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Convert ac to dc

Discussion in 'Electronic Basics' started by Victor MacQuarrie, Apr 4, 2007.

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  1. Hello, I have a power transformer that converts 110 v ac to 12 .6 v
    ac. What is easiest way to change 12.6 v ac to 12.6 v dc to run a 12 v
    fish pond pump continuously. Thx..........Vic
  2. John Fields

    John Fields Guest

    View in Courier:

    120AC>---+ +---|~ +|-----+
    P||S | | |+
    R||E | | [PUMP]
    I||C | | |
    120AC>---+ +---|~ -|-----+
  3. Jamie

    Jamie Guest

    get your self a bridge rectifier from radio shaft.
  4. redbelly

    redbelly Guest

    A rectifier will convert 12.6 Vac to 17.8 Vdc. A 12.6 Vrms has an
    amplitude of 17.8 V (multiply the rms voltage by the square root of

    You'll need to drop the 17.8 V down to 12 V. Either a 12V voltage
    regulator which can handle the pump current, or even a power resistor
    in your application, can be used.

  5. John Fields

    John Fields Guest

  6. Phil Allison

    Phil Allison Guest

    ** Groper fool Alert

    ** No it will not ~!~!!~!~

    The average DC output voltage of a perfect ( ie no diode drop) bridge
    rectifier is 0.90 times the rms input voltage.

    Look it up.

    ........ Phil
  7. Phil Allison

    Phil Allison Guest

    "John Fields"

    ** Not with a motor as the load.

    A rectified sine wave has a lower average value ( by 0.90) than its rms

    ........ Phil
  8. redbelly

    redbelly Guest

    Yeah, I was assuming a cap in there. Phil's right though, for a full
    wave bridge the average voltage is 90% of the rms value (ignoring
    diode drops). Assuming a 0.6V drop, Vic will get
    0.9*12.6-0.6 = 10.7 V
    for the average voltage.

    Chances are that's enough to turn the motor. I don't know what the
    effect of the 17V peak voltage would be, but I'm not a motor expert.

  9. jasen

    jasen Guest

    a bridge rectifier will get you 11.4 (-ish) ... close enough ?

  10. I will try the rectifier, and let you know......

    Thanks again.........Vic
  11. Ben Gustave

    Ben Gustave Guest

    The bridge rectifier will drop, say, about 1.2 volts (assuming a crappy
    silicon rectifier) off the peak ac voltage. He'll have about 16.6 volts at
    the peak of the ac cycle, which is where any filter cap he throws on there
    will charge to. Now, depending on the pump and the size of the filter cap
    he could have anywhere from your 11.4-ish up to about 14 volts, give or

    I don't think that unfiltered 120Hz dc would be any good for the pump,
    Victor, so a filter cap is a really good idea. Radioshacks tend to carry
    4700uF electrolytics for a few bucks a pair; they'd be a good fit.

  12. Hi guys.......This transformer I have has 5 leads, 2 black on the
    primary side (110 v in ) and 2 yellow and one black on the sec. side.
    The black I assume is the common, and 6.3 v at 3 A on each of the
    yellow wires.How do I get 12.6 volts??? I connect the 2 yellow and I
    get a short??? I guess this sounds silly, but I'm afraid I'm not too
    swift on electronics HELP!!! ( There was no paperwork with the
  13. wlb

    wlb Guest

    You'll not be using that black wire on the secondary side, just tape
    it off and tuck it away. Each yellow wire will go to one of the AC
    leads of your bridge rectifier and then your filter capacitor will
    hook across the + and - leads of your bridge rectifier. Take care to
    observe polarity! The capacitor - lead must go to the bridge - and the
    cap + lead to the bridge +. If you get it backwards the cap will let
    you know with a big bang. Then you hook your pump up across the cap
    and you should be in business. Good luck!

  14. Hi again guys........ Thanks for the info on the transformer Ben . It
    seems to work fine. Only problem is,..... I could only get a 3300uf 25
    volt capacitor. The tech told me it should work, only problem now is I
    have an output of 18.89 volts dc. which I think is probably not good
    for the pump. Pump is designed to run from 12 v to 16 v dc.
    Would a 4700uf 12v cap. solve the problem???

    Thanks again .......Vic
  15. Sorry, wlb in my last reply I gave credit to Ben for the info.
    Thanks again to all of you

  16. Randy Day

    Randy Day Guest

    Victor MacQuarrie wrote:

    Only if your problem is how to make something go 'bang'.

    A capacitor can only work up to a maximum safe voltage,
    after which it goes, well, bang.

    Your 3300uf should be adequate for filtering. Stepping
    the voltage down is another issue, and to suggest a
    circuit, we'd need know what current the pump needs.
  17. Try it with no capacitor.
  18. ehsjr

    ehsjr Guest

    No. Your 3300 uF cap is fine. You can drop the
    voltage down with some diodes in series between the
    pump and the supply. Use diodes with a current
    rating equal to or higher than the current rating
    of your rectifier. Each diode you add will drop
    roughly .7 volts (depends on how much current the
    pump draws).

    Start with 2 diodes in series, and measure the voltage
    across the pump while it is running. Add diodes as
    necessary to get to roughly 14 volts.

  19. jasen

    jasen Guest

    Was this measurement taken with the pump operating?
    because if it wasn't it's not telling you anything useful.

    If you want less voltage, using a smaller cap may do it.

    A little, or even a large amount of, ripple is unlikely to damage a motor.
    This sort of equipment is designed to withstand vibrations: a thyristor
    speed control gives worse ripple than a full-wave rectifier.


  20. Hello John......Disconnected the cap. and now I have a steady 13.3
    volts. Hopfully the problem is solved.... I'll let you know.

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