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+/-15 linear PSU - 2x15V or 2x18V transformer

Discussion in 'Electronic Basics' started by Andre Majorel, Sep 15, 2005.

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  1. I think that you could use a 2 x 15V transformer for a +/-15V
    linear PSU. The LM317 and LM337 need Vin to be 3V above Vout.
    Assuming the smoothing capacitors are big enough, a 15V
    secondary would provide 15 x sqrt(2) - 0.7 - 15 = 5.5V.

    Yet some people recommend a 2 x 18V transformer in this
    application. Why ? It makes the regulators work harder. Is it
    to avoid the bigger inrush current associated with larger
    capacitors ?

    Thanks in advance.
     
  2. john jardine

    john jardine Guest

    (usually factor for 2 diode drops at about 0.8V-1V)

    No straight answer, as it hinges on the maximum current drawn, the
    transformer quality and how much your're prepared to cough up for smoothing
    caps.

    Cheap transformer, cheap caps, 1A load then go for 18Vac.
    Cheap transformer, cheap caps, 100ma load then go for 15Vac.
    Good transformer, good caps, 1A load then 15Vac.

    From a sim' I already have ("good" transformer and caps) ...
    18Vac, 2200u, 1Amp. Gives 21.5V to 25.1V ripple pp. (pass)
    18Vac, 10000u, 1Amp. Gives 24V to 25V ripple pp. (pass)

    15Vac, 2200u, 1Amp. Gives 17.2 to 20.5V ripple pp. (fail)
    15Vac, 10000u, 1Amp. Gives 19.5 to 20,5 ripple pp. (pass)

    Also bear in mind the capacitor ripple currents run at about 2.8Amps rms for
    a 1Amp load. The 10000uF's can usually handle this in a 35V rating but (say)
    2200uF's may only be able to handle this up at about 100V rating. Cost of
    each capacitor is markedly different.
    Minimising the smoothing capacitor costs is a classic juggling act.

    regards
    john
     
  3. I was one who recommended considering the 15 volt winding to provide a
    (low current) 15 volt regulated output. This works pretty well if you
    have a bit of extra transformer (for low current supplies, you often
    cannot get a small enough transformer to be fully loaded), because a
    lightly loaded transformer puts out a bit of extra voltage.

    But remember to take the low line voltage case into account when you
    calculate the regulator head room.
     
  4. On top of that the mains voltage will vary too. It might be lower (or
    higher) than the "nominal" 110/220/240V

    You have to design for the worst case scenario (and then some), so with
    Linear regs you will always be throwing away a fair amount of power if
    you properly design for those worst cases. If this keeps you awake at
    night, then you should be thinking about a DC-DC converter.

    Dave :)
     
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