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Minimum current spec for power supply

Discussion in 'Electronic Basics' started by jibberjabber, Feb 21, 2006.

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  1. jibberjabber

    jibberjabber Guest

    I'm looking at the little open frame power supply (PSA-25L-201-R) that has a
    5V .2-2A rating. When my device is in idle mode it draws 750mA which is
    below the minimum Amp rating. What happens when a similar PS is run below
    the rated current? The spec sheet isn't much help.
  2. John Fields

    John Fields Guest

  3. jibberjabber

    jibberjabber Guest

  4. Chris

    Chris Guest

    Hi, Jib. You're saying you've got a small switching power supply which
    has a rating for minimum current as well as maximum, and you're running
    the power supply at below minimum current.

    The PSA number looks like a Phihong switcher. If it's the PSA-25L-201

    it shows two outputs, 5V and 12VDC. The 5V supply is shown with a
    minimum load of 0.2A, and a maximum load of 2.6A. The 12VDC supply
    shows no minimum load, and can supply up to 1A. (See page 1 of the .pdf
    data sheet.)

    Most switchers these days are built to operate properly under no-load
    conditions (like most of the PSA-25L models). Your model, though, has
    a minimum curent rating. If you run a switcher at below minimum
    current, it will not meet specifications for output regulation or
    ripple voltage, and will not respond properly to load transients.
    Worst case, this instability can cause the power supply to fail at
    turn-on, or when the load changes (i.e. from idle to run mode).

    If your load will ever fall below the rated minimum of 0.2A, load the
    power supply with an output resistor such that the current never goes
    below rated minimum.

    You're saying your load is 750mA, which is above rated minimum. In
    that case, you should have no problem. You need to be between minimum
    and maximum, which is where you are. But if you ever run into this
    again, or if your supply is something other than described above, get
    out the junkbox and scrounge a power resistor that will get you above
    minimum current.

    By the way, this is another good reason to scrounge components from any
    non-working electronics you come across. Power resistors seldom fail,
    and scrounged ones can be handy when you've got a problem like this.

    Good luck
  5. Chris

    Chris Guest

    O.K., 75mA instead of 750mA. That means you need at least another
    125mA to get to minimum current. Using Ohm's Law, that would mean you
    need lower than 40 ohms of resistance on the load. I'd go with a 33
    ohm, 1 watt resistor or a 22 ohm, 2 watt resistor. Depends on what
    you've got in your junkbox.

    Good luck
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