Transformer/Heatsink question

Discussion in 'General Electronics Chat' started by davidGG, Apr 22, 2013.

  1. davidGG

    davidGG Thread Starter Member

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    Hello everybody,
    I have a transformer rated at 36VCT 0.8A. If I use a rectifying diode and a voltage regulator (LM 317T) to lower the voltage to 5VDC to charge my kindle fire rated at 5V 1.8A, will the transformer be able to handle this current?
    Also, the rectified voltage is 10VC which means I will be dropping 5V, so the total power dissipated will be 9W! Will a standard heat sink suffice or should I design this project differently?

    Thank you for the help

    Transformer link: http://www.allelectronics.com/make-a-store/item/TX-36/36VCT-0.8A-TRANSFORMER/1.html

    Heat sink: similar to
    http://www.ebay.com/itm/2-Pcs-TO-22...866?pt=LH_DefaultDomain_0&hash=item337cba997a


    edit: yes I know I can just buy a wal-wart and simplify everything but I'm trying to use the items I currently possess before buying anything new.
  2. gerty

    gerty Well-Known Member

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  3. davidGG

    davidGG Thread Starter Member

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    Thank You.
    Just to clear things up for the future, if the transformer is rated at 36V 0.8A, I can NOT draw 1A at 5V? Meaning 0.8A is the absolute maximum I can draw (without dieing)?
  4. gerty

    gerty Well-Known Member

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    That's correct .8 amps is max. And to regulate the voltage down to charge a 5 v battery, you'll have to drop about 40 volts across the regulator, which is way too much.
  5. crutschow

    crutschow AAC Fanatic!

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    1.8A @ 5V from a 36V 0.8A source can be done using a switching buck regulator. The practical maximum including the switcher efficiency and proper transformer derating would give about 3A @ 5V.
  6. gerty

    gerty Well-Known Member

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    I was trying to keep within the posters specs,,,
  7. Potato Pudding

    Potato Pudding Active Member

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    Using a switcher as either a preregulator or for all of the regulation will help you get your voltage down AND your current up.

    That is efficient.

    Basically you can drop a switched pulse of 1.8A current across an Inductor, down from 50 to 5V.
    An input filter capacitor will smooth out that 1.8A to less than 0.8A continuous from the transformer. In fact that input current will likely average around 200milliamps.

    The magnetic field in the inductor will have enough power at a low duty cycle to provide the 1.8amp current during the off switched cycle.

    RB has a two transistor switcher design on his website.
  8. #12

    #12 AAC Fanatic!

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    With what you have, the bottom left drawing in the Hammond document is the best way to go analog. You will get .8A DC with about 24V on the first filter capacitor. That's 3 times the voltage you need, demonstrating that this transformer is really not right for your needs.
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