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Led display Digital Voltmeter question


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Did you use the negative supply in the end? If not maybe that's your problem.

Yes i did. I'm going to look at the datasheet, see if i'm missing something, and also try out the 100k oscillator resistor.

edit: 100k resistor didn't change anything.

edit2 :

The icl7660 ic is working normal. I did some new measurements :
Battery is 1,58V, icl7660 shows 0,6V. It's a ratio of 2,633 and i can't find any component that's 2,633 times different than the correct value.

Also in the datasheet they specify a 1k trimmer, on this website it says a 20k trimmer. When regulating the trimmer i don't notice any difference, so i'm guessing this may be the problem. If i change to a 1k trimmer i can adjust the reference so it shows the actual value. Or am i wrong?

In the datasheet they also mention a 200mv scale and a 2V scale, what does this mean? Is the measured voltage divided to a 0-2V scale?

The datasheet also says vref should be 1V if the scale is 2V. So for vref to be 1V i have to set the trimmer correct, and then it will show the correct value on the led display?
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The circuit in the datasheet is correct.
The circuit on this website has many errors.

The datasheet says that the IC has a sensitivirty of 200mV full-scale. It shows how to make it 2V full scale.

The circuit cannot measure its own supply voltage.

I'm going to put in the 1k trimmer this evening (if i have one). that is the only thing that still differs from the datasheet.

This is how i use the voltmeter : In my 30V dc power supply i have a circuit with a 5V voltage regulator that is connected to a transformer. That transformer is connected to another transformer i use for my 30V dc power supply. Because it's galvanic separated it should work i think. I've got another digital meter based on the icl7107 wich i bought somewhere. That one is still working.

The voltmeter i bought broke down because i didn't have a galvanic seperation back then.

edit: I used a 2k trimmer and it didn't change a thing. I still don't know what they mean with the 200mv scale and 2V scale. Does it mean when i use a 12k resistor for 0-200V i should use the 2V scale to get xxx.x reading? So the 2V scale has 2000 steps and the 200mv scale has only 200 steps?
At the moment i have a 470k resistor for 2V scale, a 12k resistor for 0-200V. And the rest of the components is exactly the same as in the datasheet. I have no clue why the icl7107 measures 2,63 times less than the actual value.

edit2: i changed the 470k resistor to a 47k resistor, no change at all. Still a solid 0.6V while it should read 1.58. Someone on posted his project on the eagle website, he had no link between pin 30 and 32 but that's even worse for me. My soldering station also broke down so i'm really getting frustrated. I checked all the connections and i checked the datasheet, i can't find any error in my design.
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Did some new tests. I replaced the 12k resistor between pin 30 and 31 with a 10k resistor. Now i get weird results.

So i leave the measuring points unconnected (floating). With the 12k resistor i had 00.0V but with the 10k resistor it rises to 38.xV.

Then i measure the 1,58V battery and it shows 34.xV. Then i switch the polarity and it shows -20.XV. When i unconnect the measuring points and leave them floating the -20.xV slowly decreases.

I'm guessing my capacitor/resistor combination is faulty. I'm using a 103 (10n) ceramic disc capacitor and a 10k(9,98) resistor at the moment.

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I noticed that too, but on this site they say you can adjust the scale with a resistor between pin 30 and 31. That's why i used a 12k resistor for 0-200V.

edit: so i took out the fixed resistor between pin 30 and 31 and switched it with a trimmer. Turns out it needed to be 31,5k. Strange but i knew something was wrong with the resistor/capacitor combo.

Thanks for your time and help.

I still have one more question though. I have a second meter wich i use as an ampere meter. But does it need and extra power supply different than the one supplying the voltmeter? Now i have a transistor that goes to a 30Vdc power supply and to another transistor. That transistor goes to a 5V and -5V power supply. Can i use that supply to power both meters?

Thanks in advance.

edit: This is getting really frustrating. I'm getting a feeling this icl7107 chip is crap. First when the multimeter was supplied by my 30vdc variable power supply it measured the 1,58V battery perfectly. Now with the icl7107 voltmeter in the 30vdc power supply it shows 3V when i regulate from 0 to 3V. The leds don't go beneath 3V for some reason. It's so weird.

I did the trimmer back to 11k so the voltage division is set for 0-200v. I connected pin 32 to GND as recommended by this guy http://www.electro-tech-online.com/electronic-projects-design-ideas-reviews/109313-icl7107-voltmeter-sensitivity-2.html#post897696
I removed the 47k resistor and put back the 470k resistor for 2V full scale. Still didn't work.
So tomorrow i'm going to put a 10k trimmer instead of a 2k trimmer to regulate the reference voltage. I couldn't get it higher than 315mV and it needs to be 1V. If that doesn't work i'm going to switch the cap at pin 27 for a bigger one (1

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I fixed it using this datasheet : http://www.datasheetcatalog.org/datasheet/TelComSemiconductor/mXrtrqt.pdf

I used the top right schematic on page 19. The only thing i added was a trimmer over pin 30 and 31 so i could finetune the voltmeter. Now it works perfectly, it's on par with my DMM. I'll post some pictures tomorrow when i got some time.

Next thing i need to fix is the shunt for my ampere meter. Does it need an extra galvanicly seperated power supply next to the one of my voltmeter? Or can i use the same?

I also put some knobs on the potmeters, but i think i broke one of them by using too much force. When i turn it it goes to 21.1V when i turn further it goes back to 15V. Before it regulated to 31V. I'll look into it later. I'm glad my voltmeter works finaly.

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I know it's not as much fun, but Harbor freight sells 3 dollar dvom's, PERFECT for the 0-30vPSU. I got one and stripped it, the signal wires even have ring terminals already attached to them which you can connect to the back of your output terminals. The only thing you need is a 9v supply (which you can easily acheive with an lm317), and the cable for the LCD is that square rubber thing, but I just used it anyway because the sub-board it was mouted to fit perfectly in the front of my psu. I cut a hole for the LCD and it look just like it's supposed to be there, and there was no designing/re-designing schematics and boards. You can also put a switch to use your leads for power on one side and on the other side of the switch have the outputs routed to only the dvom, turning the ordinary PSU into a PSU with built-in 600v DVOM.

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The datasheet for the ICL7106 for an LCD display and the ICL7107 for an LED display describes the problems caused by heating for the LED one.
The heating causes an offset voltage that could cause the circuit to be out-of-range where most of the LEDs turn off which causes an opposite offset voltage then the IC becomes an oscillator.
They recommend using an external reference voltage to minimise the problem.

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First of all I attach here the FIXED SCHEMATIC (22-2-04):



I attach also two sample power supplies for this voltmeter:



Hope this one solves all previously mentioned inconsistencies. Also the schematic at the project's page is updated:


blacksheep : To measure the current flows a load you use a small resistor in series with the load and you measure the voltage drop on it. Then with ohm's law you calculate the current pass throught the resistor and throught your load. You can't easily measure the current right away. So you can use the same circuit to measure the voltage drop across a resistor. For more info on how to do this see the pdf file i indicated previously.

I dont have accses to attachments...why?
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  • 2 months later...

Hi, im intend to build 2 LED voltmeters according to the schematics an PCB posted by M.P, i will use to measure the voltage and curent of Power supply wich i have build acording to the schematic at this site and modified to 0-30 volts 5 amps version, please if anyone can give me some calcuations for resistors value im intending to use 0.27 ohm resistor as a shunt to measure the current, ( R7 of P.S)
i have  attached the PCB and schematic in eagle format of LED voltmeter. can someone tell me how much should be the value of R3 for 0-30 volts and how much sholud be the value of R3 for voltage 1.35 volts wich i have calculated  for voltage drop across the 0.27 ohms resistor sow my panel voltmeter shows 0-5 amps current flow of P.S

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The voltmeter project still has a lot of errors. Look at the datasheet of the voltmeter IC for corrections.
I think the voltmeter must be powered by a power supply that is separate from the power supply that it is measuring.

Dear guru my power supply transformer has a 2 pair of windings 15-0-15 volts (30 volts 10 amps) and
7.5-0-7.5 volts (  15 volts and 2 amps) I'm intend to use this second winding to supply the LED voltmeters ( with rectifier bridge 4X 1N4007) also in a circuit that I'm going to use is an ICL7660 as a negative voltage converter (-5 volt). Please see attached schematic and PCB layout, i think until now this is the best and modified schematic, i hope doesent have any errors.


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  • 1 month later...

I have built the voltmeter and it works O.K when I'm supplying it with 9 volt battery, but when I'm supplying it with 7,5 volt transformer and a rectifier bridge with 4X1N4007 diodes as a rectifier bridge and 100mF capacitor the last digit is going up and down, in other words voltmeter becomes unstable, any ideas and sugestions??

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Yes i have build it , and according to IC  datasheet, actually i have build it 2 of them one will ICL7107 Maxim Dallas production and one with INTERSIL ICL7107 they are both working good as long as I'm supplying them with 9 volt battery  , only I'm having problem when I'm supplying them through transformer , i can say from my experience that the Intersil IC is working more stable when I'm feeding them through transformer, could it be the problem with smoothing the AC current?
any ideas and suggestions about the rectifier bridge and capacitor (value), and how much should be the voltage of AC voltage of secondary of the transformer? since I'm using the L7805 regulator for voltmeters.

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There are two versions of the old ICL7107 IC. The newer version ICL7107S has improved Stability.

The LED display draws up to 224mA through the IC which heats it a lot and causes it to drift.
It needs an additional -5V supply but you don't say how you produced it.

The L7805 regulator must have the two small capacitors shown on its datasheet. It needs a minimum input of +7V so the average rectified and filtered voltage must be more, depending on how much ripple is produced by the filter capacitor.
If the rectifier is full-wave then a 1000uF filter capacitor reduces the ripple to 1V peak. The transformer should be 8VAC or more at 500mA.

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