Rectifier circuit

Discussion in 'General Electronics Chat' started by sumitest, Mar 14, 2016.

  1. sumitest

    Thread Starter New Member

    Mar 14, 2016
    4
    0
    hello everyone,
    I have made full wave bridge rectifier circuit using IN4007 diodes. As per the theory we all know if my input voltage is below the threshold of the diode it will not conduct but in my case I'm using signal from function generator if I give 4V rectifier is working very well but it is also conducting when supply is 1V only. I don't the reason pls help me out from this problem.



    Mods Edit:
    Please don't do double post again, now they were merged to one thread.
     
  2. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,542
    1,251
    Look at the 4007 datasheet for the value of Vf, the forward conduction voltage. there probably is a range of values and it will say what the test current was. there also might be a plot of the forward voltage versus current. From all of this, you should be able to see what the minimum Vf is, and that it is less than 1 V.

    ak
     
  3. recklessrog

    Member

    May 23, 2013
    338
    102
    How are you measuring the voltage? if you use an RMS meter, remember that the peak voltage is 1.414 times the RMS so your diodes will just be conducting on the peaks.
     
  4. sumitest

    Thread Starter New Member

    Mar 14, 2016
    4
    0
    i'm measuring using oscilloscope
     
  5. sumitest

    Thread Starter New Member

    Mar 14, 2016
    4
    0
    The forward voltage drop is 1v and since I'm using it in full wave bridge rectifier configuration so it might be around 2V but still when supplying voltage is 1 v diodes are conducting
     
  6. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
    1,147
    204
    If you don't draw any appreciable current (e.g. just a meter), wierd things happen.
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,140
    3,054
    I agree. Add a 1K resistor as a load and see what happens.
     
  8. crutschow

    Expert

    Mar 14, 2008
    13,023
    3,236
    The forward drop of a junction diode is not fixed but is a logarithmic function of current.
    Thus at low currents the forward drop is much less than a volt.
    Here is the typical characteristic of the 1N4001-1N4007 diodes:
    The curve deviates from the logarithmic curve above 1A due to the intrinsic resistance of the diode.

    upload_2016-3-14_8-56-13.png
     
  9. wayneh

    Expert

    Sep 9, 2010
    12,140
    3,054
    I assume he's seeing TWO diode drops in his measurement, i.e. 0.5V each, which is not terribly surprising with µA current.
     
  10. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    Everyone else has pretty much covered it, I would add that a function generator can be cranked up far too fast for standard recovery diodes like the 4007.

    Anything above a few kHz and you might do better with the UF400x series.
     
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