Unknown Powersupply - Current Rating?

Discussion in 'General Electronics Chat' started by jerseyguy1996, Dec 1, 2012.

  1. jerseyguy1996

    Thread Starter Active Member

    Feb 2, 2008
    206
    9
    Good Afternoon! I did a bit of dumpster diving and snagged a couple of external hard drive shells that someone was throwing away. The hard drive had wisely been pulled but the power supply was still intact along with part of the case. The power supply puts out 12V and 5V which is perfect because I am working on an Arduino project that uses a 12V solenoid valve and relay board, and the 5V I can use to power the Arduino. I am wondering if there is any way to know how much current at 12V these power supplies are rated for. I have included a few pictures below along with some tags from the hard drive itself if they are any help.
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    IMAG0755 by jg1996business, on Flickr

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    IMAG0752 by jg1996business, on Flickr

    [​IMG]
    IMAG0753 by jg1996business, on Flickr

    [​IMG]
    IMAG0749 by jg1996business, on Flickr
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,985
    745
    Cant see the diodes on the heatsinks, possibly able to get 5-10Amps ?
     
  3. jerseyguy1996

    Thread Starter Active Member

    Feb 2, 2008
    206
    9
    So you are referring to the TO-220 packages (I guess they are probably FET's) that are attached to the heatsinks? Those will be the limiting factor?
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,006
    3,232
    Certainly the maximum total power output will be less (perhaps about 75%) of the minimum input full-load power of 100V * 1.1A = 110W for the first unit and 100V *2A = 200W for the second unit. Typically the 5V current (not power) rating is slightly higher than the 12V current rating.
     
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  5. jerseyguy1996

    Thread Starter Active Member

    Feb 2, 2008
    206
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    So at 75% of the 110W at 12V would suggest 6.875 Amps. Does this sound right? If so then I am good to go on this one as the solenoid valve pulls about 2.5 amps at 12V. If I am off, will the fuse blow before the unit self distructs?
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,006
    3,232
    Not quite. If you assume the maximum 5V and 12V currents are the same then a little math gives a max current of about 4.8A for each. Still well below your 2.5A solenoid.

    Hard to say if the fuse will blow before the until fails. I would try it with the load and check the heatsink temperatures. If one becomes too hot to touch you may be in trouble.
     
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  7. jerseyguy1996

    Thread Starter Active Member

    Feb 2, 2008
    206
    9
    Tried it out and I didn't see or smell anything funny so I think I am good.
     
  8. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,985
    745
    best way is get an ammeter and voltmeter and draw current off it using a set of 10 ohm 5watt resistors, and load them one at a time, and note the meter readings.
     
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