Why does a voltage Divider need two resistors?

Discussion in 'General Electronics Chat' started by nmcclana, Jul 8, 2008.

  1. nmcclana

    Thread Starter New Member

    Jun 30, 2008
    When I read the section on Voltage Dividers, it makes sense that the voltage potential drops as you test each successive resistor. What confuses me is when I look at the voltage divider diagram on Wikipedia.

    Electrons are coming up from the ground, some of them decide to go up to Vin, and some decide to go to Vout. In the diagram, Z1 makes sense because, it acts to resist some of the electrons from going up to Vin, diverting some of them to Vout. The part that doesn't make sense is Z2. Why does that have to be there?

    In the water analogy, the flow is diverted down two different pipes, and Z1 lets you control the size of the diverting pipe. Why must you slow down the water flow with Z2?
  2. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    What you're really trying to do with a voltage divider is produce a Vout which is largely independent of load. By choosing Z1 and Z2 so that the current through them is significantly greater than that drawn from Vout this can be achieved. e.g choose Z1 and Z2 for a current 100 times greater than drawn from Vout.

    Voltage dividers are only practical for setting Vouts where very small currents are required.
  3. HellTriX

    Active Member

    Apr 11, 2008

    What if you connected a load from Vout and up to the positive side?
    The bottom resistor would restrict the flow to that Vout load. Conversely if you connected the load from Vout to ground the top resistor would restrict the flow. The biggest problem with voltage dividers is, if you start added a load to the positive side or the negative side at the Vout. You start added parallel resistance to either the top or the bottom resistors of the voltage divider so the Vout will change voltage because of that. (not regulated)

    You would need to have a stable output load resistance and calculate the total voltage based on all the parallel resistances.

    Hope this helps.
  4. studiot

    AAC Fanatic!

    Nov 9, 2007
    Just remember in the water analogy the flow is analogous to electric current, not voltage.

    In the water analogy the analogous property to voltage is usually taken as pressure.
    This analogy is not perfect but is OK for your purposes.

    When you follow this analogy the pressure is divided between the two pipes in just the same way as the voltage is divided between the two impedances.
  5. Dave

    Retired Moderator

    Nov 17, 2003
    Without Z2, Vout is at the same potential as ground. To use the water analogy, this would mean the Vout pipe is in the ground-reservoir.

    I always feel that voltage dividers are best considered using conventional current flow.

  6. SgtWookie


    Jul 17, 2007
    Ask yourself what happens when you divide a number by 1?

    The same thing happens in electronics. If you divide an input voltage with just 1 resistor, you get the input voltage as the output.
  7. thingmaker3

    Retired Moderator

    May 16, 2005
    Actually, the electrons would be coming in to Vout and up through Z1.

    Perhaps this will help clear things up: http://www.allaboutcircuits.com/vol_1/chpt_6/1.html
  8. m4yh3m

    Senior Member

    Apr 28, 2004
    but why wouldn't they go to ground since it's the path of least resistance?
  9. thingmaker3

    Retired Moderator

    May 16, 2005
    Because electrons flow from a more negative potential to a more positive potential. The amount of charge flowing through a given path will, of course, be inversely proportional to the resistance of said path.