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Discussion in 'Electronic Design' started by Paul Taylor, Jun 14, 2007.

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  1. Paul Taylor

    Paul Taylor Guest

    Hi i was looking for a bit of advice! I have built a dual +/-15v power
    supply running from a 18v 225VA torodal on 2 separate windings, I am pulling
    4.5A on the positive side and approx 0.5A on the negative side, the positive
    rectifier is cooking i tried replacing this with an 8A one and it still does
    it has anybody else come a crossed this and know what the problem is, i have
    checked that the windings are in phase and wired correctly i can post a full
    schematic on request.

    Thanks Paul
     
  2. Eeyore

    Eeyore Guest

    The 'positive rectifier' ? What do you mean ?

    Are you not using a bridge ?

    If you're pulling ~ 4.5A then it's likely any associated rectifier will be
    dissipating ~ 4.5W What is it ? How is it mounted.

    Rectifiers *do* get hot.

    Graham
     
  3. Paul Taylor

    Paul Taylor Guest

    sorry my english is a bit bad the recifier that supplys the positive 15v
    Yes im using 2 bridge rect (8A)
    The Rectifier is a KBU6G and was board mounted
    I havent come acrossed them getting this hot before? i am unable to touch it
    with my fingers!

    Cheers Paul
     
  4. Phil Allison

    Phil Allison Guest

    "Paul Taylor"

    ** Will not do.

    Above 2 amps, small plastic bridges MUST be mounted on a heatsink or the
    chassis.

    At 4.5 amps DC continuous, best use cast metal bridge rectifier rated at 20
    amps or more & bolt it down to the case.



    ....... Phil
     
  5. Eeyore

    Eeyore Guest

    Why two ?

    If it's a centre tapped supply you on;y need one bridge.
    And how do you heatsink it ?

    The package will dissiapate ~ 4.5A (forward current) * 1V (rectifier forward
    voltage)* 2 devices ( 1 per leg ).

    That's 9W which is greater than the data sheet maximum allowable ( to my
    surprise).

    The junction to ambient thermal resistance is 8.6C/W so without serious
    heatsinking elsewhere the device will operate at 77C above ambient temp. Say >
    120C in typical use.

    You need to understand thermal management issues in electronics better.

    Make larger pads on the pcb to heatsink the device but be aware this will make
    the circuit board hot too and fit a heatsink directly to the rectifier body.

    I prefer chassis mounting rectifiers. The chassis makes a good heatsink usually.

    Graham
     
  6. Phil Allison

    Phil Allison Guest

    "Paul Taylor"

    ** Huh ?

    The KBU6G is a 6 amp bridge

    - when used on a heatsink that can dissipate 10 watts easily.



    ....... Phil
     
  7. Greg Neill

    Greg Neill Guest

    It sounds to me like he's built two separate supplies using
    two separate secondaries and tied the reference grounds
    together to (supposedly) yield independant + and - legs.
    It also sounds like a recipe for a ground loop problem through
    the bridges... he should supply a schematic.
     
  8. Paul Taylor

    Paul Taylor Guest

    Sorry Phil this is board mounted no heatsink!

    The package will dissipate ~ 4.5A (forward current) * 1V (rectifier forward
    voltage)* 2 devices ( 1 per leg ).

    I'm asking it to dissipate 9W the data sheet states 6.7W
     
  9. Eeyore

    Eeyore Guest

    Yes you are !

    It's wise to look at fundamentals like this before designing pcbs or metalwork
    etc !

    Graham
     
  10. Phil Allison

    Phil Allison Guest

    "Greg Neill"<

    ** No.

    One makes two regulated supplies using two positive regulator ICs & then
    links the ground ref of one to the + output of the other.

    Creates balanced +/- DC supply lines.

    Been standard practice for decades.




    ...... Phil
     
  11. Phil Allison

    Phil Allison Guest

    "Paul Taylor"


    ** Figure on more like 1.2 volts per diode at around 20 amps peak.

    Makes the peak dissipation 48 watts.

    The average will be about 25 % of that or 12 watts.

    If no heatsink is possible, use 2 bridges in parallel and spread them out on
    the PCB.




    ....... Phil
     
  12. Paul Taylor

    Paul Taylor Guest

    Thanks for that graham i was getting confused with 1.2v diode drop of the
    rect x 4.5A = 5.4W so i guessed i was within the data sheet spec?

    i think it is a lot clearer now thanks for you help.
     
  13. Paul Taylor

    Paul Taylor Guest

    Thanks Phil
    I'm now looking at using a chassis mount Type 36MB10A should be ok, thanks
    to you all for your help.
     
  14. Paul Taylor

    Paul Taylor Guest

    I have used one torodal with dual secondary windings and fed to 2 separate
    rectifies????
     
  15. Paul Taylor

    Paul Taylor Guest

    Sorry might sound a bit unclear! feed one secondary winding to one bridge
    for the +15vdc and the other secondary winding to the other bridge for
    the -15vdc.
     
  16. Phil Allison

    Phil Allison Guest

    "Paul Taylor

    ** One bridge is all you need for that.

    Saves on diode voltage drops.




    ........ Phil
     
  17. Eeyore

    Eeyore Guest

    And waste heat where you don't want it.

    Graham
     
  18. Paul Taylor

    Paul Taylor Guest

    I have tired the first bridge positive to the +15vdc regulator and the
    bridge minus to the positive of the -15vdc bridge this is my ground and
    the negative from this bridge to the -15vdc regulator? if that makes sense.
     
  19. Paul Taylor

    Paul Taylor Guest

    So are you saying use the torodal with the secondary windings series off and
    top winding for +15vdc and bottom for -15v with the centre common to both?
     
  20. Phil Allison

    Phil Allison Guest

    "Paul Taylor"

    ** That is the way everyone usually does it.

    Except when they want to use positive reg ICs like the LM338K for both
    rails.



    ...... Phil
     
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