Re: Ultra 1 dead, revival possible?

From: Dr. David Kirkby (nowhere_at_nowhere.com)
Date: 09/12/04

  • Next message: Dr. David Kirkby: "Re: Someone with an Ultra-1 and a multimeter ... (power supply stuff)" 
    Date: Sun, 12 Sep 2004 11:20:23 +0100

    Michael Brown wrote:

    > One other thing you may be able to help with ... what should happen if the
    > PSU is turned on without being connected to anything? If it should power up,
    > then the PSU is toast, but if it shouldn't then it leaves a few more things
    > open.

    Switch mode power supplies often have a minimum load - typically on the
    5V rail. I have known them to fail to regulate properly on a 12V line,
    since the load on the 5V rail was too low. After I added a resistor to
    meet the minimum load requirements, all was okay. It's quite possible a
    power supply will not power up properly on a continuous basis if there
    is no load. i.e. it could switch off after 0.01 sec if it detects there
    is no load.

    Someone here will no doubt know the behaviour of the PSUs with no load,
    but unless someone can confirm otherwise, I would not assume the fact
    you can't measure something with a DVM on the output indicates there is
    a fault.

    Finding a fault on a SMPS is very hard indeed. It is a closed-loop
    system, with a fault at one point, stopping another section powering up.
    Even with a circuit diagram, it would be hard. If you do attempt it, you
    should also be aware that there can be dangerous voltages present even
    with no mains input. The storage capacitors store charge, which
    depending on the design, could be for many few days if no steps have
    been taken to 'bleed' them, and there is nothing drawing current from
    them. (It would be bad design if there were no bleed resistors, so I
    doubt there are none, but there will still be a time after switch-off
    that the inside of the SMPS will be unsafe. Unless you REALLY know what
    you are doing, I would not attempt repair of the SMPS.

  • Next message: Dr. David Kirkby: "Re: Someone with an Ultra-1 and a multimeter ... (power supply stuff)"

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