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PM129B 31/2 Digital meter display problem

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yamanoorsai

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Dear All,
I am trying to display my sensor's output voltage to the PM129B 31/2 Digit LED Digital Panel Meter.

The display information is available at the following link:

https://store.kysanelectronics.com/servlet/-strse-68458/PM129B/Detail

The data sheet of the display is available at this link:

https://www.circuitspecialists.com/images/PM-129A&Bspecs.pdf

The display uses a ICL7107 IC which has A/D conversion and display utilities.
https://www.intersil.com/data/fn/fn3082.pdf

The problem I am facing is that I am not able to read the output voltage using the display. The voltage uses a 5V power supply. I have set the display's voltage range to be 0-20V.

When I switch on the power supply to the display, it shows that "0.00". So, I concluded that the display is in good condition.

Now, when I tried giving a 12V to the analog input pins, to test the display, it is displaying the character 1 and a dot. When I cut off the signal voltage, I see that the value indicated is gradually reducing to zero. I also noticed that it seems to decrease from some random higher value. For example, while decreasing, it shows values like this 16.36, 12.00 and so on until it shows zero.

There is a variable resistor "105" available in the back side of the display. I assumed it was meant for calibration of the display voltage and I tried adjusting it. There were no changes and it was showing the same character 1 when I provided the input voltage. In order to double check this, I tried giving a signal using a 1.5V battery. It seems to show the same character 1.

Is there a possibility that the calibration is wrong. I noticed an information in the website saying that if needed, we must add proper voltage dividers and decimal point wire jumper. I don't have an idea what they meant by this information.

Any help in this regard, would be appreciated.

Thanks in advance.

Regards,
Sairam
 

pranam77

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First confirm that you supply 5 volts regulated and well filtered DC to the supply terminals. Displaying a dot and 1 means the fed voltage is out of the meter's range. For setting the meter to 20 volts range, Disconnect the wire Jumper installed at RB and connect 10Meg resistor at the same place. Connect a 100K resistor at RA position. Connect 10Meg resistor at RB postion. for better accuracy, 0.5 watt metal film resistors are best suited for this.Done. Cheers. Come up with your results.
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yamanoorsai

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Hello pranam,

I shall try this and get back to you.

Thank you.
 

yamanoorsai

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Hello pranam,
Thank you very much. My circuit is working. I added an external voltage divider circuit.
 

pranam77

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You are most welcome. Do login to EDA regularely to share more ideas, doubts and knowledge. Cheers
 

yamanoorsai

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I have another issue now. When I give my sensor's output to the display, the value was displayed in negative. I changed the polarity of the input terminals and the problem was solved. Is it right to do that?

Also, my multimeter displays a stable output.

For e.g If its 0.15, it is display 0.15. Whereas my display keeps fluctuating.


I never faced any fluctuations like that when I connected a DC RPS voltage to the display. What could be the possible reason?
 

pranam77

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yamanoorsai said:
I have another issue now. my display keeps fluctuating.
I never faced any fluctuations like that when I connected a DC RPS voltage to the display. What could be the possible reason?
Yes this is the common problem with PM series of metera are that these are manufactured from China with low quality components which directly affetcs the accuracy. Every capacitor and resistor used in the pannel meter is directly or indirectly may be blamed for this. Only remedy is to use a pannel meter with high quality components which will naturally come at a good price when compared with the later. Cheers.
 

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Thanks for the reply. I read in the ICL 7607 datasheet and their trouble shooting methods. It says that the digital ground and the signal ground needs to be grounded in common. It has been done already. Will it work out if I connect the ground to the body of the enclosure?

The datasheet says that it is due to the reverse current from the LEDs which creates the noise in the A/D converter.
 

pranam77

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I think nothing special happens by connecting the ground to the body of the enclosurel untill you are using a good regulated and filtered DC source. The problem lies within the components of the pannel meter. Cheers
 

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Well, my boss is not going to accept that as a solution.. What may I do?
 

pranam77

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yamanoorsai said:
Well, my boss is not going to accept that as a solution.. What may I do?
Hmmm...your Boss is not my boss in anyway....:D:D Its your's and his problem...I have given the reply in the last post.....Cheers
pranam77 said:
Only remedy is to use a pannel meter with high quality components which will naturally come at a good price when compared with the later. Cheers.
 

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