Led display Digital Voltmeter question

[mendimano1]

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??

 

[Hero999]

I assume you mean 100

 

[mendimano1]

Yes that's correct i have used the 100 microfarad capacitor,
i will try to use 1000 microfarad capacitor instead, thanks for advice

 

[audioguru2]

Has anybody made this very old project work properly?
It will probably work properly if it is built according to the datasheet for the IC.

 

[mendimano1]

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.

 

[audioguru2]

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.

 

[PicMaster]

Has anybody made this very old project work properly?
It will probably work properly if it is built according to the datasheet for the IC.

I brought a kit from Velleman which was based om the ICL7107 and I found that this gave unstable readings and could never cure the bouncing of the readings, May be this is why they stopped selling it ?

So in the end I decided to design and build my own based on the MAX7219 and PIC16F876, This gives accurate and stable readings and is now used in my Velleman PSU 0-30V 0-10A, I used ISIS to simulate it to get everything working(Picture attached) and then designed and built the PCB for it.
I like the idea of LED'S because where my PSU sits at the back of the bench I don't have to lean forward to see the LCD version. Plus I use the green version which is betterView attachment 41262

 

[mendimano1]

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.

Im using the the ICL7107 CLP version,
i have produce the -5 volt with IC ICL7660

 

[mendimano1]

I brought a kit from Velleman which was based om the ICL7107 and I found that this gave unstable readings and could never cure the bouncing of the readings, May be this is why they stopped selling it ?

So in the end I decided to design and build my own based on the MAX7219 and PIC16F876, This gives accurate and stable readings and is now used in my Velleman PSU 0-30V 0-10A, I used ISIS to simulate it to get everything working(Picture attached) and then designed and built the PCB for it.
I like the idea of LED'S because where my PSU sits at the back of the bench I don't have to lean forward to see the LCD version. Plus I use the green version which is better

hi Picmaster is it  posible from you to post the schematic and pcb of this volmeter of yours? ( PIC based)

 

[gogo2520]

I built one over a year ago and it worked good as long as 1% resistors were used, there were two projects posted at the time so I don't know which one you are referring to.
gogo

 

[mendimano1]

gogo im refering to this project

View attachment 41264

 

[gogo2520]

Hello mendimano
Yep that looks like the one. I had to use 1% resistor onR1, R2 & R3. and the voltage divider  took a little playing around with. It worked, still have it hanging on the wall.
have fun
gogo

 

[mendimano1]

Thanks for your tips gogo, seems reasonable to use 1% resistors for more accuracy,
ill try that and i hope it will work beater then,
  and happy new year

 

[gogo2520]

I found when I was doing some web searching, maybe it will you
gogo

ICL7107_tips.pdf