Power supply

Discussion in 'General Electronics Chat' started by rougie, Dec 11, 2006.

  1. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    Hello,

    I need a power supply!

    I need to step down a 120VAC voltage to 9VAC for 750 ma. However all the transformers in the market are too big in size. Any suggestions on some special voltage regulator that could exist in the market.

    This must not take up more than 1/2" x 1-1/2" of pcb space. the height must not be more than 1/2".

    Any ideas.... anyone!

    Best regards
    Robert
     
  2. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    OOOOPPPS... I meant 120VDC to 9VDC!

    Sorry!
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    A 7809 comes to mind for the regulator, but how you're going to fit the rectifiers and filter capacitor into that space along with the heat sink the 7809 will need sounds challenging. Not to mention the transformer. How about a wall transformer to keep the bulk outside?
     
  4. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    Yes my good buddy, this is a huge challenge! Even though it doesn't seem like it.

    Its been over 6 months I have been looking all over for some small device to step down my voltage within this size. The samllest I got, was a ac to dc voltage convertor from RECOM, its 120VAC to 5VDC @ 1A... which measures appoximately 2.5" x 1" x 3/4"high. And cost about $40 last I bought one.... Maybe the price slighlty lowered.... but still its too big in size anyway!

    You see I have looked into the traditional way by using a transformer then a rectifier and filtering caps and why not a good zener to assure a steady voltage followed by another cap... you know the drill! However all this takes too much space.

    So then I tried some switchers from National, however.... I STILL NEED THAT TRANSFORMER! And their too big in size. I sort of need to do this without the use of a transformer.

    So this is why I am looking to go down a different route.

    Here is what I am thinking of doing:

    Connect my 120VAC signal directly to a rectifier, then put some sort of voltage regulator with a VDIFF of approximately 40 Volts or more if possible (Ideally 120V! I think it does exist, I have one down at the office, I just don't recall the part number) Then filter and continue with another voltage regulator... until I get down to 5VDC.

    The 7809 can only accept an input VDIFF of 40VDC max... Am I reading this right.... So I guess that's no good!

    Anyway if you know of any other type of Voltage regulator that can step down 120VDC... let me know!

    Best regrards
    Robert
     
  5. Rowan

    Member

    Jun 1, 2005
    12
    0
    Could you use a 110v 10W zener diode with a 10W current limiting resistor to get 10V? Or 100v zener to get 20VDC then just use a 7809 regulator? There's pretty much no way to get those parts (or any) to fit in a space that small.

    http://www.kpsec.freeuk.com/components/diode.htm#zener
     
  6. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    I don't know if they exist, I could look into it!

    I will let you know.... Your suggestion is Much appreciated!

    Robert
     
  7. Gadget

    Distinguished Member

    Jan 10, 2006
    613
    0
    120VDC or 120VAC to 9VDC...???
    Zeners and resistors are out... the resistor would have 111 volts across it @ 800mA (assuming the zener has a minimum of 50mA thru it), thats 89 Watts ....of pure heat (not 10watts). If your load drops to zero current, then the Zener will also have 800mA thru it, thats 7.2 watts. Add that to the 89 watts from the resistor, and thats one REAL hot 25mm x 37mm piece of PCB space.
    Most Voltage reg chips wont handle the input voltage..(normally a max of around 35 volts) and the dissipation will be similar to the afore mentioned Resistor.
    Switch Mode is the only option I can see, and it would take some genius to design a stable circuit that will fit into that space.
    I have no solution to offer at the moment, sorry.
     
  8. Pere

    New Member

    Dec 14, 2006
    2
    0
  9. EEMajor

    Well-Known Member

    Aug 9, 2006
    67
    4
    I am confused as well, are we talking about 120 VDC or 120 VAC???? If its DC to DC, you won't simply be able to use a transformer. Please let us know for sure which you are talking about. If it is DC, where are you getting 120 volts of it?
     
  10. Rowan

    Member

    Jun 1, 2005
    12
    0
    He has an AC source, but was asking if rectification to DC first would help minimize footprint, but since it won't work he'll just have to deal with a transformer and then rectification and then regulation.
     
  11. Pere

    New Member

    Dec 14, 2006
    2
    0
    Rougie says he needs to fit the converter into a 1/2" x 1-1/2" pcb space with a height not more than 1/2". My proposal does with slight changes. But if i/o isolation is mandatory, I think the trasformer must be at least an E16 (some 630mills x 630mills)... out of PCB space.
     
  12. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    Hi,

    You could go with a concept similar to what's used in energy savng lamps:
    After rectification, use a high frequency chopper (say some MHz) and then a switch mode transformer could be wound on a small ferrite ring - actual size will depend on core material and frequency of course.

    It will still be quite a job packing it to the dimensions you have laid out, but with SMD's, it should be doable.
     
  13. Johann

    Senior Member

    Nov 27, 2006
    182
    30
    Hi!
    I often build small circuit boards and have done away with the traditional step-down transformer to save space, weight and cost. In my case, for power supply to a simple temperature controller or sometimes a timer with a 12V relay output, I use a 1mfd/275VAC capacitor in line with the mains, then a W04 bridge rectifier, followed by a 12V, 1W zener diode across the DC output of the bridge and in parallel with this is an electrolytic capacitor for filtering. Of course, the circuit is not isolated from the mains (230V AC in my case here in South Africa), but that is fine if everything is housed in a plastic box! A BIG BONUS IS THAT THERE IS NO HEAT GENERATED!
    Regards!
    Johann
     
  14. KC Patel

    New Member

    Dec 19, 2006
    1
    0
  15. Gadget

    Distinguished Member

    Jan 10, 2006
    613
    0
    No Heat..?? Magic. I somehow doubt it would supply the 750ma the poster was after.
     
  16. Johann

    Senior Member

    Nov 27, 2006
    182
    30
    Try it! for more current, use higher value capacitor (at the expense of size, however!)
    Just another idea that works for me.
    regards!:p
     
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