variac + rectifier + caps = dc power supply

Discussion in 'The Projects Forum' started by pastinsain, Aug 26, 2012.

  1. pastinsain

    pastinsain Thread Starter Member

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    Here is what I got

    10amp 120 variac + 35amp 800v full wave rectifier + 450v 3500uf caps

    Will this work as a 0-120v dc power supply?

    will it provide a clean dc current?

    if not , what else is needed in addition to the above.?

    thank you for your opinion
  2. ifixit

    ifixit Well-Known Member

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    If it is going to be used as a lab supply you will need to add an isolation transformer to make it safe to use. I've had a variac supply for AC on my bench for decades. It has a 500VA isolation transformer feeding it. I likely owe my live to that isolation transformer many times over.:eek:

    Discussions of non-isolated power supplies are not allowed on this forum.

    Regards,
    Ifixit
  3. pastinsain

    pastinsain Thread Starter Member

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    wonderful ! so I need an additional isolation transformer?

    can you please recommend a forum that will discuss this circuit? "mabe a guitar amp forum"

    This forum has an excellent topic on "isolation transformers in the ac section" I just read that.

    I have repaired vintage 460v tube guitar amps for the past 10 years and are no stanger to high volt circiuts
    however, I have never used an isolation transformers before .

    I thank you for the heads up info! it could save my life next time I build a guitar amp.
    Last edited: Aug 26, 2012
  4. ifixit

    ifixit Well-Known Member

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    You will need a fuse and then a switch on the input to the isolation transformer. A fuse on the output is wise as well. Make this a fuse that is easy to change from the front panel. The whole supply needs to be enclosed in a metal box with the variac shaft sticking through the front panel. You must have an indicator light to show when the power is on.

    Since the variac doesn't have a center-tap (ct) you will need a bridge rectifier to get full wave rectification. Two ct full-wave rectifiers would do if wired as a bridge.

    With 3500uF of filtering the DC output ripple will be dependant on how much current you intend to load it with.

    What are your load requirements?
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  5. pastinsain

    pastinsain Thread Starter Member

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    My variac is a 10 amp stacor so 10amp should do.

    I just found my 120v trip lite lc 1200 regulator line conditioner.
    its got a 10 amp reset breaker. will this work as a " isolated transformer"
    it says isolated filter banks.

    maybe I could plug the stacor variac into the trip lite conditioner and
    have my cake and eat it to!
  6. ifixit

    ifixit Well-Known Member

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    I don't think the tripp lite lc 1200 has isolation.

    Regards,
    Ifixit
  7. crutschow

    crutschow AAC Fanatic!

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    If you insist on working on unisolated equipment, make sure you plug it into an outlet with a properly connected ground-fault interrupter. That should save you from getting a lethal shock between the power and earth ground (but you will still receive a good zap, as I've personally experienced after tripping a GFI by accidentally touching my finger to the hot prong of a main's plug).
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  8. pastinsain

    pastinsain Thread Starter Member

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    I think i'm ready now . sooo

    the setup is the following:

    Isolation transformer with switch ,on light and fuse. then comes the variac
    with fuse , next full wave 35 amp 800v rectifier, then 2-450v 3500 capacitors total 7000uf.

    this should make a 0-120v dc up to 10 amp.

    a great all purpose ac-dc power supply

    thank you all for the help. any more suggestions are welcomed!
  9. R!f@@

    R!f@@ AAC Fanatic!

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    Hmm !

    Not bad. But you missed ballast resistors.
  10. ifixit

    ifixit Well-Known Member

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    Connect as shown in the schematic.




    Notes:
    1. F1 fuse, 10 Amp slow-blow or time delay type
    2. S1 switch 15A AC rating
    3. T1 isolation transformer, at least 1200VA rating
    4. T2 Variac, 10 Amp rating, however, 8 Amp continuious, 10 Amp short time only
    5. All parts above must be approved by your local electrical authority. IE: CSA, UL, ETA, etc.
    6. The bridge rectifier diodes, 400V, 15 Amp, (higher rating is better) heatsink may be required for heavy loads.
    7. R1 is the ballast resister that R!@ff suggested. It will limit the inrush current to the 7000uF if the unit should be powered up with the variac set to maximum... try to avoid that. I'm not sure if this is required since there may be enough resistance in the circuit to do the job. What is the surge rating for the diodes and the caps?
    8. Suggestion: Make F2 a cartridge fuse that can be easily changed from the front panel. F2 can be any value that approximately 50% more than the worst case expected DC load, upto 10 Amp maximum. IE if you are powering a circuit that is not going to need more than 0.8 Amp, then use a 1 Amp fuse for F2. That way if there is a problem (short) then the minimum amount of damage will be done
    Anything else?

    Good Luck,
    Ifixit

    Attached Files:

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  11. shortbus

    shortbus Senior Member

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    A "bleeder" resistor across the filter caps would also be a good idea for your safety. A 15k 1W is a fair size to use. Prevents a surprise shock from an undischarged cap.
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  12. pastinsain

    pastinsain Thread Starter Member

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    the rectifier datasheet
    GBPC3508W-G - Glass Passivated Bridge Rectifier
    http://pdf1.alldatasheet.com/datasheet-pdf/view/456082/COMCHIP/GBPC3508W-G.html

    the caps are huge 8" X 3" canister polar typeBCH 3500uf 450v
    can't find the datasheet

    great idea to use the inrush resistor .

    this circuit is very similar to a typical tube amp power supply.

    the isolation transformer is unit put together by an electronics instructor and used for bare chassis radio repair
    don't have any specs on it however.
    Last edited: Aug 27, 2012
  13. ifixit

    ifixit Well-Known Member

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    The caps and diodes sound okay for the job. Can you post a picture of the transformer?

    Have Fun,
    Ifixit
    pastinsain likes this.
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