PSU trouble...please help

Discussion in 'The Projects Forum' started by magicmatt97, Sep 10, 2009.

  1. magicmatt97

    Thread Starter New Member

    Sep 10, 2009
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    Hey everybody,
    I'm new here, and was wondering if anyone could help me with a circuit diagram for a psu.

    I've attached the diagram, and i cant work out what the two transformers should be.

    Any help would be great

    Thanks in advance

    Matt
     
    • psu.jpg
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  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    T1 needs to be 12 VAC on the secondary, able to supply over 1/2 amp. T2's output is not regulated. The secondary should be around 210 - 220 VAC to get 300 VDC on the filters.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    Unless you are planning on building tube-based (valve) amplifiers, the 300 volt portion won't be of much use, will add considerably to the expense, and will represent a constant shock hazard. I strongly advise newcomers to electronics to avoid circuits that exceed 50v for their own safety.

    Such transformers used to be easy and cheap to get; there was one such transformer in every tube-type television. Nowadays, only high-end audiophile and professional musician's gear use tubes/valves; consequently the transformers are very pricey.
     
  4. magicmatt97

    Thread Starter New Member

    Sep 10, 2009
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    It is for a tube based pedal, the 300v is needed for the audio signal chain, i've built a load of guitar pedals before, but i've never come across the need for transformers like this.
    would something like this be suitable?

    would it be possible to just use to 300v secondary output? How?

    would it be possible to get separate 16, and 300v psu's to user with the two circuits independently

    Many thanks

    Matthew
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Well, yes. Looks like it has dual primaries, so it could be wired for either 115v or 230v.

    However, when you rectify and filter the 300vac output, you'll wind up with unregulated 425V DC. There just aren't many solid-state regulators that go that high.
    I suppose you could - but it seems to have both the 300v and center-tapped 6.3v-0v-6.3v outputs. Why would you not want to use them? Once rectified and filtered, you'd wind up with around 16v DC unregulated.

    I suppose you could. It's finding them at a decent price that's the problem. £52.00 doesn't seem outrageous.
     
  6. CDRIVE

    Senior Member

    Jul 1, 2008
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    Kind of low grade but considering how dificult it is to get transformers that were once very common... it's doable.
    http://www.mpja.com/prodinfo.asp?number=7840+TR

    Using two of these you can convert 120V to 12V. You then connect the 12V output to the 12V terminals on the second transformer. The second transformer will have 120v and 240V outputs. you need only 20mA.

    Yeah, I know it's kinda goofy.. but hey! Think of it as....

    Field Expedience! Old military term.

    Sort of MacGuyveresce. :D
     
    Last edited: Sep 10, 2009
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Great idea, Cdrive! I like it :)
     
  8. CDRIVE

    Senior Member

    Jul 1, 2008
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    Hey Sarg, I haven't done the math on it but magicmatt97 should be able to also supply the low voltage regulator off the secondary of the first xfmr, the one connected to the line. Actually, those are 4A xfmrs. I don't think there's any math to do! ;)
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    I realized dat! It's an ingenious no-brainer! :D
    Far cheaper, and he gets just what he needs!
     
  10. magicmatt97

    Thread Starter New Member

    Sep 10, 2009
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    This sounds excellent, although im being a bit slow this morning and am struggling to get my head round it fully.
    Would you be able to sketch out the chain from the 240v ac mains to the beginning of the rectifier stages

    The only time i have ever come across transformers is my school physics lessons, and it was all about magnetic flux, which doesn't really help :D

    Also im in the uk, so would mouser do a similar transformer?

    Thanks so much guys

    Matthew
     
  11. t06afre

    AAC Fanatic!

    May 11, 2009
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    If I understand the suggestion correct, you will be using two transformers. using the first one to transform the mains to 12 volt AC. Then using a second one to transform 12 volt AC(from the first one) up to 220 volt for the signal chain.
     
  12. CDRIVE

    Senior Member

    Jul 1, 2008
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    Yes, like this.
     
  13. magicmatt97

    Thread Starter New Member

    Sep 10, 2009
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    Ahhh right ok, looks, good.

    Here mains voltage is 240vac, so would the same sort of thing work with the 12v one of these, so 240v into one side, tap one 12v secondary for the low voltage bit, and the other 12v secondary to go through another identical transformer to step it back up to 240v.

    would this work out amperage wise?

    Thanks

    Matthew
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    No, those sub-mini transformers are only capable of 100mA current. You need 20x to 40x that much.

    On this page:
    http://www.maplin.co.uk/Module.aspx?moduleno=46757
    There is a listing for #N05CF, 0-6V, 0-6V @ 1.67A which would probably be adequate, if they only had an option to buy it.
    They do have an N23CF, but they are £21.79 each; that's $36.40 USD; rather spendy.
    You really need something in the 24VA to 48VA range.
    They do have some toroidal mains transformers, but they're about as spendy as the N23CF.

    Digikey has these: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=595-1076-ND
    12.6v, 2.5A CT secondary, 115/230 primary, $13/ea. I don't know if they're in the UK.
    Took a look on Farnell UK's site, but didn't see anything promising.
    Sorry, MPJA won't ship outside the USA or Canada.
     
  15. magicmatt97

    Thread Starter New Member

    Sep 10, 2009
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    OK i thought it was looking too good to be true :D

    So i'm after a xfmr with 240vac primary with 2 12v secondaries with between 24VA and 48VA?

    Here there is both 20va and 50va (88-3432 and 88-3440 respectively) would these work? which would be best the 50? theres also a 25va one here ( 88-3720 )

    Aside from finding the right transformer would the idea work?

    Thanks again

    Matthew
     
  16. SgtWookie

    Expert

    Jul 17, 2007
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    Most good things usually are. :)

    No, 230VAC or 115/230VAC primary and either two 6v or a 6v-0v-6v (center-tapped 12v) secondary, somewhere in the range of 24VA to 48VA; that's roughly 2A to 4A output.

    Chassis Mains Transformer 25VA 0-6V 0-6V, 88-3718 is what you'd want.
     
  17. CDRIVE

    Senior Member

    Jul 1, 2008
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    If you're in an area that uses 240V mains isn't your frequency 50Hz?
     
  18. magicmatt97

    Thread Starter New Member

    Sep 10, 2009
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    ok so mains into 230vac primary then wire the two transformers as per cdrive's diagram, tapping off 12v and having the second transformer's out as 240v total (as its not tapped) then run the 12v out and 230v out through the rectifier stages etc that i posted to begin with.
    does that sound good?

    also the fuse f1, i should be slow blow but what rating should it be?

    Thanks
    Matthew
     
  19. magicmatt97

    Thread Starter New Member

    Sep 10, 2009
    7
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    yes frequency is 50hz
     
  20. CDRIVE

    Senior Member

    Jul 1, 2008
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    The schematic that I drew pertained to the transformers that I provided a link to. If you can purchase two 240V/12V @ 50Hz transformers the hookup will obviously be much simpler. Follow the current requirements that Sgt.Wookie gave you.
     
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