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Thread: Dummy Load

  1. #1

    Default Dummy Load

    A quick question: once you start a PSU using a dummy load will the PSU stay on if you disconnect the dummy load? I am wondering if there would be any point in putting a fuse inline in case of too much current draw. Thanks.
    Current Wish List: 1. IBM 7531 Industrial Series PC 2. NEC MultiSync XL (JC-2001) Monitor 3. MicroSolutions UniDOS card 4. Compaq 14" VGA CRT Monitor (the one that came with the SystemPro). 5. Stacker HW CoProcessor Board MCA BUS. If you have any of the above for sale please PM me. Thank you!

  2. #2

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    It might or it might not stay on.

    With the current drain from the dummy load, this tends to store some energy in the the filter inductors and cause a small voltage drop from the smps transformer supplying it, even though the duty cycle of the drive voltage to the primary of the smps main transformer is increased to allow for that via negative feedback.

    So what could happen, possibly, with the dummy load "suddenly disconnected" there could be a positive going voltage transient that might possibly shut down the supply (be detected as an over voltage error).

    But it would depend on the design of the particular supply.

    I would think though, once the supply was started and stable, if you removed the dummy load, say if it was in small increments, from a number of load resistors disconnected one by one, then probably you could get away with it and have the supply sitting there running with no load.

  3. #3

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    What do you use for a dummy load?

  4. #4

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    Quote Originally Posted by Hugo Holden View Post
    I would think though, once the supply was started and stable, if you removed the dummy load, say if it was in small increments, from a number of load resistors disconnected one by one, then probably you could get away with it and have the supply sitting there running with no load.
    Well I was thinking more of taking the load off all at once (i.e. if a fuse blows and load is cut off). In my very scient... errr experiment with an n of 1 once the PSU was turned on disconnecting the load did not shut off the PSU. This is in a generic AT class PSU.

    Quote Originally Posted by whartung View Post
    What do you use for a dummy load?
    I am using a nonworking Seagate ST-225 but you could use anything you want. IBM used to specify a 50W 5ohm resistor for their dummy loads and other members on this board use automotive light bulbs.
    Current Wish List: 1. IBM 7531 Industrial Series PC 2. NEC MultiSync XL (JC-2001) Monitor 3. MicroSolutions UniDOS card 4. Compaq 14" VGA CRT Monitor (the one that came with the SystemPro). 5. Stacker HW CoProcessor Board MCA BUS. If you have any of the above for sale please PM me. Thank you!

  5. #5
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    Consumer power supplies are alleged to be more forgiving, but I was of the understanding that power supplies can be damaged w/o sufficient load. I for one am not going down that road.

  6. #6

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    Quote Originally Posted by tipc View Post
    Consumer power supplies are alleged to be more forgiving, but I was of the understanding that power supplies can be damaged w/o sufficient load. I for one am not going down that road.
    they may not start or regulate properly, but I've never heard of one being actually damaged by a lack of load

  7. #7
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    Default

    Save yourself some time without all the fuss and just grab an old mobo and an old HD or floppy and go from there.
    Surely not everyone was Kung-fu fighting

  8. #8
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    that approach won't work for me as I want to test a replacement p/s I just received, one that's rated for 800 watts. Not looking to torture test it, but would like to run it for 24 hours drawing 400-600. The first one they sent was as dead as a door knob. These units are known for turning up dead. But allegedly are good runners if you can get them started ... so therefore, well not sure what I'm going to do just yet.

    I'm planning a new build, one which might make this endeavor easier. The designated setup has 2 redundant 1600 watt supplies. Now I doubt I would even need half that, as I'm not building a server per se, But it's a hog. But I'm not ready to take the plunge just yet.

  9. #9
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    Just curious as to why you insist on cut rate stuff when for a few more bucks you can get a good nights' sleep? I can understand that with peripherals it's more or less a crap shoot, but the P/S is the heart of the build.
    Surely not everyone was Kung-fu fighting

  10. #10
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    Quote Originally Posted by whartung View Post
    What do you use for a dummy load?
    Quote Originally Posted by Shadow Lord View Post
    I am using a nonworking Seagate ST-225 but you could use anything you want. IBM used to specify a 50W 5ohm resistor for their dummy loads and other members on this board use automotive light bulbs.
    I have seen that '50W 5ohm resistor' specified at [here], but not specified elsewhere.

    A 5Ω/50W resistor is going to be unsuitable for some power supplies. For example, putting one on the +12V line of the IBM 5150's stock power supply will result in 2.4 amps (12V 5Ω) being drawn. Per [here], 2.4 amps exceeds the rating of that power supply's +12V line.

    There a 'quirky' power supplies such as the stock one fitted within an IBM 5155. See [here]. I doubt that a Seagate ST-225 alone will allow that supply to start.

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