/4A" )~50Vdc output (on zero load) to a "ground-referenced" (well actually "C10's negative pin referenced") small aux voltage like 5V and you can use that for both meters, AND if you place the Ammeter like i show below, BEFORE R22/R24, then you will have zero error from its tiny onboard shunt!

*example drawn is with a combo V/I meter but the same works for two separate 5-digit meters.

[Dr.Krieger]

Better schematic, with the 5-part Zener Regulator.

Transistor and Zener dissipation depends on the current the meter/s draw, a 500mW Zener is enough but the BJT can have a Vce of over 40V so even a 30mA current means it will dissipate ~1.5W so a heatsink is needed.
If you use two meters it is possible need more than 50mA so maybe use a bigger Zener value (like 12-15V instead of 5.6V) and let the meter's onboard 3.3V reg do some of the work.
Like i said i have used this circuit to put those cheap and inaccurate but "close enough" 3-digit red-blue V/I meters on the output of 48V PSUs and it works fine as long as you put a heatsink on that TO-220 (i use BDX53C).
*"close enough" for general monitoring of a PSU that powers a 3D printer or something like that, not for a Lab PSU ofc!

OFC a switch-mode DC/DC step down would be way more efficient but those cheap eBay LM2596 modules can't handle a 50V input...

[tunk]

I have seen 4-digit "red-blue" V/I LED Panel Meters but i can't seem to find any on eBay atm...

You could try to search eBay with this: ammeter 4bit

[Dr.Krieger]

I have seen 4-digit "red-blue" V/I LED Panel Meters but i can't seem to find any on eBay atm...

You could try to search eBay with this: ammeter 4bit

Well damn, it was that easy!
I don't know why i couldn't find them, i guess i was using both "voltage" and "current" in the search bar of something like that...
In any case thank you sir!

This variety seems to be in the 100/200V range, i don't know if <30V accuracy suffers from that higher overall range, and i haven't used one of these to know how accurate they are, just remember vaguely seeing them.

Anyone used them before?

[gabri.peic]

Wow thanks for all that feedback.
I will read all of the suggestions later or tomorrow (its getting late over here).
But i just wanted to quickly ask if anyone had any experience with these meters...

https://www.ebay.com/itm/323892139594?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=t_-H60NrTzC&sssrc=2349624&ssuid=AvwScfSORWO&var=&widget_ver=artemis&media=COPY

[fkfaraz]

some Panels available on Ali Express

Claiming to be OLED Displays
https://www.aliexpress.com/item/1005002950979761.html?spm=a2g0o.detail.1000014.24.1ef27dc2bJF4Ej&gps-id=pcDetailBottomMoreOtherSeller&scm=1007.40050.281175.0&scm_id=1007.40050.281175.0&scm-url=1007.40050.281175.0&pvid=46f64d2b-8ad2-4fb9-b72b-409d46a031d2&_t=gps-id:pcDetailBottomMoreOtherSeller,scm-url:1007.40050.281175.0,pvid:46f64d2b-8ad2-4fb9-b72b-409d46a031d2,tpp_buckets:668%232846%238113%231998&pdp_ext_f=%7B%22sku_id%22%3A%2212000022928175654%22%2C%22sceneId%22%3A%2230050%22%7D&pdp_npi=2%40dis%21PKR%21%211133.5%21%21%21%21%21%40210312ef16526125808314290ecf5c%2112000022928175654%21rec

https://www.aliexpress.com/item/33019908970.html?spm=a2g0o.detail.1000014.16.7eb657a9zTrdgX&gps-id=pcDetailBottomMoreOtherSeller&scm=1007.40050.281175.0&scm_id=1007.40050.281175.0&scm-url=1007.40050.281175.0&pvid=e226f384-4815-40ba-903b-c49ec2ebdc76&_t=gps-id:pcDetailBottomMoreOtherSeller,scm-url:1007.40050.281175.0,pvid:e226f384-4815-40ba-903b-c49ec2ebdc76,tpp_buckets:668%232846%238113%231998&pdp_ext_f=%7B%22sku_id%22%3A%2267203017554%22%2C%22sceneId%22%3A%2230050%22%7D&pdp_npi=2%40dis%21PKR%21%211139.29%21%21%21%21%21%40210312ef16526126331334775ecf5c%2167203017554%21rec

4 1/2 Voltage Meter display
https://www.aliexpress.com/item/32856546181.html?spm=a2g0o.productlist.0.0.c1a33c95A7Yheh&algo_pvid=b86ad202-d697-4cec-a189-f66fea443ba0&algo_exp_id=b86ad202-d697-4cec-a189-f66fea443ba0-12&pdp_ext_f=%7B%22sku_id%22%3A%2265256055499%22%7D&pdp_npi=2%40dis%21PKR%21%211542.18%21%21%21%21%21%402101e9d016526123633072192ee7a4%2165256055499%21sea

[gabri.peic]

Ok, i read all the suggestions and decided to go  with the pre made panel meters.
As far as powering the meters i will have the auxiliary probably 12v transformer anyways for the temperature measurement, overheat protection and fan speed control circuit so i willl probably just take 12v from that supply and send it through isolated dc to dc converter to keep two channels isolated.

[David Hess]

There are many models of 7 segments panel meters on Aliexpress, cheaper than one can build.  Various colors and segment sizes, most of them with 4 digits.  Some are single indicator, some are grouped in 2-3 lines for V, A, W.  They usually have a trimmer to calibrate them.

If you can trust the specifications; it is not trivial to achieve 4 digit performance.

ICL7135 4.5 digits
slow display refresh rate (weak CPU)
slow ADC sample time (may hide REF or input noise problem/immunity)

The ICL7135 is a real instead of toy ADC intended to have 1 count accuracy over 20,000 counts.  Typical applications would require a low sample rate to suppress noise and achieve that sort of resolution.  The datasheet recommends 2.5 or 3 samples per second maximum to still deliver 1 count accuracy.