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Experiments with dummy loads

Discussion in 'Homebrew and Kit Projects' started by K4VBB, Aug 4, 2021.

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  1. K4VBB

    K4VBB XML Subscriber QRZ Page

    This is the "Fail" documentation of my dummy load experiments.

    I decided to make a high-power dummy load for use with my AL-80B. My goal was to make a dummy load capable of taking the output from my AL-80B for about 30 seconds using "mostly" parts that I have laying around.

    The resistors are RF-Florida, flange mount, 250 watt termination resistors at 50 ohms. To string them together in the correct configuration, I had to put some in parallel and some in series. I came up with a 3X3 configuration that would result in a 50 ohm impedance at the end. 3 banks in series, each bank containing 3 50 ohm resistors.

    The first go-round was semi successful. Even without dunking the thing in oil it was taking the 1Kw output power like a champ. Heat dissipation was pretty even as measured by my IR thermometer.
    However, due to the configuration I used to mount the thing inside of a paint can, there were some inductance issues. SWR started out good, but climbed to an unacceptable level quickly.

    My second attempt was much better as far as SWR goes. I inverted the mounting plates so that the resistors are at a closer distance from each other, and used a "center-fed" design in an effort to minimize inductance. This minimized the wire inside the dummy load, and effectively solved the inductance issue.

    Problem solved? Not so much.

    Now there is an RF issue (which I can't put my finger on at the moment). One of the resistors started arcing within about 5 seconds of testing. Chalking it up to a bad solder job, I promptly replaced it with a spare. Testing again, the new one arced itself to death within about 10 seconds at reduced power output.

    Guess I'm going to have to start from scratch. More to come as soon as more resistors get here in the mail.

    Attached Files:

  2. AE1N

    AE1N Ham Member QRZ Page

    B.DUMMY LOAD.jpeg

    ---Layne AE1N
    WA9UAA and KI4ZUQ like this.
  3. N7EKU

    N7EKU Ham Member QRZ Page


    I suspect most of these issues are with not enough heatsinking. The datasheets for the resistors should give designs and specifications on the required area and type of heatsink. I'm sure even for a single resistor it is MUCH bigger than what you have there. If, from the start you had it submerged in oil, it might have been OK for awhile, but in air not. Think of what the old large Pentium 4 CPU's used for heatsinks -- pretty large and with a fast fan on them. That's the kind of heatsink and air flow you want for each resistor (if you want each one to take 250W).

  4. AA3EE

    AA3EE Ham Member QRZ Page

    Where did you get the resistors? We built a large solid state TV transmitter (100KW) that used the Florida RF terminators in the many many splitters and combiners. We had lots of problems that we traced to a bad lot of the terminators. We had to remove all of the heat spreaders and have them machined to take parts from a different vendor. About 6 trips to an island in the Yellow Sea with a 4 hour ferry ride (aka barf fest) each way. Not a fun time.

    Just wondering if you got some of the bad parts too...
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  5. AC9YY

    AC9YY Ham Member QRZ Page

    Blurry, but you get the idea:

    N7EKU likes this.
  6. WN1MB

    WN1MB Ham Member QRZ Page

    Sketch out the schematic and post. I'm not seeing enough detail in the photos.
  7. KI4ZUQ

    KI4ZUQ Ham Member QRZ Page

    Nice job! Too bad, you know, the frustration of burned parts. I made one for a hundred watts that did not seem to have surprizes but another ham developed the basic concept and proved it. 20210622_130247.jpg
    WN1MB likes this.
  8. KA9JLM

    KA9JLM Ham Member QRZ Page

    Good thing. Resistors can get very Hot !
  9. K4VBB

    K4VBB XML Subscriber QRZ Page

    Here's a screenshot from KiCad that resembles the electrical connection configuration I used first:


    My problems are RF related. The connection locations I used resulted in lengths of wires (and their routing) that created a high-inductance condition. I re-routed them to lower the inductance, which worked, but it then apparently created a high voltage RF field which caused some arcing on one of the resistors. It's likely that this is caused by feeding the resistor banks at the end. Probably would not have arced if I'd have suspended it in oil before testing, but I'd still have an unbalanced load on each resistor bank, and then I'd have a gloppy mess to work with correcting that issue.

    I've since re-configured the design to feed all resistor banks from the center, and I'm also feeding the N-connector with a short length of LMR-400, also from the center of each resistor bank. This should minimize inductance at the points where the individual resistors connect to each other (~2" piece of copper wire), minimize inductance caused by the parallel feed wires (shielded cable), and more evenly distribute the RF load to all resistor banks. Connections from H1 and H2 are using LMR-400.

    Electrically, here's the new design:

    Since I'm down 1 resistor, I can't put it on the analyzer at the moment. I'll have to wait until the "new" ones get here.

    Heat is definitely an issue. Each resistor can handle 250W. In total, and with the appropriate heat dissipation material, in theory the dummy load will wind up handling 2250W. However, at maximum output I'll only ever be running this at half that rating. I can run it in air for about 30 seconds before it gets too hot for my comfort. In oil, it should easily handle the heat for the time necessary to tune up the amp, and then some. If I ever upgrade from the 1KW amp I'll need to make a new one with larger/more fins and a larger can.
  10. K4VBB

    K4VBB XML Subscriber QRZ Page

    I got them off of Flea-Bay. It's possible that one of them was dead, but unlikely. I tested them quite a bit with the first iteration and they worked fine. It was about $12 for 5 of them, so not big monetary loss. It wasn't until I started shortening the connections, feeding the banks from the end when the arcing issue manifested itself.

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