# ICL7107 Problem

#### jj_alukkas

Joined Jan 8, 2009
753
I have breadboarded a voltmeter circuit using ICL7107. When I tested it, on power on itself it shows readings like 340 and 120 instead of zero.. My doubt is if this IC had been affected by electrostatic charges during handling, what would happen to the output? Due to breadboard limitations, I guess some component is not seated perfectly.. It has about 50 wires running to 4 seven segment displays... Should I go for soldering this and what could be its problem and how to solve it??? Varying the variable resistor doesnt help..

http://www.electronics-lab.com/projects/test/014/index.html

#### beenthere

Joined Apr 20, 2004
15,819
Do you have the input shorted to ground? That is usually necessary to get a zero reading. What happens when you apply a known voltage input?

#### bertus

Joined Apr 5, 2008
21,600
Hello,

What version of the ICL7107 do you have ?
There are versions with an R extention, those are mirrored in the pin counts.
See datasheet.

Greetings,
Bertus

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#### jj_alukkas

Joined Jan 8, 2009
753
@Beenthere

Do you have the input shorted to ground? That is usually necessary to get a zero reading. What happens when you apply a known voltage input?
I did not short the input to ground as this chip datasheet says it has 2 dedicated input pins which are seperated from ground.. I shorted them together as per said in the calibration part of that link. And it shows zero as said by them. I didnt need to adjust the preset. But when I open it, display goes high to some hundred digit values. Could my chip be damaged?

What version of the ICL7107 do you have ?
There are versions with an R extention, those are mirrored in the pin counts.
See datasheet.
@ bertus
I had checked the version before connecting the chip. It doesnt have an 'R'... Also the seven segments wouldnt have shown a readable display I think.

#### jj_alukkas

Joined Jan 8, 2009
753
Also is the resistor R3 connected between pin 30 and 31.. i made it as a line over 30..

#### Ron H

Joined Apr 14, 2005
7,014
Also is the resistor R3 connected between pin 30 and 31.. i made it as a line over 30..
R3 needs to be connected to pin 30. If it is not, you will get readings like 120 and 340 when the inputs are floating.

#### bertus

Joined Apr 5, 2008
21,600
Hello,

Pins 30, 32 and 35 are connected together, see also the typical connection schematic.

Greetings,
Bertus

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#### jj_alukkas

Joined Jan 8, 2009
753
Thanks Ron H and Bertus.. i am half way soldered now.. Will check back and report results tomorrow.... Thanks a lot..

#### jj_alukkas

Joined Jan 8, 2009
753
one more doubt.. In the datasheet, they have stated 2 schemes one in which the IC ground (pin 21) is connected to input low (pin 30).. the other one is the one listed above.. Should i ground it? This circuit is to be built into a VRPS.. So always will have a voltage input..

#### bertus

Joined Apr 5, 2008
21,600
Hello,

There is a note in this schematic of the datasheet on the GND connection.

Greetings,
Bertus

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#### jj_alukkas

Joined Jan 8, 2009
753
Could u please explain that for me.. I didnt get it.. Anyway I'll make my necessities clear.. My input wont be floating as it is built inside a power supply unit. And so will always be fed with a voltage.. I'm feeding power for this chip from a dual supply which is seperate from the main power supply. Which schematic should I choose now.. The first or the last with GND??? Please suggest..

#### jj_alukkas

Joined Jan 8, 2009
753
Also I have not considered to use that reference diode in it..

#### jj_alukkas

Joined Jan 8, 2009
753
Ok .. Finally I plan to use this circuit..

It looks similar to my original one.. But I have a doubt.. Can i ground pin 30 (input lo) as I will have to bolt my 7805 to case as a heatsink.. and so the GND of my main power supply will be in direct contact with the GND of this ckt.. will it cause a problem??

#### Audioguru

Joined Dec 20, 2007
11,249
Circuits on hobbiest websites are frequently wrong. The one you found last is from the datasheet and is correct. Always look at the datasheet.

#### jj_alukkas

Joined Jan 8, 2009
753
Ok guys, I need one more help... The range selection for this IC is done through R3.. Only the first website I stated gives light on this.. I cant find clear details on the datasheet.. I need a 20v range.. what should I do?? Connect a 1.2k resistor across 30 and 31 input pins??? Help please..

#### jj_alukkas

Joined Jan 8, 2009
753
I finally soldered the whole thing on a common PCB.. It works and I calibrated it with my multimeter and found that it is working in the 2000mV range.. I didnt use R3 to get this result. how can I adjust it to 20v range??

#### Ron H

Joined Apr 14, 2005
7,014
I finally soldered the whole thing on a common PCB.. It works and I calibrated it with my multimeter and found that it is working in the 2000mV range.. I didnt use R3 to get this result. how can I adjust it to 20v range??
I'm assuming you are still referring to the circuit you referenced in your original post.
Did you see this table? It is totally wrong.
Rich (BB code):
0 - 2 V ............ R3 = 0 ohm 1%
0 - 20 V ........... R3 = 1.2 Kohm 1% THIS TABLE IS WRONG!
0 - 200 V .......... R3 = 12 Kohm 1%
0 - 2000 V ......... R3 = 120 Kohm 1%
It should look like this (assuming R4=1Meg):
Rich (BB code):
0 - 2 V ............ R3 = ∞ (e.g., no resistor)
0 - 20 V ........... R3 = 111 Kohm 1%
0 - 200 V .......... R3 = 10.1 Kohm 1%
0 - 2000 V ......... R3 = 1.00 Kohm 1%
For the least error on the 20V range using standard 1% resistors, use R4=1.02Meg and R3=113k.

#### jj_alukkas

Joined Jan 8, 2009
753
I refered to the first circuit I posted. But didnt use R3. And Ron I really thank you for your help. None had been replying to this post though I was stuck in the middle of my work.
One last question. Is resistor R3 connected across the 2 input pins? The diagram is not clear. Thats why. Also would like to know how you knew all this.

#### Ron H

Joined Apr 14, 2005
7,014
I refered to the first circuit I posted. But didnt use R3. And Ron I really thank you for your help. None had been replying to this post though I was stuck in the middle of my work.
One last question. Is resistor R3 connected across the 2 input pins? The diagram is not clear. Thats why. Also would like to know how you knew all this.
Yes, R3 is across the input pins, as Bertus and I both stated previously.
I know all this because, many years ago, I went to college and got an EE degree. You don't need an EE degree to calculate voltage dividers. All you really need to know is Ohm's law. You can use this knowledge to calculate voltage dividers.