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Building a bench power supply

Discussion in 'Electronic Design' started by quietguy, Oct 24, 2005.

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  1. quietguy

    quietguy Guest

    Hi All

    I am building a dual supply for general hobby work - 5v fixed and 0 to
    15v variable. Suit microchips, TTL stuff, and small motors etc etc

    Basically 2 separate supplies in the one box, with the monitoring meter
    switchable between V and I, and between supply A and Supply B

    No sweat with that.

    What I am not sure about is whether to have the supplies fully floating
    or earthed or floating with a removable link to earth or a removable
    link for the negs.

    ie both neg output terminals earthed? Both just joined - ie common negs
    -

    The variable supply will be based on a switch mode power pack I happen
    to have - output fed to an adj voltage regulator - hassles?

    The other on a xformer power pack feeding a 7805 etc

    Housing is a plastic box - can fit with a 3 pin lead/plug if I need the
    earth

    Will appreciate views, ideas, and advice

    David
     
  2. budgie

    budgie Guest

    Generally I avoid building power supplies. Not because I'm scared of them, but
    usually I can find what I want at a decent price where someone else has done the
    hard yards, and my bech time can then be spent on more interesting/challenging
    projects for which there is no COTS solution.

    Having said that, there is certainly a place for building to get exactly what
    you want, and a satisfaction when (if) it all powers up and performs to your
    expectation.

    Starting with the variable part, unless the max load current is low the use of a
    linear following the SMPS module is going to create a heat dissipation issue.
    I'd investigate whether the SMPS output voltage can be directly controlled over
    a suitable range.

    For the 5V module, again knowing your max load current info would have helped to
    make more meaningful comment. I'd certainly consider running the 78X05 off the
    SMPS module if the current draw is modest. It would save using a second
    stepdown device, although it has common ground implications.

    The most flexible grounding solution is to have both the outputs floating and
    the provision for ground links to each supply's negative terminal. This is
    widespread among COTS power supplies.

    Be aware that there are safety impications wher the outputs are fully floating -
    a fault in the unit can result in line voltage on the output terminals.
     
  3. You'll need more than that eventually. As the old adage says, you can
    never have too many power supplies!

    Tracking +/-15V variable and fixed 5V and 12V would be an absolute
    minimum I would recommend. Although if you are doing digital only stuff
    then maybe you can get away without a extra negative supply.

    Variable current limiting is essential for experimentation, so you
    don't blow stuff up. This is a fair bit extra complexity, but well
    worth it. In this case it's probably bets to get a pre-designed kit or
    commercial unit so it's all done for you.
    You'll need fully floating with an earthing option.
    Fully floating allows you to combine the outputs to give different
    voltages and negative rails etc.
    Nope, fully isolate the two.
    Just excess power dissipation, make sure your heatsink is big enough.
    At least you are doing it the safe way with existing packs, that's
    good.
    I think you'll quickly find this basic supply quite limiting (no pun
    intended).

    As Budgie said, commercial units with nice displays and current
    limiting etc are available at good prices on eBay etc.
    Although it's fun to build your own occasionally and get exactly what
    you want. The more outputs the better!

    Dave :)
     
  4. message
    Do what is done on most better bench supplies. Have the units seperate,
    and arrange the outputs so you can change the interconnection terminals.
    You will want a -ve rail sometime. Add a ground connection somewhere
    though.
    If you are going to drive motors, this implies a reasonable current. Think
    carefully about the amount of heatsinking involved.
    Also, consider whether the regulators can survive having their outputs
    driven 'above' the incoming voltage. This is what will happen if the two
    supplies happen to get joined in parallel, and the variable unit is set
    above the voltage on the fixed system.

    Best Wishes
     
  5. quietguy

    quietguy Guest

    Thanks for the replies guys - very helpful

    I agree that a 'bought one' would be nice, but I just don't have that sort of
    cash to spend. ie $200 - $300+ for a 3A dual supply unit

    Will give some thought to including current limiting - hadn't considered it
    so thanks

    Not sure whether the SMPS can be modified (be me) re o/p - they are tricky
    things that I have had litte (read none) experience with, but...

    The possibility of a fault raising the o/p to mains level is scary -
    especially in that much of the stuff I will be playing with will also be
    connected to my Windows computer, my CRO input, and my fingers as well. Mmmm
    - don't like the smell of burning flesh, but then I may not be in a state to
    smell it or anything else!

    However, just had a thought about earthing the o/p of the supplies - is this
    likely to be a hassle in that the plug packs then have 240v between prim and
    (if earthed) secondary? I suppose not but as a very old time TV tech (B&W
    w/valves days) I have a grave mistrust of anything that doesn't have an
    earthed metal chassis

    Cheers

    David
     
  6. kell

    kell Guest

    This thread has some info about modifying a pc power supply to make a
    variable power supply:

    http://groups.google.com/group/sci....pc+power+supply&rnum=1&hl=en#b28481d4e14b282d

    Using the information I gleaned there I disabled overvoltage protection
    on a salvaged pc power supply. Then to make the voltage adjustable I
    looked for the inverting and noninverting input pins on the pwm chip (a
    tl494). A potentiometer (or a transistor controlled by a pot) from one
    of these pins (I forget which one) to ground causes the output voltage
    to vary.

    Current limiting is another matter. You want it inside a voltage
    regulation loop. So the simple potentiometer scheme outlined above for
    setting voltage has to be abandoned.
    Use feedback from a voltage divider on the output into the base of a
    transistor controlling one of the input pins on the tl494, and this
    will set the voltage. Then you can put the current limiting part of
    the circuit (a sense resistor and transistor) inside the voltage
    feedback loop.
     
  7. Jasen Betts

    Jasen Betts Guest

    Float them, that way you can set the variable to 5+ and hook it's positive
    to the neg of the other and hava a +/-5V supply for playing with op-amps
    etc...

    Bye.
    Jasen
     
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