Question about PIC Voltmeter 0-30V

[maharadga]

Hello.
I maked Voltmeter with PIC16F876A in range 0-30V.
But in range 0-~20V , voltage, displayng on LCD is about ~0,1 V less that real value. In range ~20-30V is Ok.
It's error on ADC or it's my mistake?

[medelec35]

Hi maharadga,
Have you tried on real hardware?
I would not have thought it would work at all.
Reason being, for calculations you're using INT variables.
You have got a calculation:

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volt = 0.293255 * adc

As volt is an integer you can only multiply and divide by whole numbers only.
For example if

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volt = volt * 0.99

then whatever the volt variable starts at, the result will always be 0
So if volt starts off with a value 250 then result will be

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250 * 0 = 0

as bytes and intergers are always rounded down and not up.
Try

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volt = adc * 10 / 34

martin

[maharadga]

Hi, medelec35.
Yes, of course I tryed in hardware. Exactly in hardware I see difference 0,1V bellow ~20V.
In your formula " volt = adc * 10 / 34" difference is 0,2V over measured voltage.
Better result is with formula "volt = 0.0300097 * adc" where "volt" is floating point and "adc" is integer.