Stabilizing / Averaging a volt meter

Discussion in 'The Projects Forum' started by trimoon, Apr 1, 2010.

  1. trimoon

    Thread Starter New Member

    Apr 1, 2010
    4
    0
    Hello. I'm trying to build a self powered, isolated volt meter to measure a 12 volt boat battery. I built the volt meter using this kit: http://www.canakit.com/universal-digital-panel-meter-kit-ck221-uk221.html
    Then I bought a 5V isolated power supply to keep it stable: http://parts.digikey.com/1/parts/733021-conv-dc-dc-6w-sngl-12v-5v-pcb-cc6-1205sf-e.html
    The +/- comes into the isolated power supply input leads, and onto the test leads of the kit, and out the other side of the isolater to the power input leads on the meter.

    It works... but its too sensitive. It jumps up and down about a volt very quickly.

    I'm looking for an easy way to average the input current so its not so jumpy.

    Thanks for any ideas!

    - Peter
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Hi Peter,

    Try using a resistor and a capacitor to make a simple low-pass filter, like this:
    [​IMG]
     
  3. trimoon

    Thread Starter New Member

    Apr 1, 2010
    4
    0
    So the blue box on the right is the meter and the V1 is the source right?

    I just tried that using a 6.3v 1 uF radial cap and a 10k resistor. Still fluctuates the same (Before i said 1v, its actually less, about .08 volts its jumping up and down). Didnt seem to make a difference.

    I'm using a 12v transformer to test this which does fluctuate .03v or so, but i don't want it to show up on the meter.

    If i put the testing leads to a different source, i.e a 9v battery. It behaves and stands still because this voltage is consistent.

    If i put the meters power source from the 9v batt and the testing leads to the 12v transformer, it fluctuates a tiny bit, about .03v, which is correct, because the transformer does move that much. However i still want to average this and not have it jump up and down.

    If i put both the power source and test leads to the 12v transformer, it fluctuates a little more, about .08.

    I think i need something that stores more power and lets it out slower. A bigger filter maybe?

    I don't know that much about electronics as im a beginner. Would I want a bigger capacitor / smaller resister to do this?

    Unfortunately i just hooked up my isolated power supply chip backwards and will have to buy a new one before i try this again :(

    Thanks for your help!

    - Peter
     
    Last edited: Apr 1, 2010
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Ouch. :(

    Always use a cap with a higher voltage rating than you're going to place across it. You should've used a 16v or higher cap. I'm surprised the 6.3v cap didn't explode on you.

    Your DC-DC converter may have a good bit of noise on it's output. Adding a 0.1uF and 100uF cap across it's output will help to quiet that down.

    You can also use a small and a large cap in parallel for C1 in the diagram.
    Yes, V1 is the battery, and the blue box is the volt meter.
     
  5. trimoon

    Thread Starter New Member

    Apr 1, 2010
    4
    0
    Typo, cap was rated 63 v not 6.3

    The output isn't where the noise guilty of making the gauge jump around is coming from though as its the input leads on the volt meter that are placed before the DC-DC converter.
    Ill experiment using the 0.1uF and 100uF both before and after it using your filter diagram. Thats a good idea.

    I just add in the extra cap beside C1 and in the same direction, still using the 10k resistor, correct?

    Thanks!
     
  6. trimoon

    Thread Starter New Member

    Apr 1, 2010
    4
    0
    OK I put a 1 uF and 100 uF in parallel and im down to a fluctuation of .01v from the transformer. Waaay better!

    Im curious how this works though... Why does adding the 1 uF and 100 uF beside each other work? why not just the 100 uF?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The large cap takes care of the low frequency transients.
    The small cap takes care of higher frequency transients.

    If the large cap is of a low-ESR (equivalent series resistance) type, you probably wouldn't need the small cap.
     
Loading...