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Multi-output regulated PSU

Discussion in 'Electronic Basics' started by Wally, Nov 19, 2004.

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  1. Wally

    Wally Guest

    I want to make a regulated bench PSU with three adjustable outputs
    (LM317/337). Do I need to use a separate smoothing circuit for each output,
    or can I connect the inputs of the regulators to a single smoothing circuit?
     
  2. You can connect the inputs of all the regulators to a single smoothing
    circuit.

    If the regulators will provide different output voltages, you may have
    to worry about excessive power dissipation on the lower-voltage
    regulators.
     
  3. Wally

    Wally Guest

    In the sense that they might be damaged, or that power is being lost? The
    transfomer is rated at 24-0-24v, 1A, and the spec of the regulators is 20W,
    1.5A.
     
  4. Power will be "lost" in the regulators in the form of heat. The power
    dissipated in each regulator will be the voltage across it, times the
    current through it.

    Full-wave rectification from that transformer will give you about 33
    volts DC. If a regulator is set to give you 12 volts out, it will
    have 21 volts across it. If it has to deliver 100 mA, it will be
    dissipating 2.1 watts. Without a heatsink, it will get HOT!

    Although the LM317 is spec'd for 20 watts, you do have to provide an
    adequate heatsink to achieve that performance.
     
  5. Rich Grise

    Rich Grise Guest

    And just for the sake of monkey wrench, if you stack them, the high ones
    have to pass _all_ of the current.

    Cheers!
    Rich
     
  6. Wally

    Wally Guest

    So, if I want the transformer to be able to deliver its maximum current of
    1A, then I might be approaching the limit for the regulators. The (vague)
    plan is to have panel voltmeters on the outputs and an ammeter between the
    rectifier/smoothing and the regulators - maybe a current limiting circuit
    here would be a good idea.
     
  7. Wally

    Wally Guest

    What do you mean by stacking them? Using more than one device to get around
    the input voltage limitation, and get a higher o/p voltage?
     
  8. circuit?

    You can connect the inputs to the same filter cap, but it's *MUCH* more
    useful to have three electrically isolated outputs. i.e. each regulator
    circuit has it's own transformer (or transformer tap), it's own
    rectifier and filter cap, and it's own adjustable regulator. The
    grounds are not connected and the three circuits are totally isolated.
    You could veen put in a switch to connect the ground when needed.

    It's also very useful to have a direct adjustable AC output. Simply put
    a muti-tap transformer in the box and feed the output straight through
    through a selector switch.

    Dave :)
     
  9. Wally

    Wally Guest

    Is having three rectifier/etc circuits hanging off the same secondary a
    no-no? Methinks a multi-tap transformer might be more expensive than three
    of the single centre-tapped secondary that I have in mind (I'll do some
    searching and see what I can find).

    A ground switch had already occured to me.

    How is the AC made adjustable? Multi-tap xformer, some sort of resitive
    divider, active components? (I don't know much about this stuff, just
    conversant with the bits and putting them together.)
     
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