Help reading tiny voltage on analog voltmeter (car's tore apart in the driveway)

Discussion in 'General Electronics Chat' started by dyingbreed, Oct 19, 2008.

  1. dyingbreed

    Thread Starter New Member

    Oct 19, 2008
    2
    0
    First off Hello all...

    Here's my problem if anyone could Please help out.

    Changing the TPS in my car and the book says it should be adjusted to between 0.48 and 0.52 volts.
    From what I have already seen on here, I have a pretty standard Multi-meter (although kinda cheap) with the ranges on the face...
    ohms
    0-250
    0-50
    0-10
    dB

    Selector has...

    DC mA
    0.5
    50
    250

    and

    DCV
    10
    50
    250
    500

    So, what selection on the dial, which scale, and what should it read to be between 0.48 and 0.52 volts?

    Any help would be most appreciated because as the title says, my car is tore apart in the driveway, and this is the only thing holding me up. :)
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    The DC 10 volt scale is the only one that will register anything, but probably lacks the resolution you need. If your needle is accurately on 0 with the leads shorted, you can try to get an indication of half a volt, but you won't be able to do better than that.

    If the needle is not accurately on 0 with no voltage applied, you can pry off the meter cover and use the small screw there to move the pointer to 0. Then you look for the indication of the sensor voltage. You really won't be able to see a small deviation from the upper or lower limit. Then again, if it is working, the reading should be good. A problem will usually be way off, like 0 volts or pegging the meter on the 10 volt scale.
     
  3. dyingbreed

    Thread Starter New Member

    Oct 19, 2008
    2
    0
    So I am think that I should have the selector on 10, and get the needle dead in the middle of 0-10 on the 0-50 scale, this should give me 0.5... correct?

    ya, the needle is calib. to dead on 0

    Thanks for your help :)
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You always read the scale that corresponds to the range set. Read 0 - 10 volts on the 0 - 10 scale. A .5 volt reading will put the pointer exactly between 0 and 1 volt on the 10 volts scale.

    The pointer displacement is proportional to the voltage measured. .5 volts is 5% of full scale in the 10 volt range. The reason that you can't see a small deviation is that your meter movement is probably only 3% accurate.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You really should be using a DMM (digital multimeter) for this adjustment. A good analog meter will have a resistance of about 10,000 Ohms per volt, however inexpensive meters may be as low as 1,000 Ohms per volt. This resistance will offset what you are reading (with the additional load of the meter applied) vs what is there when there is no load. When you combine this offset error with perhaps another 3% meter movement error, then combine that with the fact that you are nearly at the end of the scale, your error margin is not acceptable for this adjustment.

    Compounding the problem is that I don't know what the resistance of your TPS is, so I can't give you a good estimate of how much you need to offset the voltage reading to get an accurate no-load setting.

    You can buy inexpensive multimeters at quite a few places. Harbor Freight frequently has cheap digital multimeters on sale for between $2 - $5. They have a 2000mV scale (0.000mV-1.999mV) which would be perfect for your purpose. Digital multimeters have extremely high input impedance, so you don't have to calculate the resistance of the meter in with your reading to get the offset.
     
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