LCD Panel meters - the good, bad and ugly - your thoughts please.

[nzo]

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 are expensive, between $NZ30 and $NZ40.

In a moment of stupidity I bought three 3-digit LiPO meters from Deal 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. These can be 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 at eBay and 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 a DC to DC converter from 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!

[EEVblog]

Some panel meters have jumpers or some other way to enable common ground, even if that feature is not advertised.

Dave.

[ciccio]

This one on Ebay has common ground:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=170483294158
It'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

[thilo]

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.

[Zero999]

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.

[ciccio]

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.

Most panel meters (3 1/2 digits) are built around an ICL 7106 / 7107 converter (or a clone).
http://www.intersil.com/data/fn/fn3082.pdf
If 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.
 

[thilo]

Thanks, Hero and ciccio.

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.

Most panel meters (3 1/2 digits) are built around an ICL 7106 / 7107 converter (or a clone).
http://www.intersil.com/data/fn/fn3082.pdf
If 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.

Like the circuit on page one of this pdf? http://www.hestore.hu/files/pm438.pdf

And 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.

[johnwa]

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

[ciccio]

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.php

The circuitry in the pdf posted by thylo or a DC-DC converter like the one proposed by johnwa are probably an overkill.
Regards

[nzo]

Thanks everyone for your responses and ideas. I decided to get a couple of these eBay common-ground LCDs for a start. They're still a bit prehistoric and boxy but they'll do for now.

[NiHaoMike]

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.)

[nzo]

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!

[NiHaoMike]

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.

[nzo]

With proper design, the power usage can be lowered to the point where it can be left on continuously.

Now 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 wireless

If 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.

[NiHaoMike]

With proper design, the power usage can be lowered to the point where it can be left on continuously.

Now 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 wireless

If 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.

Microcontrollers 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.

[alm]

Microcontrollers with low enough power usage are very common products nowadays. The MSP430 is the most well known example.

Note 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.

[EEVblog]

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.

[Uncle Vernon]

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.

What do you consider as being cheap?  Futurlec are flogging some 0-200mv 5V supply ones but they do not quote current consumption http://www.futurlec.com.au/Panel_Meters.jsp.

[Bored@Work]

Emphasis by me

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.

What is is, LCD or LED? If LCD, http://www.mpja.com/prodinfo.asp?number=16177+ME

Configurable for 5V common ground or 9V isolated power supply
1 mA supply current (if the datasheet is to be trusted)
3 1/2 digits
Switchable 200V, 500V AC range
Switchable 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.

[Psi]

I dunno if they meet all your specs but here are some lcd panel meters for USD$4-7 each
http://www.hebeiltd.com.cn/?p=panel.meter

Googling 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.

[Uncle Vernon]

Emphasis by me

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.

What is is, LCD or LED? If LCD, http://www.mpja.com/prodinfo.asp?number=16177+ME

"Canadian orders must be phoned or fax. Sorry we do not accept orders from any other countries at this time." <--- the joy continues.