Any reason not to use this?

Discussion in 'The Projects Forum' started by ebeowulf17, Jan 3, 2015.

  1. ebeowulf17

    Thread Starter Active Member

    Aug 12, 2014
    I've got a project I've been wanting to try for quite some time now, and one of the things holding me back was a 24VDC supply. Coincidentally, I just had a laser printer poop out on me, and when I took it all apart to scavenge parts and recycle, I found this:


    It says it provides almost 4A of 24VDC power, which is double what I was planning on buying, so it would give me room to expand. Obviously I'd need to put this into an enclosure for safety, but assuming that gets done right, any reason not to use this as my 24VDC supply?

    Also, I checked the wires on the 9-wire bundle coming out of the board for voltage with the board hot and continuity with it dead, and found these readings, relative to chassis ground:

    white: 24.3VDC
    black: 24.3VDC
    black: continuity to ground
    black: continuity to ground
    black: continuity to ground
    black: continuity to ground
    black: 5.2VDC
    black: 5.2VDC
    black: 23.5VDC

    Any guesses as to why there would be so many wires with apparently redundant signals? Distributing amp load over more wires to avoid larger gauge wire? Separate outputs, each with its own regulator and max amp load? Any guess as to what I can reasonably expect to safely draw out of any individual one of those lines?

    Finally, any idea why there would be two lines just over 24V and one line just under 24V? You'd expect all of the nominal 24V lines to be at the same actual voltage. I rechecked all three of those measurements several times, and the difference is quite consistent.

    Thanks in advance for any feedback. I'm really excited to maybe/hopefully have a beast of a power supply for free!
  2. ericgibbs

    Senior Member

    Jan 29, 2010
    Its common practice to have the Common/Ground/0V to be connected at single point on a power supply in order to reduce ground loop problems.
    The four black: continuity to ground leads are used for that purpose.
    if one of your peripheral devices had say a 24v motor, you would connect a +24V line and a dedicated 0V to that motor module and for the motor logic +5V say, you would use another dedicated 0V.

    Off Load its possible that the 24V will be a little high, as for the lower 23.5V its possible its powering some circuitry on the secondary section of the power supply.

    You maybe already aware of the high voltages on the PCB marked out by the dividing printed hash lines identifying the Primary and Secondary sections.
    The voltages on the Primary section are at Mains potential, so take care.!:eek:

    Looks a neat little PSU.

  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
    Scavenging PCBs from computers is done all of the time. You fell into a very lucky situation and I would definately use it if I could hobble a nice enclosure together. Try to use the original plug connector and as much of the original enclosure frame as possible. Also, some power supplies require some minimum load to maintain voltage regulation. Usually, 50 to 100 mA on the 5 volt section.

    Finally, the 23 volt terminal may have additional short-circuit protection for a motor drive section (in case of paper jams, for example).
  4. ebeowulf17

    Thread Starter Active Member

    Aug 12, 2014
    Awesome! Thanks for the tips and explanations. Hadn't thought about minimum currents for regulation, short circuit protection, or ground loops. All good stuff.