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AC to DC

  1. Dec 13, 2015 #1
    How to get pure DC from an ac whose voltage or amplitude is increasing by time until it reaches a particular fixed value and then drops to zero

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  3. Dec 13, 2015 #2


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    You can't. If the AC voltage is increasing and you wish to represent this as a DC voltage then the DC voltage will be increasing as well. This implies it is no longer a DC voltage. I would say you are after a peak detector with filtering.
  4. Dec 13, 2015 #3


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    and regardless of if the AC is varying in amplitude or not ... you are pretty much never going to get pure DC from it
    there will always be some ripple
  5. Dec 13, 2015 #4


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    Well, I have a problem with "you can't". Generally the answer should be "you can do that, but it is complicated"

    First off, let's assume you are using said AC as a power source and want a constant voltage DC output that can supply some current. (you did not explicitly state that)

    You can rectify the AC and smooth it with capacitors. That will get you to a DC that generally tracks the AC. Then, assuming the AC doesn't really go to zero, you can use DC to DC converter technology to convert the DC to a constant DC. Or, use the input to charge a battery that drives a regulator.

    The DC to DC converter architecture becomes complicated if your input DC can be either higher or lower than the output DC. If the input is always lower, you need a boost architecture. If the input is always higher, you need a buck architecture. If it is both, you can use a sepic converter. (and don't ask me how they work --- they just do)

    You said nothing about the relationship between the AC and the desired DC, what voltage ranges you are dealing with, and what power levels to need to supply.
  6. Dec 13, 2015 #5
    I guess you could use some combination of zener diodes and capacitors to smooth the peak input voltages while trying to keep the average voltage output even.
    Might need a good heat sink for those diodes though.
  7. Dec 14, 2015 #6


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    You don't say what you want the "DC" to do? Are you just trying to detect the AC burst, so the "DC" is really just a signal indicating that AC is present, or are you trying to power something with it?
  8. Dec 14, 2015 #7
    Basic rectifier and then a Buck/Boost DC to DC converter - is the simple answer, low tech - after that we need more info. When you say Pure DC, well how pure? What is the input V range and then the desired output DC?
  9. Dec 15, 2015 #8
    Guys I am really sorry for the delayed reply.... A big thanks for helping me
    Last edited: Dec 15, 2015
  10. Dec 15, 2015 #9
    I am trying to charge a battery by using that converted dc like a car battery
    I just do want to know if this is possible or not

    And one more thing what if that ac is 3 phase.

    I am just up with some basic ideas for a collage project pls do help me .
    Last edited: Dec 15, 2015
  11. Dec 15, 2015 #10


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    Battery chargers can be made to work with 3-phase AC. But it does not sound you have enough experience to do it safely. I think your best option would be to buy a battery charger designed for the AC source you have, and for the type of battery that you're charging.
  12. Dec 15, 2015 #11


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    You said nothing regarding the power levels, voltage levels, range of AC variation, etc.

    You need to specify the project before you can conclude whether it is possible, feasible, easy, hard, what approach, etc etc etc
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