Author Topic: Sharing LCD panel meter supply with measurement input  (Read 1364 times)

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Online Analog KidTopic starter

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Re: Sharing LCD panel meter supply with measurement input
« Reply #25 on: January 13, 2025, 01:45:10 am »
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.
 

Offline pqass

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Re: Sharing LCD panel meter supply with measurement input
« Reply #26 on: January 13, 2025, 03:16:53 am »
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.

I 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).  See here for the EDN article, simulation here and snap attached.  You will need 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.
« Last Edit: January 13, 2025, 03:19:02 am by pqass »
 

Online Analog KidTopic starter

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Re: Sharing LCD panel meter supply with measurement input
« Reply #27 on: January 13, 2025, 03:29:31 am »
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.
 

Offline Vovk_Z

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Re: Sharing LCD panel meter supply with measurement input
« Reply #28 on: January 13, 2025, 01:54:47 pm »
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).
Say 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?
I 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.
 

Offline Kleinstein

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Re: Sharing LCD panel meter supply with measurement input
« Reply #29 on: January 13, 2025, 03:05:31 pm »
If the whole thing is battery powered a real option would be to just use a 2nd battery, if the space permits.
 

Offline TimFox

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Re: Sharing LCD panel meter supply with measurement input
« Reply #30 on: January 13, 2025, 04:33:28 pm »
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.
« Last Edit: January 13, 2025, 04:35:12 pm by TimFox »
 

Offline shabaz

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Re: Sharing LCD panel meter supply with measurement input
« Reply #31 on: January 13, 2025, 05:44:30 pm »
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.


 


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