LCD Panel meters - the good, bad and ugly - your thoughts please. - Page 1
Forum section: electronics / general

author: nzo
author_rank: Regular Contributor
timestamp: August 13, 2011, 05:53:53 am
content: Since I got my new debit card I've been doing a little online buying of, in this case, LCD panel meters. I don't know about you but in New Zealand these panel meters areexpensive, between $NZ30 and $NZ40.In a moment of stupidity I bought three 3-digit LiPO meters fromDeal Extreme, thinking these could be modded to be simple DC voltmeters which I could use in my solar station and my portable emergency end-of-the-world solar/radio/lighting pack. Thesecanbe modded but the expertise is beyond me at the moment. Still, I can probably resell them to the R/C folks.I took a look ateBayand there are more panel meters than you can shake a stick at. The most common appear to be in the DC 7v to 20v range. In many cases what they don't tell you is these meters do not have a common ground. You have to feed the meter an isolated DC +5v supply independent of the voltage being measured. There are 100v meters available but again these do not have a common ground.I looked at using aDC to DC converterfrom DX, but the simplicity of one little meter running off of a 12v DC solar battery seems to be defeated.For those here who have walked the walk, what are my best options for doing/buying what I need?Thanks and appreciation!

author: EEVblog
author_rank: Unknown
timestamp: August 13, 2011, 06:00:28 am
content: Some panel meters have jumpers or some other way to enable common ground, even if that feature is not advertised.Dave.

author: ciccio
author_rank: Frequent Contributor
timestamp: August 13, 2011, 01:50:22 pm
content: This one on Ebay has common ground:http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=170483294158It's from the same supplier of the one you have posted the link.The dc-dc converter you link to does not seem to be an insulated type.regards

author: thilo
author_rank: Regular Contributor
timestamp: August 13, 2011, 02:45:13 pm
content: What's the issue with common ground? Or to put it another way: why can't all of them operate with a common ground/how does a digital panel meter work?Thilo.

author: Zero999
author_rank: Super Contributor
timestamp: August 13, 2011, 04:50:09 pm
content: Because the input stage of a panel meter is normally a differential amplifier made with op-amp who's common mode range doesn't include 0V or one of the inputs is actually floating at half the meter's supply voltage.

author: ciccio
author_rank: Frequent Contributor
timestamp: August 13, 2011, 05:37:39 pm
content: Quote from: thilo on August 13, 2011, 02:45:13 pmMost panel meters (3 1/2 digits) are built around an ICL 7106 / 7107 converter (or a clone).http://www.intersil.com/data/fn/fn3082.pdfIf you look at the above PDF, you'll see that the negative input (IN LO : pin 30) or the input common (pin 32) are not connected to the negative power (V- : pin 27).Additional circuitry is required to have common power and negative input grounds.

author: thilo
author_rank: Regular Contributor
timestamp: August 13, 2011, 10:17:28 pm
content: Thanks, Hero and ciccio.Quote from: ciccio on August 13, 2011, 05:37:39 pmLike the circuit on page one of this pdf?http://www.hestore.hu/files/pm438.pdfAnd how do those panel meters work that can work with a common ground? Do they integrate such a circuit, or are there other ways to measure voltage (digitally)?Thilo.

author: johnwa
author_rank: Frequent Contributor
timestamp: August 14, 2011, 11:02:52 am
content: Hi,You might be better off using a DC-DC converter if you aren't too sure of the connections required for a common ground. Something like this:http://au.element14.com/recom-power/rm-1205s/dc-dc-unreg-0-25w-12v-05v-sip4/dp/1793301. Although this is not necessarily the most elegant solution, hooking it up is as simple as you can get - two pins for power in, and two for power out! I recently used one of these in a similar application at work. Although they are a little on the expensive side, it still was a better option than spending time building something. You can probably find a cheaper supplier than Farnell if you look, too.John

author: ciccio
author_rank: Frequent Contributor
timestamp: August 14, 2011, 06:14:59 pm
content: There are some ways to overcome the problem of common ground, such as those reported in the ICL7106 - ICL71017 data sheet.They require generating the negative supply voltage V- with some CMOS inverters (page 10) then connecting the GND and V- pins (page 11) or using an external reference (page 12), or both..A schematic of the first soluton (using a 7660 voltage inverter IC instead of the simple CMOS gate proposed by Intersil) is here:http://electronics-diy.com/ICL7107_volt_meter.phpThe circuitry in the pdf posted by thylo or a DC-DC converter like the one proposed by johnwa are probably an overkill.Regards

author: nzo
author_rank: Regular Contributor
timestamp: August 15, 2011, 11:50:58 pm
content: Thanks everyone for your responses and ideas. I decided to get a couple of these eBaycommon-ground LCDsfor a start. They're still a bit prehistoric and boxy but they'll do for now.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 16, 2011, 03:01:28 am
content: If absolute accuracy and reliability are not important, buy some cheap DMMs and use them as panel meters. Harbor Freight sometimes has some for $2 or so each.Another alternative is to use a microcontroller and LCD display. That's especially useful if your application has many measurements sharing a reference or something other than a simple linear scaling. (Actually, just about every modern DMM, except maybe the cheapest ones, is based on a microcontroller of some sort.)

author: nzo
author_rank: Regular Contributor
timestamp: August 16, 2011, 04:38:07 am
content: Ni Hao Mike. I wanted the LCD to be small and simple. Simple is good. Then I can leave them in-circuit for me and other people to monitor the voltage in the solar emergency packs. But thank you!

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 16, 2011, 12:48:58 pm
content: Use a low power microcontroller and a LCD. With proper design, the power usage can be lowered to the point where it can be left on continuously.

author: nzo
author_rank: Regular Contributor
timestamp: August 16, 2011, 02:10:51 pm
content: QuoteNow you're cookin' NiHaoMike - you're just the man/woman to design something like that. I'll buy some if the price is good. So will many others.With nano technology it should be possible to assemble the circuitry on the head of a pin. The biggest items would be the LCD and the in/out wiring connection pads, unless you made them wirelessIf you can't access the electron microscope and atomic tweezers to put it together, you could simply design a circuit that would meet the specs in your above quote and throw it open to finer electronic brains than mine. I don't yet have the skill to design something like that, unless you have a blue pill I could take.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 17, 2011, 01:11:58 am
content: Quote from: nzo on August 16, 2011, 02:10:51 pmMicrocontrollers with low enough power usage are very common products nowadays. The MSP430 is the most well known example. (I own a watch that has a rather high end MSP430 in it, yet it runs for years from a little coin cell.) The entire development kit for some of the lower end MSP430s only costs $4.30 and the chips themselves are available for well under $1 each in quantity.Another benefit is that it's very easy to program the microcontroller to display an estimated SOC level instead of just a voltage, and if you want to display current as well, just add in a shunt and sense amplifier. It can also operate as a charge controller by switching off the charging circuit or switching on a load to dump the excess current, or even perform MPPT using the microcontroller's built in PWM to control a buck converter.

author: Unknown
author_rank: Unknown
timestamp: August 19, 2011, 10:05:28 pm
content: Quote from: NiHaoMike on August 17, 2011, 01:11:58 amNote that if you're already experienced in other MCU's like PIC or AVR, there's no reason to change. Both Microchip and Atmel offer MCU's with similar low power usage as the MSP430, the large difference in power usage is long gone. You'll find some appnotes from the manufacturers arguing that their part uses less power under certain conditions, but the truth is that they all make parts suited for low power applications.Not that there's anything wrong with the MSP430, and the Launchpad is probably the cheapest way to start, but there's nothing special about it's power usage either.

author: EEVblog
author_rank: Unknown
timestamp: August 19, 2011, 10:46:01 pm
content: Now I'm in a position of needing a cheap common ground LCD panel meter, preferably 0-2.00V and 0-20.00V switchable.I found those LED one's on ebay that are nice, but 20mA consumption is a bit on the high side for me.Anyone found any others?Dave.

author: Unknown
author_rank: Unknown
timestamp: August 19, 2011, 11:08:16 pm
content: Quote from: EEVblog on August 19, 2011, 10:46:01 pmWhat do you consider as being cheap? Futurlec are flogging some 0-200mv 5V supply ones but they do not quote current consumptionhttp://www.futurlec.com.au/Panel_Meters.jsp.

author: Bored@Work
author_rank: Super Contributor
timestamp: August 20, 2011, 05:07:09 am
content: Emphasis by meQuote from: EEVblog on August 19, 2011, 10:46:01 pmWhat is is, LCD or LED? If LCD,http://www.mpja.com/prodinfo.asp?number=16177+MEConfigurable for 5V common ground or 9V isolated power supply1 mA supply current (if the datasheet is to be trusted)3 1/2 digitsSwitchable 200V, 500V AC rangeSwitchable 200 mV, 2V, 20V, 200V, 500V DC range(I won't trust the isolation of that thing to handle 200 V or 500 V)It has your 2 V and 20V range, although the 2V range is 2.000V, not your desired 2.00V. However, your 2.00V range would be the same as as the 20.00V range when it comes to resolution, so I guess you anyhow want 2.000 not 2.00.

author: Psi
author_rank: Super Contributor
timestamp: August 20, 2011, 05:17:15 am
content: I dunno if they meet all your specs but here are some lcd panel meters for USD$4-7 eachhttp://www.hebeiltd.com.cn/?p=panel.meterGoogling the order numbers (PM-438, PM-128, PM-129-A1) seems to find more info about them.I've ordered UFO LEDs from the company before and the goods arrived without any problems and performed well.No idea on the quality of their panel meters though.

author: Unknown
author_rank: Unknown
timestamp: August 20, 2011, 06:19:43 am
content: Quote from: BoredAtWork on August 20, 2011, 05:07:09 am"Canadian orders must be phoned or fax. Sorry we do not accept orders from any other countries at this time." <--- the joy continues.