Full bridge rectifier

Discussion in 'General Electronics Chat' started by berms03, Sep 24, 2012.

  1. berms03

    Thread Starter New Member

    Sep 24, 2012
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    I am confused, I currently have a 24VAC supply going through a full bridge rectifier. But the output is not what it should be, I am seeing a half wave rectifier, why is this? I have used four diodes as well as an full bridge IC and the output is the same. I Would expect to see, /\/\/\/\/\..., but i am seeing /\_/\_/\_... Can someone explain this?
     
  2. t06afre

    AAC Fanatic!

    May 11, 2009
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    Given that your setup is correct. One of the diodes may be a goner.
     
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  3. berms03

    Thread Starter New Member

    Sep 24, 2012
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    I have switched out all four diodes with new diodes, and I have used multiple Full bridge ICs, and I still get the same output.
     
  4. cork_ie

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    Oct 8, 2011
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    Sounds like you are connecting your AC input between an AC input and - output terminals
     
  5. berms03

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    Sep 24, 2012
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    Don't believe so, This is my schematic, [​IMG]
     
  6. t06afre

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    Your schematics look correct. As I can see it
     
  7. berms03

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    Sep 24, 2012
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    I believe that atleast one of the diodes is blown, but how is this happening? in rush current?
     
  8. t06afre

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    Well how much current do you want to draw from this setting. And do you connect a capacitor to the bridge. Also what kind of diodes do you use
     
  9. berms03

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    Sep 24, 2012
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    I have this signal going to an opto-iso, that only should draw 10mA. I am using 1N4001. And I have used a bridge IC thats rated for 400V 1A. I have taken the opto-iso out of the circuit to look at the output of bridge and it's only a half wave and not a full wave, even with new diodes.

    There is no capacitor on this circuit, i want to know when the sine wave goes to zero.
     
    Last edited: Sep 24, 2012
  10. berms03

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    Sep 24, 2012
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    I am not planning on using this circuit as a power supply. I am using the bridge rectifier to an opto- iso, which will allow me to know when the AC crosses 0, so when i hook the other side of the circuit to a uC i can tell if the signal is 50 Hz or 60 Hz. Regardless of load, I am not seeing a full wave, only a half wave. I am not getting the negative portion of the sine wave to be positive.
     
  11. Dodgydave

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    Jun 22, 2012
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    what do you see on the AC pins?

    are you using an isolated supply for your ac, like from a transformer?
     
  12. berms03

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    Sep 24, 2012
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    On the live side I see a nice sine wave, and the neutral line is relatively straight.

    I am currently using a Variable AC (VAC) so if something is wrong it will blow the fuse in the VAC.

    if I use a scope using the earth ground, on the + after the bridge i see, /\_/\_/\_...(half wave) and on the - after the bridge i see \/-\/-\/-... (negative half wave)
     
  13. Dodgydave

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    Jun 22, 2012
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    remove the earth from the scope and try that, or have you got a battery powered scope is better, i think the scope earth is shorting the bridge out which is why you dont see a sine on the negative side.
     
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  14. berms03

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    Sep 24, 2012
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    When I measure the output of the bridge I connect the scope to the + and - side of the bridge
     
  15. Kermit2

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    The ( - ) side of your O'scope is almost certainly connected with EARTH GROUND if it has a three prong power cord.

    Lift the ground on the scope, and you'll get a proper Ac signal
     
  16. berms03

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    Sep 24, 2012
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    I isolated the earth ground from my three prong cord with a three prong to two prong adapter, so there is no long an earth ground for the scope. The Sine wave is now somewhat a square wave and I still don't get the full rectifier output. Also I plugged in the Three prong for my scope and lifted the ground from my scope and still saw the same half wave rectifier output as before.
     
    Last edited: Sep 24, 2012
  17. crutschow

    Expert

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    Post a diagram with all the connections including the VAC. (What is providing the VAC?)
     
  18. berms03

    Thread Starter New Member

    Sep 24, 2012
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    I have the VAC plugged directly to my wall outlet. I know my drawing isn't great, but should give you an idea what im trying to do. I am having an output with a frequency of 120Hz, and it should be 60 Hz. This is what lead to me look at the output of the bridge, and noticed it wasn't acting like a full bridge rectifier.
     
  19. vk6zgo

    Active Member

    Jul 21, 2012
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    With the whole thing disconnected from the mains,take a DMM on the ohms range,& check for continuity between the two DC output leads of the bridge & the AC input connections to the bridge.
    There should be no continuity.

    If you haven't somehow,got a direct connection between one of the DC outputs & the Neutral line,I would guess that you have one or more of the diodes in the bridge incorrectly oriented.


    To check this,disconnect the AC input leads to the bridge,& measure with the DMM in diode test position between each bridge AC input in turn,& the +ve DC output connection(with the black lead on that point).
    You should see a forward based diode in each position.

    Do the same with the -ve Dc output connection,this time with the meter connections reversed.
    Again,you should see two forward biased diodes.
     
  20. t06afre

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    May 11, 2009
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    Do yo use any transformer in this setting. I hope so
     
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