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Variable AC (0 to 240) source how to take fixed DC (110 or 220)

  1. Dec 23, 2014 #1

    i am looking for fixed DC (110VDC and 220VDC) from my variable AC source (0 to 220V).
    please support me.

  2. jcsd
  3. Dec 23, 2014 #2


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    Buy separate fixed DC output sources.
  4. Dec 23, 2014 #3


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    How much power? How about some real specifications, and some ideas you might have had. What are you really trying to do. Some context makes it easier to come up with ideas.

    At first glance, it seems difficult to get 110VDC from 0V AC, but I might be missing something.

    Why do you expect us to do more work than you are willing to do. I already spent more time on it than you spent posting your question.
  5. Dec 24, 2014 #4

    thank you for your support on my topic,
    equipment input supply 240V,50Hz,1ph, output 0 to 220VAC is variable dimmer source available, using this source i want produce fixed type at out put section 110,220VDC &3 to 5Amp.
    here we have used rectifier bridge for make convert of AC to DC source,
    i looking here need 110V and 220V dc it is for our safety to test the components those DC levels only.
    (while i testing time i struggle to set the desire voltage, some times this variable nob get increase or decrees is happens, due to i facing failure our components at testing time)
    i expecting any circuit is available for desire voltages select?
    if available please help me.

  6. Dec 24, 2014 #5
    I've seen people put an isolation transformer after a variac and simply rectify and filter. Be sure to use fuses that are rated for HV DC. Common AC fuses arc badly when attempting to break DC.
  7. Dec 24, 2014 #6


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    Actually, Bystander nailed it. Buy two DC power supplies and switch between them. Are you trying to do something that won't provide?
  8. Dec 24, 2014 #7
    The main reason they use isolation transformer is probably to eliminate/reduce potential problems with DC premagnetization current.
  9. Dec 24, 2014 #8
    The isolation transformer is for safety. Given that an AC neutral ties back to ground, There's no means of attaching the rectified + or - DC to ground without either fireworks, or an isolation transformer. Folks who design offline power equipment regularly keep the transformer / variac combination upstream. I used to keep a 10 kW pair upstream with a breaker bar on the variac because my wrists and forearms weren't up to turning the dial.
  10. Dec 25, 2014 #9
    Function is to "serve" and to "protect". Protection role/safety increase of isolation transforners is obvious, as already explained here. I was referring to their advantage in service with rectifier circuit topologies.
  11. Dec 25, 2014 #10

    jim hardy

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    Is that what you are using? or are you asking if one is available?

    A Variac, also known as "Adjustable transformer" or "slider" , will work well for you with a rectifier
    but an everyday household lamp dimmer will not..

    I agree with others - you should buy an isolated power supply or use an isolation transformer.
    Variac doesn't isolate the neutral wire and folks get into trouble using one with a bridge . In a US system, earthing either side of the DC bypasses half the bridge.. making sparks and smoke.
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