Effect of adding parallel resistor

Discussion in 'Homework Help' started by metelskiy, Nov 4, 2010.

  1. metelskiy

    Thread Starter Member

    Oct 22, 2010
    I need help with lab report that i need to do. Or should i say i need to be able to explain effect of parallel resistor. Here is the situation. I had to design a simple circuit voltage devider with 10V power source using only 2 resistors and output voltage to be 6V. So i figured out two resistors would be R1=3.9k and R=5.6k
    lab 4_6V.png
    Than in next lab we had to modify this circuit to make output voltage 5V instead of 6V by simply adding parallel resistor R3 across R2. By calculation i figured out R3 to be 11k.
    Now in my lab report i need to explain everything. But i don't understand how adding 11k in parallel with R2 dropped voltage from 6V to 5V? Can someone please explain in details how did this process go? I thought at point A voltage should be 10V minus voltage drop across R1. Or if we have main source 10V and we need 5V between to resistances i thought those 2 resistances should be equal.
  2. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    Were you restricted on what Resistor group (E12, E24, E96) you could use?

    You can get the output closer to six volts than your choice of resistors.

    Is there any limitations?
  3. hobbyist

    Distinguished Member

    Aug 10, 2008
    Not trying to be sarcastic, but how did you come up with the calculation of 11K for R3, you had to use a voltage divider equation, as well as parrallel resistance equation, and using a logical approach to drop the voltage to half the supply voltage, so just explain what process you used to come up with your calculated answer for 11K (R3).
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    Putting the extra resistor in parallel with the lower resistor in diagram reduced the overall resistance, which increased current, which increased the voltage drop on the first resistor in the divider by 1V.
  5. zgozvrm

    Active Member

    Oct 24, 2009
    Your values for R1 and R2 result in 4.11 volts across R1 and 5.89 volts across R2.
    Why not start with values of 4K and 6K instead? That'd give you 4.0 v and 6.0 v respectively.
  6. metelskiy

    Thread Starter Member

    Oct 22, 2010
    Forgot to mention that we had to use best available resistors in lab so that was the closest i could get.
    thatoneguy, thanks for explenation, now i can see what's going on with parallel resistor added.
    Is there a way to explain why total parallel resistance is always smaller than any one value of resistors in this particular circuit? Lets say 5.6k with 11k results in Rt=3.7k? Or how did 11k resistor lowered total resistance when connected in parallel with 5.6k? I understand technique how it works in series but not in parallel.
  7. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    It is an additional pathway. Thus, more total electrons find a path from ground. More current. lower resistance. same voltage. ???

    metelskiy likes this.