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Led display Digital Voltmeter question


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thanks MP

a isolated dc-dc converter might work??
somebody can helpme with a low cost circuit dc-dc isolated??
i edit the board file from eagle and added:
on-off-on swch
grid ac led
inverter led
the bridge for ac readings
becouse dc-ac inverters usually have big battery banks , will be nice to power itself the voltmeter.
as soon i finish i will post here

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i have a dc-ac inverter 3kw 110v from 12v battery bank.
i would like to monitor the battery voltage, but i dont want to use 9v
alkaline battery nor 110ac power supply for power the voltmeter.
becouse i can not feed the vmeter from same source i'm monitoring,
i want to use a circuit to be able to do that , a dc-dc converter or any suggestions.
9v do not last enough for always on situation.

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i already contacted www.beta-dyne.com who make a 9v to 5v dc converter 2watts , 400 ma isolated and they are on their way .
As soon i have them i will post results and my modified board layout.
becouse mostly dc-ac inverters use 12v or 24 v configurations, i will
use 9v to 5v beocuse at 9 v the dvm will work besides the inverter who stop working at 10.7 v. i will feed the dc-dc from lm317 to220 1.5 amp variable regulator at 9.2 volts , so does not matter if is 12v or 24 volts battery bank .this is important becouse i built dc ac inverter up tp 10kw for 110 or 220 vac , and that will be a nice feature


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  • 1 month later...
Guest Kasamiko

I've successfully constructed this project and set the range to 20VDC by using a 100k input resistor..I connected it to my power supply LM317 but notice that my last digit wont lock, let say i'll set the voltage output to 3.6 vdc, my display says 3.61, 3.63, 3.65....dirty pots?

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Hi Rhonn,
You are trusting the stability of your components:
1) How much do the values of you project's and your LM317's resistors change as they warm up?
2) How much does the voltage references in the IC for this project or in your LM317 change as they warm up?
Your IC for this project is driving many LEDs so will not remain cold.

My purchased digital meters are stable maybe because their IC is driving a very low current LCD display, and/or maybe because their manufacturers used very stable 1% or better metal-film resistors.

Most meters are calibrated after a long warm up period. See if yours becomes stable after both it and your LM317 have been turned-on for about 1/2 hour.

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  • 2 weeks later...

I have been reading these posts, and I must say there are some very bright individuals in here:)

anyways, I would like to use this cIrcuit to measure the voltage of a power supply output, however it is 0-25.2 VAC not DC, is there a mod I can implement that will allow this?? I will use a separate power supply for the + and - 5v. I saw in the spec sheet in figure 21 that a AC to DC convertor with the ICL7106 can be used?? Can i simply implement that circuit??

I'm a newbie at this stuff, so please forgive my ignorance!!!

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Hi Ante,
Your full-wave rectifier circuit works fine at low frequencies, I have used it many times. Its "full-wave" becomes assymetrical above about 2KHz, unless you use a wide bandwidth opamp like a TL074, which rectifies well up to about 25KHz in this circuit.
The TL074 has FET inputs so RV2 is unecessary (input pin 10 connects directly to ground), but an offset voltage adjustment is needed with any opamp for very low-level inputs.

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  • 2 weeks later...
  • 2 weeks later...

I do not think this device will handle the current you are wanting to measure. Please look at the maximum ratings on the data sheet to be sure.

These two capacitors are the power supply bypass caps. There is a wide variety of what will work and it is dependent upon the power supply you use. A good common value that should work in most normal cases is a 10 uf.


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  • 3 weeks later...

I am looking to do the same project as nice ride....couldn't you just use the same battery that you are measuring with a regulator circuit regulated at 5 volts?

also i need to know how to get the 0-20 volt display scale...
what resistors and capacitances are needed????


Thanks everyone for the help ahead of time.... I have read everything, and reviewed everything, and I am ready to build this circuit; however has this question been answered?

I basically am building this circuit to measure the voltage of a car battery and would like it to just be a two wire hook up. If I can
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