Sharing LCD panel meter supply with measurement input - Page 1
Forum section: electronics / beginners

author: Analog Kid
author_rank: Super Contributor
timestamp: January 11, 2025, 11:15:04 pm
content: In another thread I suggested using an inexpensive LCD panel meter. Tim Fox pointed out that the supply for the meter (typically ~9 volts) needs to be isolated from the input terminals on the ground side.I'm planning a project where the measurement circuitry would use the same supply (a 9-volt battery) as the meter itself, so I'm wondering if this would work to isolate the two:Would that diode be sufficient to isolate the two grounds? Or would I not be able to "fool" the meter this way?

author: TimFox
author_rank: Super Contributor
timestamp: January 11, 2025, 11:48:36 pm
content: No, that probably wouldn't work. The circuit supplied does a cheap job of splitting the 9 V isolated supply into a split supply to allow the bipolar input range.My experience is with the 4.5 digit models, which seem to be all based on the TC7129 or equivalent ICs.You can look up the datasheet for the 3.5 digit IC used in your panel meter and get more information than the meter guys gave you.The panel-meter guys don't give you much information, but the ones I bought seem to follow exactly the typical application drawing in Fig. 3-1 (with cheap R-C oscillator in Fig. 4-7 instead of crystal).The input R-C filter in the app note, with 150 k\$\Omega\$, also limits fault input current to the max 0.4 mA for reasonably high overvoltage (60 V).Seehttps://ww1.microchip.com/downloads/aemDocuments/documents/APID/ProductDocuments/DataSheets/21459E.pdffor the 4.5 digit TC7129.

author: squadchannel
author_rank: Frequent Contributor
timestamp: January 12, 2025, 12:56:08 am
content: meas side and power side GND are probably common.they are connected internally maybe.need to isolated dcdcs

author: TimFox
author_rank: Super Contributor
timestamp: January 12, 2025, 04:29:23 am
content: Quote from: squadchannel on January 12, 2025, 12:56:08 amNo. Internally, there must be a "split" supply to use with bipolar (+ or -) input for the dual-slope circuit.Check the manual: for most of these, the power supply must be "independent", as I discussed above.There are some meters with power supply and input common, but not these.

author: Analog Kid
author_rank: Super Contributor
timestamp: January 12, 2025, 05:27:36 am
content: Quote from: TimFox on January 11, 2025, 11:48:36 pmYou say it probably won't work, but is there a chance it might?I've got a meter coming soon, so I'll probably just try it and see what happens.By "the circuit supplied" do you mean the circuit in the LCD meter or my circuit?Quote from: TimFox on January 12, 2025, 04:29:23 amI looked at the TC7219 datasheet. Can't say I really understood it.The word "independent" doesn't appear anywhere in that document.So how would a guy create an independent power supply for this, using a single 9-volt battery?

author: DavidAlfa
author_rank: Super Contributor
timestamp: January 12, 2025, 06:28:37 am
content: Get a cheap isolated dc/dc module like this:https://www.aliexpress.com/item/1005006216595025.htmlOr find a simple module that goes straight to the battery, measuring the same power it uses.

author: inse
author_rank: Super Contributor
timestamp: January 12, 2025, 06:58:42 am
content: Those DC/DC converters have a bad efficiency, draining a 9V block rather fast.Two-pin LCD voltmeters do exist and can be had e.g. at Aliexpress for single €/$ amounts.While GND separation is not mentioned verbally in the TC7129 datasheet, I think it’s the CMVR parameter (page 3) that says (V-) +1.5V for Vin=0

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 12, 2025, 08:12:03 am
content: IDK about tbis exact LCD meter, but for usual LED meter you typically don't have to connect power GND. Just connect measurement input GND and it'll be good (at least for most cases).

author: Analog Kid
author_rank: Super Contributor
timestamp: January 12, 2025, 09:15:59 am
content: Quote from: Vovk_Z on January 12, 2025, 08:12:03 amSay again? What you wrote doesn't make sense.There are two sets of connectors, one for power (+ and -), the other for measurement input.How can you not connect the power ground (the minus terminal) and have it work?

author: inse
author_rank: Super Contributor
timestamp: January 12, 2025, 09:44:18 am
content: Can we simply agree that there are two types of LCD+LED modules or controllers, one allows common GND while the other doesn‘t?

author: DavidAlfa
author_rank: Super Contributor
timestamp: January 12, 2025, 10:55:59 am
content: True, I forgot it was battery-powered!I think you don't really need isolation, only negative voltage from Gnd, so a charge pump will do?

author: Kleinstein
author_rank: Super Contributor
timestamp: January 12, 2025, 11:20:29 am
content: A negative supply is possible way. It still depends on the LCD module. There is a good chance that one input (typically the neg side) is connected to some intermediate voltage (e.g. the COM at ICL7106). So one may also have to separate that link.An alternative could be adding a differential amplifier or INA before the LCD. The LCD module neg input would likely be the ref. input signal.

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 12, 2025, 02:43:32 pm
content: Quote from: Analog Kid on January 12, 2025, 09:15:59 amYes, all cheap VA-meters typically have two sets of connectors (power and measurement), with GND lines connected internally (for LED-meters).If you don't have isolated power supply but have to supply it from the same power lines which you have to measure you don't have to connect both TWO GNSs to avoid possble issues.Upd.: I'm talking only about LED panel meters. LCD panel meters possibly are a totally different thing (totally different design, with different power lines design, ICL71xx-like).Upd2.: I now realize that almost all LCD panel meters have totally different design, ICL71xx-like, so all my comments above are not relevant for TS question.

author: TimFox
author_rank: Super Contributor
timestamp: January 12, 2025, 02:58:01 pm
content: Again, the meters that I am familiar with, which came from Jameco as did the OP's, specify "independent" power supply: nominal 9 V but 7-12 V usable.I have used both of the models from them that have 4.5 digits: I believe he is considering the 3.5 digit units that use a different IC, but still require "independent" power supply.I had to check the datasheet from the IC manufacturer to understand the details: you cannot connect the negative end of the power supply to the negative input on their circuit board.The chip itself has a differential input, but the circuit board has an onboard R-C filter that is single-ended.For my first application, with two meters to look at the two outputs from a quadrature lock-in amplifier, I simply mounted two meters in a plastic enclosure with input voltage dividers appropriate to the signals and two isolated 9 V batteries: worked perfectly.The second application is line-powered, with op-amps running on a commercial +/- 15 V supply.I decided to use anisolatedDC-DC converter with nominal 9 V (unregulated) output from a 24 V regulator powered from +15 to -15V.The converter is similar to the one used by David Alfa above.Note that the specified current drain of this module is quite low, so I had to add an extra load resistor to meet the minimum output current requirement of the DC-DC converter.That project is in progress.My other question in that design was the possibility of the input going to +12 V on the 0.2 V full-scale range. From the IC datasheet, the 150 k\$\Omega\$ resistor on the input filter will limit the current to a safe (non-smoking) value.Since these cheap meters have limited documentation, the user must resort to the datasheet for the IC to obtain this information.The TC7129 chip is good for the application, and includes some functions (such as logic outputs for scale up and scale down for external autoscaling) not used in these inexpensive modules.Again, the 4.5-digit module I use follows the app-note suggested circuit, except for changing from crystal to R-C oscillator, that explicitly requires a floating battery/power supply.

author: TimFox
author_rank: Super Contributor
timestamp: January 12, 2025, 04:26:18 pm
content: Quote from: Analog Kid on January 12, 2025, 05:27:36 amThe word "independent" may not appear on the TC7219 datasheet, but iscapitalizedon the short spec sheet that comes with the PM-328 module : "INDEPENDENT 7-12V DC" for "Supply Voltage".From the TC7219 datasheet, the app note drawing (used by the module manufacturer) shows a 9 V battery connected between pins 23 and 24 of the chip, but input negative goes to pins 28, 32, and 35.I think these modules from Jameco are a good economical product, but the power supply issue must be obeyed.I looked up the Jameco spec for your PM428. It states “cannot be used with common ground”, equivalent to “independent power supply”, but works with bipolar input voltage.However, their CX102B (with LED display) is “suitable for common-ground applications”, while the CX102A uses an independent 9 V. The CX102B connection shows a parallel RC network between power supply negative and input negative.

author: pqass
author_rank: Super Contributor
timestamp: January 12, 2025, 07:26:07 pm
content: See attached isolated* power supply schematic for an (ICL7106-based) LCD panel meter.After building it I found that it was noisy** so I removed the inverter.It produces about 5mA with a 12V supply (vs. 9V as shown).* may not technically be galvanically isolated but does work.I found ithere; see very bottom of page.** noisy due to the slow gate. I saw it on the oscope.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 12, 2025, 07:34:44 pm
content: The charge pump supply is not isolated, though it can be shifted by a DC voltage. So it could work to shift the supply for something like ICL71xx. However it also injects quite some AC current to the new GND point. This could be more than the ICL7106 likes. Added capacitance could help, but the capacitance can be an issue at start up.

author: Analog Kid
author_rank: Super Contributor
timestamp: January 12, 2025, 09:46:39 pm
content: Quote from: pqass on January 12, 2025, 07:26:07 pmWellll, I'm hoping the same may be true for the circuit I originally posted. Worth a try, anyhow:Either it works or it doesn't. Won't damage anything.I'll report back with results.

author: lakis70
author_rank: Contributor
timestamp: January 12, 2025, 10:15:54 pm
content: If you talk about the PM-328 module, by independent they don't mean to separate the grounds but not power from the measurement voltage, use your power supply and make a voltage divider for the measurement voltage. This module haven't voltage regulator like some othershttps://www.rpelectronics.com/Media/Data/Specifications/S-PM-328.jpg

author: TimFox
author_rank: Super Contributor
timestamp: January 12, 2025, 10:26:31 pm
content: The PM328 module uses a bias network on the PCB to establish the correct "common" voltage for the chip.The chip's input voltage has a narrow range (+/- 200 mV), but there is a finite limit for that range with respect to the extremes of the external power supply, and the negative input cannot connect to the negative power terminal.For better treatment, see the chip's datasheethttps://ww1.microchip.com/downloads/aemDocuments/documents/APID/ProductDocuments/DataSheets/21459E.pdfand note that the PCB for the PM328 uses exactly the same circuit, with independent floating power supply/battery, as shown in the application note, except for substituting a cheap R-C oscillator for the crystal oscillator.The short datasheet for the PM-328 module specifies "independent" twice for the power supply.

author: TimFox
author_rank: Super Contributor
timestamp: January 12, 2025, 11:01:50 pm
content: Quote from: Analog Kid on January 12, 2025, 09:46:39 pmI suggest you try your module first with a floating 9 V battery (per instructions) to verify how it works, then try alternatives for the power supply.

author: Analog Kid
author_rank: Super Contributor
timestamp: January 13, 2025, 12:10:07 am
content: Will do. I want to do a proper experiment here.

author: pqass
author_rank: Super Contributor
timestamp: January 13, 2025, 12:12:03 am
content: FYI: see attached and livesimulation.The 1.8K resistor is a stand-in for your LCD panel meter. Any more than a 9mA load and the voltage will drop below 9V (given 12V CLK).

author: Analog Kid
author_rank: Super Contributor
timestamp: January 13, 2025, 12:49:38 am
content: Quote from: pqass on January 13, 2025, 12:12:03 amMaybe not a problem, as the specs state a power voltage of 7-12V.

author: pqass
author_rank: Super Contributor
timestamp: January 13, 2025, 01:26:46 am
content: Quote from: Analog Kid on January 13, 2025, 12:49:38 amThen you can drop the zener altogether.The output won't be regulated; can be as high as 11V with no load, 9.9V with a 5.5mA load, or 8V with a 12mA load.Just make sure the clock high doesn't exceed 12V.

author: Analog Kid
author_rank: Super Contributor
timestamp: January 13, 2025, 01:45:10 am
content: What are you talking about, clock, 12 volts?This thing'll run from a single 9-volt battery. All that stuff is internal to the DMM.

author: pqass
author_rank: Super Contributor
timestamp: January 13, 2025, 03:16:53 am
content: Quote from: Analog Kid on January 13, 2025, 01:45:10 amI did not realize that your main power source is only 9V.The power supply that I confirmed will work will only produce about 1V less than the input clock max at no load.With a 3.2mA load it will barely get to 7V output with no margin as the battery goes lower.You can explore the original circuit and re-introduce an inverter to drive the bottom 100nF (instead of grounding it). But you'll have to get rid of the glitches it may produce. Possibly by choosing a faster inverter (74HC04 or 74HC14) or more capacitance (vs. 10uF) or a parallel fast 10nF (or so) cap to remove spikes. You'll have to play with it a bit while visualizing on an oscilloscope.Alternatively, for another single chip solution, you can choose a (CMOS) 556 timer as the clock (first half being the clock with the other half acting as inverter). Seehere for the EDN article,simulation hereand snap attached. Youwillneed a 12V zener to make sure you don't exceed the LCD panel meter power max. In the simulation you can vary the 556 power supply from 7-10V via slider on right margin.You'll probably be better served with a properly spec'd isolated boost DC-DC module that can tolerate inputs going down to 7V or as high as 9.5V while producing a fixed 12V output.

author: Analog Kid
author_rank: Super Contributor
timestamp: January 13, 2025, 03:29:31 am
content: Thanks, but I'm just going to try using the 9V battery both for power input and for my measurement circuitry, with some attempts at isolation (diodes and a voltage divider at the DMM input). Maybe it'll work, maybe it won't. We'll see soon enough.

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 13, 2025, 01:54:47 pm
content: Quote from: Analog Kid on January 12, 2025, 09:15:59 amI now realize that almost all LCD panel meters have totally different design, ICL71xx-like, so all my comments above (which are about LED, but not LCD) are not relevant for TS question.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 13, 2025, 03:05:31 pm
content: If the whole thing is battery powered a real option would be to just use a 2nd battery, if the space permits.

author: TimFox
author_rank: Super Contributor
timestamp: January 13, 2025, 04:33:28 pm
content: Certainly the simplest way to power the LCD meter modules that want an "independent" power supply is with a floating 9 V battery.The current drain is generally quite low, around 0.5 mA for the PM-328 4.5 digit unit.

author: shabaz
author_rank: Frequent Contributor
timestamp: January 13, 2025, 05:44:30 pm
content: For about the same price, there are nice panel meters from AliExpress, with shared ground connection.I used a couple of them to build some milliohmmeters.The photo of the rear of the module shows what the PCB should look like (there are some with a noticeably different PCB, which have an inferior circuit, so those should be avoided by comparing with that photo).There's an old discussion here:https://www.eevblog.com/forum/projects/$5-voltmeter-with-5-digit-(0-1mv)-resolution/Also, if desired, with a slight mod, it's possible to convert the input to differential, for a bit more accuracy.