Voltage divider - Wrong volt.....

Discussion in 'General Electronics Chat' started by TheDag, Mar 27, 2012.

  1. TheDag

    Thread Starter Member

    Jan 1, 2010
    94
    1
    Hi,
    I Have Output voltage of 7.67v and i put 240 and 270 ohm resistor to divide the voltage...
    but instead of 3.7v output, its 2.2v...
    Why ??
     
  2. evilclem

    Member

    Dec 20, 2011
    118
    16
    Double check your input voltage. If it's from a battery then it will drop when loaded.
     
  3. TheDag

    Thread Starter Member

    Jan 1, 2010
    94
    1
    What do you mean its will drop when loaded ?
    its from 7v of 555 timer
     
  4. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    Post your circuit. Where is the 7V coming from? A 555 timer is not a voltage source. If the source is a battery and it is old or weak, when power is requested from it, the voltage output may go lower. Think of a flashlight with weak batteries. The light gets dimmer because the weak batteries can not supply the proper voltage/current.
     
  5. TheDag

    Thread Starter Member

    Jan 1, 2010
    94
    1
    Its output from 555 as monostable....
    And the battery stay stable,
    The voltage divider splits the voltage to 5.44 and 2.23 (together 7.68), instead of 3.622v and 4.058v....
     
  6. mrmount

    Active Member

    Dec 5, 2007
    59
    7
    I would double check the resistor values then.
     
  7. TheDag

    Thread Starter Member

    Jan 1, 2010
    94
    1
    I Double checked it, its 233 ohm and 261 ohm,
    Still the voltage is not make sense....
     
  8. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,013
    1,531
    Measure the out put voltage at the 555 with the divider in use and see what that voltage is(this will be the actual input voltage to the divider, you can then refigure you divider using that voltage.)
     
  9. TheDag

    Thread Starter Member

    Jan 1, 2010
    94
    1
    Dont understand what you said...
    The circuit is ok, why its different volts??
     
  10. crutschow

    Expert

    Mar 14, 2008
    13,014
    3,234
    The 555 has an output impedance of its own which will cause its output voltage to drop some when it is loaded. As shortbus said, measure the output voltage of the 555 with the divider connected.
     
  11. TheDag

    Thread Starter Member

    Jan 1, 2010
    94
    1
    I Just did!
    its 7.68v....
     
  12. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
    2,807
    834
    I still want to see a schematic...
     
  13. Eric40McCowan

    New Member

    Mar 27, 2012
    1
    0
    If the source is a battery and it is old or weak, when power is requested from it, the voltage output may go lower.

    Thanks,
    mail forwarding
     
  14. TheDag

    Thread Starter Member

    Jan 1, 2010
    94
    1
    the voltage is not going lower....
    The resistors divide it into 5.44 and 2.239...
    Thats mean the battery is full!
     
  15. TheDag

    Thread Starter Member

    Jan 1, 2010
    94
    1
    Problem Solved!
    I Used LED indicator, and I think its do some trouble while measuring volts...
     
  16. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    Proper test methods means a lot. How many of us old timers have never done anything like that????

    By the way, ...... What kind of LED voltmeter do you have that will read 3.7 volts or 2.2 volts or do you mean you had an LED tied to the circuit? If so, the LED was loading the circuit to its forward voltage drop.
     
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