Switching adapter into 120v DC

Discussion in 'General Electronics Chat' started by aguywithfeet, Jun 6, 2014.

  1. aguywithfeet

    Thread Starter New Member

    Jun 6, 2014
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    I am not saying I will do this, I am trying to understand the operation.

    In a switching adapter, the 120v goes through a transformer to go down to low volt ac, then rectified, then filtered, correct? I read something that said they do not always use transformers, hire could this be?
     
  2. aguywithfeet

    Thread Starter New Member

    Jun 6, 2014
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    Along those lines, why is 120v not just rectified, filtered and us as ,120 dc. Wouldn't this allow for more itemsin series with less resistors etc?
     
  3. timwhite

    Member

    Apr 10, 2014
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    7
    You are correct in your steps of a Linear AC/DC adapter. The adapter first transforms the voltage down, then rectifies it to a pulsating DC, and finally it filters that to a steady DC. This was the original design of an AC/DC adapter. This process was pretty inefficient, as all of the excess energy was let off as heat through resistors and the like. Back in the early twentieth-century though, people started using switched-mode power supplies . However, SMPS's still utilize a transformer. Unless you're talking about a device that simply converts the 120v A/C to 120v D/C, you would have to use a transformer. I'm pretty sure that Non-isolated AC/DC Converters do not use transformers, but I can't remember if the output voltage is the same as the input or not. If it is, that would explain it.

    To answer your second question, I'll ask you another. Why do we have transformers at all?
     
  4. BobTPH

    Active Member

    Jun 5, 2013
    782
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    Because if you did that the output would be 170V.

    Most SMPS do actually rectify and filter the incoming AC. The then switch this voltage at a high frequency through a transformer primary, and rectify and filter the transformer secondary to get the DC output.

    The transformer is there for isolation, not just for voltage reduction, so your DC supply does not connect you directly to line voltage, which could be fatal.

    Bob
     
  5. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the transformer is to step down the high frequency switched dc to the output rectifiers, isolating the output. they dont isolate the input, in fact, both + and _ are above ground a lethal amoount.
     
  6. aguywithfeet

    Thread Starter New Member

    Jun 6, 2014
    13
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    I understand that many things in electronics are cheaper at lower voltages and ratings, I suppose I was asking why transform it down relating to simple loads such as led lights. However in writing this, I'm thinking led a19's probably don't transform the voltage down correct?

    Back to the adapter, why is the voltage so much higher off the rectifier? Sorry for the basic questions, I'm an electrician trying to learn about electronics. I have read a bunch recently but these were the things I don't quite grasp.
     
  7. alfacliff

    Well-Known Member

    Dec 13, 2013
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    because the filter caps charge to the peak voltage of the ac input. 120 vac gives 170 vdc. peak volts = 1.414 times the rms (average) value.
     
  8. aguywithfeet

    Thread Starter New Member

    Jun 6, 2014
    13
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    Alright, got it for now. Thanks!
     
  9. BobTPH

    Active Member

    Jun 5, 2013
    782
    114
    Because white LEDs run at about 3.6V. That said, you can put them in series and run them directly from rectified line voltage, and the cheaper LED bulbs do this. In this case, you have to insulate them well.

    Bob
     
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