Multimeter test lead

Discussion in 'The Projects Forum' started by roro36, Sep 7, 2010.

  1. roro36

    Thread Starter Member

    Sep 7, 2010
    52
    0
    I,ve written and made a multimeter using C and a PIC microcontroller that can read in just about any voltage, I've set it to 60V. When I remove the voltage test lead however which runs into the ADC port of the micro the voltage which i have displayed on an LCD, jumps around. How can I stop this?
     
  2. AMIT_GOHEL

    Member

    Jul 13, 2010
    67
    7
    Hello roro36,
    first of all Welcome to AAC..

    To get solution of your problem post schimeric and c code..
    then n only we can help you..
     
  3. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    605
    24
    You could try some decoupling caps on the input.
     
  4. Dyslexicbloke

    Active Member

    Sep 4, 2010
    420
    19
    It sounds as though you are leaving the input floating, which you shouldnt.

    Try a simple resistive devider so your input cant float while the leads are disconnected.
    Use the bigest resistor to ground that you can get away with and still stay imune from noise then sacle the input with software.
    You might also considder buffering the input with a high ipedance opamp but the comment above would still apply.
    Hope this helps
    Al
     
    roro36 likes this.
  5. roro36

    Thread Starter Member

    Sep 7, 2010
    52
    0
    Thanks, yes I know the input is floating but all voltmeter leads are until they are put in a circuit to measure. So by basically tieing them to ground through a large resistor will have little effect on the voltage measured as such a tiny bit of current would pass through?
     
  6. Dyslexicbloke

    Active Member

    Sep 4, 2010
    420
    19
    Sorry bud ... but no amp will work, in practice, if its input is alowed to float.
    - I dont know much but I know that. -
    Ther is always stray voltage, parasitic inductance and capavcitance to considder.

    Your are right its tiny so the currents involved are also tiny which is exactly why a big resistor will be capable of stabilising things for you without having too big an effect on your measurmrnt capability.

    Why dont you simply clip one acros your leads as they are now and see what diference it makes.
    Al
     
  7. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    I'm puzzled.

    The PIC ADC input can only accept 5V or less so you definitely has some form of voltage divider or conditioner connected before the input to the PIC pin.

    If it is a voltage divider, then it should acts as a bypass for the PIC pin voltage to near zero when there is no external input.

    You need to tell us exactly how you manage to get the 60V voltage signal into the PIC pin. What kind of resistor values are you using if it is through a voltage divider.
     
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  8. roro36

    Thread Starter Member

    Sep 7, 2010
    52
    0
    You're right, when I was testing, I kept taking it out before the divider circuit. My bad. Thanks.
     
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