Tie the top end of your voltage divider to +5V instead of to ground. When
your supply is at -21V, you'll have 26V across the voltage divider.
Calculate the resistor between +5V and the PIC A/D when you have -21V on
the supply to give you 5V. When the supply goes to 0V, you'll have 5V -
5/26 V across the resistor between +5V and the A/D input. The full range
on the input to the voltage divider would be:

+5V = ADC Full Scale
-21V = ADC Zero

Do the rest in software!

Harold


> I've built a triple variable power supply with two 0...+21V outputs and
> one 0...-21V outputs. I want to finish it with a dedicated triple
> voltmeter,
> using an 16f877 and a 2x40 character LCD.
>
> I'll use a voltage divider to get each supply into the + or -5V range
> and then multiply in software to get the actual value to display.
> Just one problem - how to change the minus voltage to positive voltage,
> so that I can measure all three sources using a single 16f877?
> (Each 16f877 ADC input must be 0...+5V.)
>


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