What kind of transformer do we use for rectifiers

Discussion in 'General Electronics Chat' started by Mohamed87, Apr 15, 2015.

  1. Mohamed87

    Thread Starter New Member

    May 1, 2010
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    Hi,

    Does anyone know what kind of transformer do we use in rectifiers ? Is it a power transformer ?

    Thanks !
     
  2. cornishlad

    Member

    Jul 31, 2013
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    If you mean "what kind of transformers do we use WITH rectifiers" then you use a POWER transformer if the context is a POWER supply.
    But you need to give more information to get answers.
     
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  3. Mohamed87

    Thread Starter New Member

    May 1, 2010
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    Hi
    You are right !
    I want to rectify the alternative voltage to get only positive half-cycles which I will apply on an Opto isolator via a resistor. So, I need to reduce the amplitude of grid voltage (using a transformer) and rectify it (using a diode bridge).
    So, I'm wondering what kind of transformer should I use for this ! I hope it's clearer now.
     
  4. cornishlad

    Member

    Jul 31, 2013
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    It will be the smallest of mains (grid) transformers that you can buy..possibly 3VA and output over 4 volts.. In practice any mains transformer with any output up to say 15v. (adjust the resistor) You are only needing 50ma absolute max so personally I would not even consider making one, but would use an old phone charger which usually give 5v stabilised. In the UK 50p from a charity shop if you don't already have one lying about.
     
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  5. RRITESH KAKKAR

    Senior Member

    Jun 29, 2010
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    In rectifier we simply use iron core copper wire layer on it etc etc
    advance have high freq fer rite core.
     
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  6. cornishlad

    Member

    Jul 31, 2013
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    A step down mains transformer (50 or 60Hz) could have a laminated soft iron core with many turns on the primary and fewer on the secondary. they are also built using toroidal ferrite cores.
    Depending on the frequency, high frequency transformers may have ferrite or air cores. The ferrite will have properties appropriate to the design frequency. If you last post was a question, does that help ?
     
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  7. Mohamed87

    Thread Starter New Member

    May 1, 2010
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    Thanks for your answers,
    Actually, I didn't intend to make a transformer, just wanted to know how to choose one, thanks anyway for the details you gave me.
    I found a 8 VA transformer of output 15 V, considering the limit of 50 mA cornishlad talked about, I suppose this should do it for current limit value.

    Thanks again :)
     
  8. cornishlad

    Member

    Jul 31, 2013
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    In practice I believe an opto-isolator will work ok at as little as 10ma. 50ma is an absolute max for the ones I've come across. With an analogue power supply such as we have been discussing (now with 15vac tfmr) , if you started initially with 10Ma current, expect the power supply output to be nearer 20 volts DC. - depending on value of the filter capacitor you use. I would suggest a 1.8K series resistor to start with and reduce as necessary. A lot depends on what else the circuit may supply. Use of a three terminal regulator chip to 12 or 5 volts may be a good idea.
     
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  9. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    If you only want the positive half cycles, then you don't need a bridge rectifier.
    If you want DC output, then use a bridge rectifier and a filter capacitor.
    The first image shows the difference between half-wave and full-wave rectification.

    half-and-full-wave-rectification.jpg

    The second image show half-wave rectification with a filter capacitor.

    plus-9v1.gif

    And this image is full-wave rectification with a filter capacitor.

    CMWmx.gif
     
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  10. Mohamed87

    Thread Starter New Member

    May 1, 2010
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    Thanks,
    I actually wants a full-wave rectification, that's why I used a bridge.
     
  11. gerty

    AAC Fanatic!

    Aug 30, 2007
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    Here, maybe this will help you make some decisions..
     
  12. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    I can buy used AC-output Wall-Wart plug-in transformers (not SMPS) at my local thrift store for next to nothing. Typically, these are rated at 6Vac to 24Vac at a few VA,
    Even some of the ones rated as DC output contain a transformer and full-wave bridge rectifier with no filter capacitor, so put out the full-wave rectified wave...
     
  13. crutschow

    Expert

    Mar 14, 2008
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    If you have a transformer than you don't need an opto isolator as the transformer already provides isolation.
     
  14. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Just as a comment on to the title:
    There was a power transformer once used in car radio's that transformed 12VDC to 200VDC without the use of rectifiers.
    Google synchronous vibrator if more info needed.
    Max.
     
  15. Reloadron

    Active Member

    Jan 15, 2015
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    Many fond memories of those early 6 volt and 12 volt car radios which used vacuum tubes or valves depending on which side of the pond you are on. However Max, they used the vibrator to convert DC to an square wave which drove a transformer to step the voltage up for the plate supply. That stepped up voltage was then converted to DC and filtered as in the schematic shown below.

    Car Radio Vibrator.png
    The OZ4 tube in this case rectifies the high voltage. Just about all of the early car radios I am familiar with used this scheme. The Vibrator converted DC to AC, the transformer stepped up the AC and there was rectification to convert the high voltage AC back to DC for the tube plate voltages.

    Ron
     
  16. MaxHeadRoom

    Expert

    Jul 18, 2013
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    That was the non-synchronous vibrator, I was referring to the synchronous version that switch the primary and secondary at the same time, NO rectifier.
    Max.
     
  17. Reloadron

    Active Member

    Jan 15, 2015
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    The synchronous vibrator (about 1/2 way down the page). :)

    Really another pretty slick design from long ago. I always saw it as what they mention, a mechanical type of rectifier. :)

    Ron
     
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