Voltage divider Q

Discussion in 'General Electronics Chat' started by protomor, Dec 30, 2008.

  1. protomor

    Thread Starter Member

    Dec 23, 2008
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    If I use an R1 resistor of 100 ohms and an R2 resistor of 200 ohms, I get 6v from a 9v source (for example). The equation becomes (200/300)*9. or 2/3 of the voltage in (12v in would give 8v out; in this scenario).

    Since the ratio is what gives the over all output. What (other than heat) would be the down sides of using 2k ohms and 1k ohms? Or 20k ohms and 10k ohms? or even 1 ohm and 2 ohms?
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    Typically, the values of resistors in a voltage divider circuit are chosen to keep the current consumption of the circuit to a minimum without compromising the reliable function of the overall circuit in which the voltage divder is being used.

    hgmjr
     
  3. protomor

    Thread Starter Member

    Dec 23, 2008
    22
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    so lower the better.

    Can you give an example of what compromising the reliable function of the overall circuit would be?
     
  4. hgmjr

    Moderator

    Jan 28, 2005
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    Lower current consumption from the power source is generally desireable. Lower current consumption corresponds to higher values of resistance.

    One example is the bias network on the input of an common-emitter transistor amplifier. If the values of the resistors are too low then the effective input impedance to the amplifier is lower which is generally undesireable. If the values of the resistors are chosen too high then changes in the base current of the transistor will have a greater influence the bias voltage setpoint.

    hgmjr
     
  5. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    It will work if the "load" has a very high resistance.
    With low resistance the voltage wil change.

    Here is an example without and with load.

    [​IMG]

    Greetings,
    Bertus
     
  6. protomor

    Thread Starter Member

    Dec 23, 2008
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    so bertus, in your example how would you get it back to 8v? use 1k and 2k instead of 100 and 200?

    @hgmjr. that example kinda went over my head. I don't know what a bias network is or an emitter transistor amplifier. I'm really new to this.
     
  7. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    When you change the voltage divider to 1 and 2 Ohm the voltage will go back to 8 Volts.
    This will give a lot of current in the divider that is converted to heat and will shorten battery life.

    Greetings,
    Bertus
     
  8. protomor

    Thread Starter Member

    Dec 23, 2008
    22
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    I think I get it.

    Lowering the resistance values in a voltage divider raises consumption while higher resistance lowers consumption.

    So, generally, you want to use higher valued resistors unless (like in Bertus' example) it disrupts the circuits normal function.
     
  9. hgmjr

    Moderator

    Jan 28, 2005
    9,030
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    It sounds like you have grasped the concept.

    Study and practice DC circuit analysis to explore the topic in greater depth.

    hgmjr
     
  10. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Another possebility to keep the voltage at higher resistances is using a buffer.
    This can consist of an opamp.

    [​IMG]

    Greetings,
    Bertus
     
  11. protomor

    Thread Starter Member

    Dec 23, 2008
    22
    0
    wow that sounds like a great idea if I ever run into that problem. I just looked up opamps and it sounds really useful.
     
  12. bertus

    Administrator

    Apr 5, 2008
    15,645
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    Hello,

    Keep in mind that opamps can only deliver upto 50 mA
    (depending on the type, take a look at the datasheets).

    Greetings,
    Bertus
     
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