Another Potentiometer Problem

Discussion in 'Homework Help' started by yan500, Jul 14, 2011.

  1. yan500

    Thread Starter Member

    Jul 12, 2011
    48
    0
    Hey all,

    Man I don't know if it's just me but I am really terrible at solving voltage divider problems. This particular problem is just confusing me: http://www.allaboutcircuits.com/worksheets/e_divide.html

    It's question 18. I keep getting 9.090 kΩ as the total resistance but apparently that isn't right (but it is kind of close). Could someone help explain how I would go about finding the total resistance? I think I can formulate the voltage output, I just can't find the total resistance.

    Thanks!
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
    2,936
    488
    If you divide the potentiometers into 2k5 resistors it gets clearer. You have 2k5 + 2k5 = 5k in series with 5k || to 100k.. gives you the right answer, 9.762 kΩ
     
  3. yan500

    Thread Starter Member

    Jul 12, 2011
    48
    0
    Thanks for your prompt response! How did you know that the first resistor wasn't in series with the other 5k? The way I'm reading the schematic (which is probably wrong) is that the two 5k resistors are in series with each other and are both in parallel to the 100k resistor. So what I did was just divide everything by two and then added the two series resistors together. So then I just got 5k || 50k. Obviously this is wrong, but can you explain why?
     
  4. praondevou

    AAC Fanatic!

    Jul 9, 2011
    2,936
    488
    At the bottom is written "Note that all potentiometers in this circuit are set exactly to mid-position (50%, or m = 0.5)."

    that means it's the same as having 2x 2k5 resistors in series.
     
  5. yan500

    Thread Starter Member

    Jul 12, 2011
    48
    0
    That part I understand, but why didn't you just add the four in series and then combine it with the resistor in parallel? It gives you the wrong answer but it seems to be schematically correct.
     
  6. praondevou

    AAC Fanatic!

    Jul 9, 2011
    2,936
    488
    No it's not correct. The 100k resistor is not in parallel with the potentiomers, because the middle point is adjusted exactly at 50%. You were right if the upper middle point was UP and the lower one was DOWN. Then you had 10k in parallel with 100k.
     
    yan500 likes this.
  7. yan500

    Thread Starter Member

    Jul 12, 2011
    48
    0
    Ah, I see, thank you very much for your help.
     
Loading...