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Newbie to Electronics, Pull up resistor

Discussion in 'General Electronics Discussion' started by skip0pot2mus, Nov 29, 2010.

  1. skip0pot2mus

    skip0pot2mus

    2
    0
    Nov 29, 2010
    Hey all,

    I'm a newbie here, and I am looking for some advice as I am totally lost.

    I am installing a temperature gauge in a vehicle, the gauge input is in volts.

    I have a one wire thermistor which is tapped into the block, trying to get a data sheet on that but it's proving difficult.

    The problem I have is with the pull up voltage.

    I am currently using a 471 Ohm pull-up resistor. I have a 5.02V stable output voltage.

    Using the formula V=R1/[R1+R2]x5.02 I should get the voltage pulled from between the two Resistors...right? I'm not.

    For example.

    R1=630 Ohms
    R2=471

    So output voltage should be 2.872V on a wire between the two resistors...right?

    I'm getting 4.00V

    What am I doing wrong? I've tried a different sized resistor and it's the same problem, the math doesn't tie in with the practise. Earths are good, voltage is stable... no resistance in the line.

    With the wire removed from the thermistor I get 5.02V on it. I get the same voltage output between the thermistor and earth as I do at the back of the gauge.

    Please help, I've spent hours and hours on this and getting nowhere

    Thank you for reading
    Matt
     
  2. Resqueline

    Resqueline

    2,848
    1
    Jul 31, 2009
    Can you provide some links to the products in question?
     
  3. skip0pot2mus

    skip0pot2mus

    2
    0
    Nov 29, 2010
    Unfortunately, no.

    The thermistor is from a mid 90's Honda.

    I have found this morning, that when the thermistor is grounded, it's resistance changes, the same thing happens when using a fixed resistor of 461 ohms, it changes to 310 ohms. That's whats making the math bad. I don't know why it's doing that though.
     
  4. Resqueline

    Resqueline

    2,848
    1
    Jul 31, 2009
    I'm having a hard time understanding what kind of products you are dealing with, what the problem is, what your measurements are, and what you are trying to do/accomplish.
    Ordinarily thermistors use only the gauge as a "pull-up resistor" and current sensor. Are you employing some kind of (homemade) digital gauge/scheme?
    If commercial then a brand name & type and/or a picture/diagram/description would help.
     
  5. neon

    neon

    1,325
    0
    Oct 21, 2006
    did you forget the loading on your divider? anything across is a load to the divider
     
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