cx101bg panel meter alternative - Page 1
Forum section: electronics / beginners

author: jgalak
author_rank: Regular Contributor
timestamp: February 12, 2018, 03:11:24 am
content: I'd like to build a constant current load like the one in Dave's video (#102) and am stymied by the panel meter he uses. It's a CX101 (presumably the BG version that works with common ground). But this thing doesn't seem to exist anywhere. I found one site that claims to sell them, but it's not a site I know (https://www.circuitspecialists.com/lcd-panel-meter-cx101bg.html). Is this thing discontinued? If so, are there alternatives that would do the job?I can't seem to find a reasonably priced panel meter that'll read in the appropriate range (from 0 to "less than 2V"). All the stuff on digikey seems to either have a non-zero low-voltage or it seems to be on the expensive side (in the $30 range and up).

author: floobydust
author_rank: Super Contributor
timestamp: February 12, 2018, 09:57:24 pm
content: It looks like a CX101 is a vanilla digital panel meter with GND referenced power/input, and is configured to be a 0-1.999V voltmeterThe current-sense shunt is 1R so 1V/A for 0-1.999A display.There are many offerings on eBay or Aliexpress for such a thing, up to 5 digits. Search for 2V or 1.999V voltmeter.ICL7107 based DVM's need +/-5V power, so they don't work here; with single supply, as I recall.

author: jgalak
author_rank: Regular Contributor
timestamp: February 13, 2018, 03:31:37 pm
content: A 2V would be fine, easier to use than the 200mV in the video.Problem with the eBay ones is no datasheets or wiring diagrams, so can't tell if they can use common ground or not.... For the $6-9 they want, I suppose I can buy one or two and try it, but I really like knowing what I'm getting...

author: tooki
author_rank: Super Contributor
timestamp: February 13, 2018, 04:23:36 pm
content: Quote from: jgalak on February 13, 2018, 03:31:37 pmThen you’re really not looking hard enough! Tons of listings have wiring diagrams.All the cheap Chinese panel meters I’ve seen and tried have common grounds. (If anything, I wish they had models that are isolated!)FWIW, the 4 and 5 digit models are preferable in my experience — they seem to be spot-on for voltage, using parts that are seemingly inherently linear. Most 3 digit ones (and many 4 digit) often use trimpots for calibration, and they’re often not very linear. Just look for listings with good pictures — if you see trimpots, run!FWIW, another option, which gives you total control, is to use a TI INA226 IC (buy it as a module from eBay, it’s way cheaper) and an arduino to read it and drive a display. The INA226 is insanely good — its native voltage range is 0.000-36.000V, with millivolt resolution at 120 readings/second (!) that agrees within three counts of my 6.5 digit Keithley 2015 DMM. (Seehttps://www.eevblog.com/forum/projects/ode-to-the-ina226-voltagecurrent-sensor/)The Chinese panel meters all seem to have update rates of around 3 per second. If faster is desirable, the INA226 becomes a really, really good option.If you need current measurement, then use the INA226’s current sensing. It’s pretty flexible. (Though I am fairly certain I’ve run into bugs in the INA226 library for Arduino, giving me less resolution in current than it’s supposed to.)

author: jgalak
author_rank: Regular Contributor
timestamp: February 13, 2018, 05:41:56 pm
content: I did a quick look through ebay, and didn't find any with wiring diagrams. Maybe I need to go to Aliexpress, but I've never used them, so have been somewhat hesitant.The INA226 + arduino approach seems like overkill to me. It'll significiantly increase the complexity of the project, and I just want it to display the voltage (as a current proxy). But I'll definitely keep that module in mind for other projects, I can see quite a few uses to it.

author: tooki
author_rank: Super Contributor
timestamp: February 13, 2018, 09:15:07 pm
content: Quote from: jgalak on February 13, 2018, 05:41:56 pmOK, I have to take back my comment: right now, I can’t find any listings with wiring diagrams, either! Very weird, cuz when I was shopping for panel meters maybe a year or two ago, lots of listings had them.In any case, this is the kind of meter I suggest:https://www.ebay.com/itm/Green-LED-5-Digit-DC-0-4-3000-33-000V-Digital-Voltmeter-Voltage-Meter-Car-Panel-/400491885769?_ul=CHIt has 4 wires (+ and - input, and Vcc and Gnd), expressly capable of operating with just Vcc, Gnd, and +in, but with better accuracy when using a separate power supply. These are extremely accurate in my experience.This version (unlike my slightly older one) is nice in that it actually has two ranges, giving you 0.1mV resolution in the 4.3V range!Aliexpress is fine, but has weaker buyer protections. But sometimes you find things there that you don’t find on eBay.Quote from: jgalak on February 13, 2018, 05:41:56 pmOh, there’s no question that it’s gilding the lilly!! But it does give you really high performance, and the ability to manipulate the measurement before display, and total flexibiity in the display device and format.One of my never-quite-finished projects is pimping a cheap HY3005-style lab PSU, which came with crappy 3-digit panel meters. I had originally planned on simply using eBay 5-digit panel meters like the one above, but several things made me look further: 1) the relatively small digits, 2) the slow uppdate rate of 3Hz, and 3) many meters “stabilize” to 0.000 when they’re close to zero (e.g. <30mA on a 10.000A meter), meaning they effectively blank out low readings.Using an INA226 and Arduino means I can choose effectively any update rate I want (I find about 10 per second to be a nice balance between adjustment responsiveness and visual stability), and combine it with any display I want (I’m looking at both 7-segment and LED dot matrix, both in white, bought from AliExpress, where there’s a vendor with a huge selection). And since the Arduino has computing power to spare, it means I could easily add more mods in the future and just use the same Arduino.

author: jgalak
author_rank: Regular Contributor
timestamp: February 13, 2018, 10:08:57 pm
content: That should work. I'd lose half the range, but that's perfectly fine. Thanks!

author: jgalak
author_rank: Regular Contributor
timestamp: February 13, 2018, 10:10:45 pm
content: Except from that seller deliver is estimated as Wed. Mar. 21 and Tue. Apr. 10Oh well, it gives me a better idea what to search for.Thanks!

author: tooki
author_rank: Super Contributor
timestamp: February 14, 2018, 01:57:43 am
content: Quote from: jgalak on February 13, 2018, 10:10:45 pmIt’s Chinese new year. All the vendors are on holiday. Some are just kind enough to actually mention it...