Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Capacitor/Transformer Voltage Rating for a 12V Power Supply?

  1. Jul 25, 2010 #1

    About me...so you know where I am coming from. If you don't want to read, skip to the end-ish...

    I'd like to start off my saying that I know next to nothing about electronics although I am eagerly reading the All New Electronics Self-Teaching Guide right now and it is confusing but I am slowly understanding it.

    There's a lot of equations in that book. Perhaps some of you may have heard of it, read it. But, I am going to attempt to do a very small project and that is building a simple 12V power supply for...anything that might require 12V.

    So I am searching over at Mouser.com, the site which I have had the best luck with and I am looking for some nice capacitors. I am an avid PC hardware enthusiast trying to get into electronics and nothing draws my eye quite like aluminum organic polymer caps. These caps are pretty much worshiped on the PC spectrum. (AKA solid state caps).

    But, don't really know squat-diddly about electronics. What kind of voltage rating do I need to use on a 12V supply sipping power straight from the wall. No batteries or anything.

    Solid state caps only go up to about 50V or so I have seen but standard electrolytics can go up to 900-1000v.

    What the heck do I need? I have some schematics, just trying to determine the parts. This is the schematic: http://www.instructables.com/files/orig/F6Z/DVT6/FKVQJ2RJ/F6ZDVT6FKVQJ2RJ.jpg

    Also, and I am sorry if I am asking too much, the primary/input rating on a transformer should be like equal to the wall? And the output maybe around 35V?

    Last edited: Jul 25, 2010
  2. jcsd
  3. Jul 25, 2010 #2
    you are using 7812, which will give 12V output. Input side voltage should be 15-18V. so you need capacitor <50V ratings.
  4. Jul 25, 2010 #3
    http://www.mouser.com/ProductDetail/Panasonic-Electronic-Components/EEU-FC1E472/?qs=sGAEpiMZZMtZ1n0r9vR22SCZt3APDM9SqIKsF5pj5H4%3d [Broken]

    Last edited by a moderator: May 4, 2017
  5. Jul 25, 2010 #4
    Primary voltage on the transformer can be up to 35V?
  6. Jul 25, 2010 #5
    ^ you mean secondary voltage of transformer (i.e. voltage input to rectifier circuit) ? as primary voltage on the transformer is 240V as per circuit diagram.

    why to use 240/35 V transformer? it would create problems of low efficiency, over heating, high temperature. 240/15 (or 240/18V) is sufficient.
  7. Jul 26, 2010 #6
    Thank you very much aks786. I read about 50 pages on transformers yesterday before I crashed for the night, in hopes of trying to make a little bit of sense.

    Here's the one thing I am not getting. I understand how transformers work but on the schematic it says 240V. I live in the US and is not a standard wall outlet only 100-120V? Why does it say 240V then? Am I missing something.

    Also, regarding power rating. VA = Volt Ampere. Is there a reason why they put VA instead of Watt? This is the one I am considering: http://www.mouser.com/ProductDetail/Triad-Magnetics/F372P/?qs=sGAEpiMZZMv4oUrzpPKU3Ka3fUoV036URXm0H0/Wt4U%3d [Broken]

    Thank you very much for your help.
    Last edited by a moderator: May 4, 2017
  8. Jul 26, 2010 #7
    240V is also standard in some countries like in india. (higher distribution voltage is dangerous but economical)

    like all other electrical machines, transformers rating is limited by temperature rise, which depends on losses. in transformer there are mainly two type of losses

    1. copper losses - depends on current through windings and resistance of winding (I²R). keeping resistance constant this part is restricted by current.

    2. iron losses - depends on core (size, material, lamination thickness), flux density and power frequency. here physical properties of core and power frequency is constant, so iron losses will depend on flux density. which will depend on voltage applied.

    in this way transformer heat generated will depend on voltage applied on transformer and current flowing through windings, it has nothing to do with power factor. so you will find transformer ratings such as 300VA, 315KVA, 100 MVA ....
  9. Jul 26, 2010 #8
    Thanks you have very clear descriptions.
  10. Jul 27, 2010 #9


    User Avatar
    Science Advisor

  11. Jul 27, 2010 #10
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook