Voltage divider circuit -- why do you get zero volt output if end resistance isn't zero?

Discussion in 'The Projects Forum' started by brent44, Sep 2, 2015.

  1. brent44

    Thread Starter New Member

    Sep 2, 2015
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    We have an application for a potentiometer used as a voltage divider to control the speed of a motor. The 5,000 ohm pot is causing the customer's motor to still be spinning a bit in the "at-rest" position, because the end resistance (maybe called residual resistance) between the wiper and the low terminal (at that position) is about 400 ohms, quite a bit above zero or just a few ohms (which you'd be getting from a higher-quality conductive plastic pot such as Clarostat 380 series). The pots are a linear slide pot and the pot manufacturer (BI model 404 pots) says when used as a voltage divider the higher residual resistance shouldn't matter. In any case that's the situation.

    My question is the math... if you use the standard equation, such as v(out) = (r2/(r1 + r2)) x V(in), and if r1 = 500 (the pot's resistance between wiper and low terminal in at-rest position) and r2 = 4500 ohms, with a 10 V input, then the voltage reading should be 9 volts (and 1 V), but the meters always read 10 V and zero V in the at-rest or "off" position. Remember, the pot doesn't go all the way to zero; it goes in a linear manner from 500 up to the max pot value.

    What am I doing wrong or thinking wrong? The math seems clear but the voltage meter reading consistently shows 10V and 0 V, no matter what pot value, and perfectly divides it through the range (the pot values actually at the low end are typically 167 ohms, 220 ohms, etc., and the high end is your typical +/-10%). Unfortunately I'm a mechanical engineer and not very smart at this apparently...
     
  2. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Mathematically I'm lazy; So what is total current, 10/ 5000 = 2 mA; V drop across 500 ohm @ 2 mA, E = IR,= .002 X 500 = 1 V
    If the bottom reading is 0, then at that time the residual R is 0. Try a different pot.
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    What is the input impedance of your meter when being used as a voltmeter?
    What is the input impedance of the motor control circuit?
    Are you measuring the wiper voltage with or without the downstream stuff connected?
    Any chance that the motor controller circuit is sourcing current?
     
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