Measuring negitive voltages as positiveand sensing that it is netitive?

Discussion in 'General Electronics Chat' started by spinnaker, Dec 19, 2009.

  1. spinnaker

    Thread Starter AAC Fanatic!

    Oct 29, 2009
    I am working on my first project in a very long time. A bench power supply with built in digital voltmeter. The voltmeter will have a PIC 18F45K20. The PIC will also operate the front panel.

    After ordering everything, I quickly realized that it is probably cheaper to buy a bench supply rather than build it on my own. But I sort of already knew this, from what others have said on this forum. But it is hard to put a value on the challenge, the education and the exercise to my old brain cells. :)

    Anyway, I knew I could not directly measure negative voltages with the PIC. So I thought what I really need to do is to measure absolute voltages and I probably would need to use an op amp to do this. So I did some searching and sure enough I found an op amp circuit to measure absolute voltages.

    But what I can't figure out is how the PIC would know a negative or positive voltage was being measured. How would I do this? Maybe a voltage divider and a diode on the input to the op amp?

    Or is there an easier way to measure both negative and positive voltages altogether?
    Last edited: Dec 19, 2009
  2. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    The simplest way is to use a resistive divider with it's 'common' end to the PIC +5V.

    eg. If you want to read somewhere around -12V, calculate the divider to cover 17V, say a 3:1 ratio giving 1/4 input to the ADC, full scale 20V (= -15).

    Invert the reading from that ADC channel and subtract the appropriate offset for the +5V bias. With the 3:1 example, the offset would be 1/4 of 256 or 64.

    -12V would give an ADC input of 5 - (17 / 4) = 0.75V
    The ADC reading would be about 38

    Invert: 256 - 38 = 218
    Remove offset: 218 - 64 = 154

    (Allowing for the 1/4 input from the divider)
    ADC steps, based on full scale of 20V: 20V / 256 = 0.078125V per step.

    0.078125 * 154 = 12.03125V

    About right.
  3. spinnaker

    Thread Starter AAC Fanatic!

    Oct 29, 2009
    Thanks. Do you mean like this?

    Seems like a very useful site.

    I will have to but that little circuit through spice. I'm sure I have come across circuits with two different voltage sources way back in my electronics technician days but never really thought about how they really work. It would be nice to know how this really works.