Project : Electronic Load

Discussion in 'The Completed Projects Collection' started by Wendy, Sep 16, 2014.

  1. Wendy

    Thread Starter Moderator

    Mar 24, 2008


    At some point we all have a power supply to test. I built this piece of test equipment specifically for the job.

    Basically all it is is a constant current source, nothing more. I was very paranoid about the transistor / heat sink dissipating 150W (its max rating), so I used the metal lid from the Radio Shack box to make a heat shield.

    While I allowed for the option, I have not installed heat sink fans. I'm going to wait and see how badly they are needed first. It is a very good idea not to have the heat sink on any valuable surfaces like the counter top or kitchen table when you are using this gadget full bore, lest you leave scorch marks.

    The modulation input will allow you to vary the load current with a square wave, you can check the two current set points with an oscope on the DVM output. This will allow you see how good the power supply under test really is.

    Complete drawings, including mechanical drawings, are in the attached zip file.

    One innovation, which will soon be released as a separate Completed Projects entry, is the X4 AA battery bucket. It is super glued in the enclosure to keep the batteries put and stable. As an added bonus, I taped some spare 10A fused underneath the batteries. See images 2.JPG and 3.JPG if you want to see how I did this.

    Frankly the panel meter was a disappointment. It is not very stable, and not real accurate. I will use it, it will get me in the ballpark, but for precision I will use external metering if I feel it is needed.
    Last edited: Mar 1, 2017
    Metalmann likes this.
  2. BeerBelly

    New Member

    Dec 16, 2013
    Nice project, thanks for sharing.
    The resistors inside the enclosure can get pretty hot.
    I use a circuit similar to this as a dump load for my solar panels. The array of transistors and resistors are mounted to a water tank to supply hot water.
  3. Wendy

    Thread Starter Moderator

    Mar 24, 2008
    The power supply shown in the picture has steadily been getting weaker, and I hadn't really noticed because it was so slow. It is almost 30 years old, and home brewed. The max it would put out before the regulation would shut down was 0.1A, and it should have been almost 2A. When I opened it up I saw the PCB shown below, it had baked itself pretty thoroughly. It was a Radio Shack PCB they sold way back when that I modified for my own use, leaving off a lot of unnecessary parts. So I laid wire to replace most of the baked out traces (which were more oxide than metal) and it worked like it used to, though it was incredibly ugly. Even one of the #6/32 screws used to hold the TO-3 case style LM317K was corroded, and replaced.

    Sometimes having the right piece of equipment is just what you need.

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    Last edited: Mar 1, 2017
  4. paulktreg

    Distinguished Member

    Jun 2, 2008
    A small errors in your parts list for R7 to R10 and one or two other resistors. ;)
    Last edited: Oct 6, 2014
  5. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
    Nice job... I was working on one using opamps constant current and voltage with pwn current load..