Help understanding circuit

Discussion in 'General Electronics Chat' started by rgehrig, Sep 10, 2014.

  1. rgehrig

    Thread Starter New Member

    Sep 10, 2014
    Hi all

    I have modeled this circuit in LTSpice IV and it gives me results that are consistent with measured results from the actual circuit.

    I am seeking help in understanding how it operates.

    Some facts about the circuit.

    The 9620 ohm resistor is a 20K trim pot set to that value.

    The 2201 ohm resistor is a 5K trim pot set to that value.

    The 960 ohm resistor is a Simpson 50uA meter and that is its equiv resistance.


    purpose of 20K trim pot?
    purpose of 5K trim pot?
    purpose of Germanium Diode 1N34?

    Any assistance what be welcome.

  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    Hi neighbor,

    It would make a poor thermometer?

    Add this directive to your LTSpice sim:

    .STEP TEMP -50 50 10
    Last edited: Sep 10, 2014
  3. wayneh


    Sep 9, 2010
    The trimpots presumably are for adjusting zero and span. Not that I understand that circuit, but I understand how you calibrate a meter.
  4. rgehrig

    Thread Starter New Member

    Sep 10, 2014
    That it does.

    What it is doing is measuring the variation in the amplitude of an oscillator (whose output amplitude is varied by eddy currents affecting the inductance of the coil in the oscillator) The sine wave is half wave rectified by an emitter follower and filtered to become V1 source on the circuit.

    I have simulated the 20K pot going from 0 to 20K in 1 ohm steps and at the set value I get 25uA through the 960 ohm resistor leaving the 5K pot at the set value.

    Leaving the 20K pot at the set value and varying the 5K pot in 1 ohm increments seems to do nothing to the current through the meter (960 ohm ).