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Volt Amps and Watts — Parallax Forums

Volt Amps and Watts

piguy101piguy101 Posts: 248
edited 2012-04-23 16:52 in General Discussion
I was lookin' at http://search.digikey.com/us/en/products/241-7-28/595-1062-ND/952932, it is a 28VAC, 2A CT transformer, so it is 56VA. If I were to rectify its output, I should get around, 28*1.414-1.1=38.5VDC. Can I still get 2 amps out of it when it is rectified? This would be 77W, of course a smoothing capacitor would be needed, but could I get 77W out of it?

Comments

  • Dr_AculaDr_Acula Posts: 5,484
    edited 2012-04-22 17:21
    You probably could get more amps out, but above the rated value, the heat output of the transformer goes up. Plus as you draw more current, the voltage drops so then you can't get as much power out. Probably best to stick to less than the max ratings.
  • kwinnkwinn Posts: 8,697
    edited 2012-04-22 17:38
    Sorry piguy101, there's no free lunch. If you put a large capacitor in the circuit to try to get 2A at 38V you will get very high peak currents. Since power dissipation is proportional to the current squared the power dissipation of the transformer will be higher than it would be with a resistive load that draws 2A.
  • piguy101piguy101 Posts: 248
    edited 2012-04-22 18:01
    Aww...Looks like I have to find a bigger transformer.
  • kwinnkwinn Posts: 8,697
    edited 2012-04-22 19:05
    piguy101 wrote: »
    Aww...Looks like I have to find a bigger transformer.

    Sorry bout that. For power supplies I usually use one that has a current rating at least 1.25 times more than the expected current draw.
  • davejamesdavejames Posts: 4,047
    edited 2012-04-22 20:39
    Pi - go here for info on "rollin your own" supply, section 8 in particular:

    http://www.onsemi.com/pub_link/Collateral/HB206-D.PDF

    And if you can find/buy an old copy of the National Voltage Regulator Handbook (cica 1977), there's a section that delves into the details of transformer considerations.
  • piguy101piguy101 Posts: 248
    edited 2012-04-23 12:13
    Thanks, Dave. I'm actually trying to build my first real power supply from scratch with the switching regulator, LM2678T-ADJ. (My current supply is just an LM317 with just a 1.5A output)
  • davejamesdavejames Posts: 4,047
    edited 2012-04-23 16:52
    Pi - you're welcome!

    It'd be interesting to hear about your efforts as you progress.
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