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need to determine max power supply current

Discussion in 'Power Electronics' started by rebeltaz, Oct 23, 2014.

  1. rebeltaz

    rebeltaz

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    Nov 22, 2012
    I have a switch mode power supply board that was removed from an unknown piece of equipment. I know that the output is 5v, but I need to be able to determine the maximum current that this supply can provide. I need at least 2 amps. I now how to load the supply and measure the current used, but I don't want to load it to 2 amps if it can't actually handle that and risk damaging it. Is there any other way to accomplish this?

    Thanks.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Can you post a photo of the power supply?
     
  3. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    You might try pulse loading it with a very low duty cycle and low repetition rate, monitoring the output for any sign of sag or fold-back current limiting using an oscilloscope, while increasing the load upwards toward two amperes. Also listen for sounds of distress, overheating, and smoke. Letting the smoke out usually means failure of course, so best to have a spare on hand.
     
  4. rebeltaz

    rebeltaz

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    Nov 22, 2012
    I' m sorry.. for some reason, I'm not getting alerts about replies to this thread. Here are a couple of pictures of the supply.


    DSCF0993.JPG DSCF0992.JPG

    @hevans1944 -> If I had a couple of these, I'd be more than willing to just ramp it up until it fails and then I'd know how far I could push the other one. Sadly, this is the only one that I have, though, and I really don't want to risk letting out the magic smoke. That stuff is murder to get back into those little components! lol :)
     
  5. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    @hevans1944 -> If I had a couple of these, I'd be more than willing to just ramp it up until it fails and then I'd know how far I could push the other one. Sadly, this is the only one that I have, though, and I really don't want to risk letting out the magic smoke. That stuff is murder to get back into those little components! lol :)[/QUOTE]

    I didn't suggest that you "ramp it up until it fails". Use a pulsed load perhaps a few milliseconds in duration with a pulse repetition rate high enough to allow you to see the power supply output on an oscilloscope. It would be desirable to also monitor the pulsed current in the load with another channel on the o'scope. Start at low current and increase the pulsed load on the power supply while paying attention to how the output responds. If you can reach your desired current level, you can then increase the duty cycle (longer pulse widths, same repetition rate) while again monitoring for signs of distress, like especially overheated components or sag in the output voltage or sag in the pulsed current. You would need a linearly variable load such as a 2N3055 BJT with an adequate heatsink operating in its linear range of current.

    BTW, is that a fuse covered in insulation on the left side bottom of your second picture? I would try to unsolder it, cut away the insulation, then see if there is an amperage rating on it. If it is a fuse, try to determine whether it is in the input or in series with the 5 V output. If in series with the output, the fuse rating will give you a clue about how much current the power supply is designed to deliver. Unfortunately, from its placement on the board, I suspect the fuse is in series with the input power; so not much help there unless you can manage to measure the efficiency of the power supply. The efficiency typically starts out low and improves as more current is drawn, then it decreases as the supply approaches overload.
     
    Last edited: Oct 24, 2014
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    That's a pretty low power supply. The transformer is very small, as is the input capacitor. I would recommend that you look at the diodes on the output side and see if you can find a part number. That should allow you to determine their current carrying capacity. This will give an upper limit on what the current rating could be.

    Note that there are no heatsinks. This also indicates that the power rating is low. The switching device is probably internal to the 8 pin device. If you look it up, you may find some specs that will also give an upper limit on power.
     
    KrisBlueNZ likes this.
  7. rebeltaz

    rebeltaz

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    Nov 22, 2012
    Oh, I wasn't saying that you actually said that, just that that is what I would do if I could. When you said "Also listen for sounds of distress, overheating, and smoke" I just went from there. While I do have several scopes, I do not have a "pulsed load" and that would lead me to a completely different thread :)

    It is a fuse, but you are correct, it is in the AC line, so...

    I will look up the specs on those parts. It probably won't be what I am looking for.

    Thank you both. If I come up with anything I will report back.
     
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