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Rectifier Circuits a little help please

  1. Jan 31, 2008 #1
    Hi I'm new here. I'm trying to obtain the max power possible from a standard 120VAC 15A outlet, but the wave must be rectified so it is always positive. I always see a bridge rectifier circuit paired with a transformer, but if I'm going for max power, and not trying to achive a specific voltage, do I have to have a transformer?
    The output also doesn't have to be DC, so no filtering is required. How would you go about getting the max positive power from an AC outlet in the most inexpensive way possible?

    Thanks
     
  2. jcsd
  3. Feb 1, 2008 #2

    berkeman

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    Welcome to the PF, ronmann. First off, please be careful in working with AC mains voltages. How experienced are you in working with high voltages? What kind of safety rules and precautions are you familiar and practiced with?

    Next, the answer to your question depends a bit on the application. Honestly, the max power would be to power a heater coil. Turn all 1800W into heat.
     
  4. Feb 1, 2008 #3
    I'm an instrumentation technician, but first I had to be an electronics technician so I've been brainwashed with safety. The problem is I can troubleshoot but I can't engineer. The application is for an experiment with an electrolysis cell (seperate H and O out of water). I can change the load of the cell so I won't switch the circuit breaker. Transformers are very expensive so I was wondering if I could do it for cheap with a bridge rectifier. If not, I will use a power diode to make a half wave rectifier.
    THANKS
     
  5. Feb 1, 2008 #4

    berkeman

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    The electrolysis uses what voltage and current? How do you plan on changing the load of the cell? By varying the platinum plate areas somehow?

    To get the maximum power to the load, you would use a power factor correcting switching power supply. The configuration of the two stages in the supply would depend on the final output DC voltage that you want to have. You can use a transformer followed by a full wave bridge rectifier, but that is a fair amount less efficient because of the lower power factor (you only pull power through the system when the rectifier output voltage is higher than the load voltage (with its DC storage capacitor), so you waste the parts of the AC cycles that are not at the peak voltages.

    Here is a good article on power factor correction that I was reading recently for something else:

    initial wikipedia.org article: http://en.wikipedia.org/wiki/Power_factor_correction

    which referenced this ON Semi paper: http://www.onsemi.com/pub/Collateral/HBD853-D.PDF

    (check out page 7 of the ON Semi paper to see the issue with the current flow in a bridge rectifier circuit)
     
  6. Feb 1, 2008 #5
    The electrolysis uses any voltage and any current-but the more the better. There are no electronic components in this electrolysis cell. The load can be varied by changing the distance between the plates. I have a switching ps but they are still expensive for the amount of power. Thanks for the links but I know about power factor correction. I guess a half wave rectifier is my best bet.
    THANKS for your help.
     
  7. Feb 1, 2008 #6

    berkeman

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    A full wave rectifier will give you more power throughput. Is there a reason that you want to only use half-wave?
     
  8. Feb 1, 2008 #7
    Thats what I'm trying to figure out. I know a half wave rectifier does not require a transformer. I've only seen schematics of full wave rectifiers paired with transformers so I'm not sure if you can use a full wave rectifier without a transformer. If you can, then I would certainly use a full wave rectifier.
     
  9. Feb 1, 2008 #8

    berkeman

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    AFAIK, the rectifier issue is independent of the transformer. You use a transformer to step down the AC voltage if the application requires it, and the transformer supplies safety isolation (assuming the transformer is approved as an AC Mains safety barrier) for ground faults.

    The input to the transformer needs to be AC of course, so the full wave bridge goes after the transformer. Whatever storage/smoothing capacitor you put on the output of the bridge rectifier needs to be rated for the peak output voltage, and also the ripple currents that it will see.
     
  10. Feb 1, 2008 #9

    berkeman

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    BTW, in building this apparatus, you really need to consider safety issues carefully. You say that you've had a lot of exposure to safety training, so keep in mind that you need to physically isolate all conductive things that are running above 42V (in the US or 60V in Europe) so that they cannot be touched by a person. This means planning your enclosures appropriately, and providing enough creepage and clearance distances in your apparatus so as to prevent anybody being able to accidentally get zapped. You should also be sure to provide a shutoff switch (in the hot, right?), and a breaker or fuse (also in the hot). You should not be relying on the breaker panel to provide that protection.
     
  11. Feb 1, 2008 #10
    Yeah safety is definatly an issue here especially because I'm dealing with water. I plan to use proper enclosures. There will be a switch and fuse aswell. I will use a bridge rectifier with some large power diodes.

    Thanks for all your help.
     
  12. Feb 1, 2008 #11

    berkeman

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    Your welcome. BTW, be sure to use a GFCI panel breaker, and maybe even make your own breaker GFCI as well.
     
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