Voltage Divider Issue

Discussion in 'The Projects Forum' started by bpmccain, Jun 29, 2012.

  1. bpmccain

    Thread Starter New Member

    Jun 29, 2012
    1
    0
    I'm having a problem with a reference voltage I am trying to provide to an LM339 comparator.

    It is a simple voltage divider consisting of a 200K resistor, a 50K Pot, and 100K resistor, with a 5V supply voltage. The center tap of the Pot goes to the LM339 as a reference voltage. Looking for approximately a 1.75V reference voltage.

    The strange behaviour is that I get 0V at center tap of the pot. I have tried replacing all the parts, and all resistances test fine outside of the circuit.

    Without power, but installed in the circuit the resistance of 200K resistor is about 100K, the 100K resistor is measuring 50K and the 50K Pot is measuring 45K.

    When I apply power, all 5V are dropped over the first 200K resistor.

    I've attached my schematic.

    Any thoughts? I'm confused.
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    Disconnect everything connected to the center tap (i.e remove the LM339 and C16) and measure the voltage again at the center tap.
     
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