650 Watts CW Dummy Load workshop project !

Discussion in 'Homebrew and Kit Projects' started by CX3CP, Jan 18, 2019.

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  1. CX3CP

    CX3CP Ham Member QRZ Page

    this idea has nothing special, it's just about putting all the parts together.

    Whit a KDI 650 watts 50 ohms RF resistor. my idea is to mount it on a CPU heat sink and turn on the FAN when it is in operation.
    I think it can be a good and cheap option to consider. Obviously if you feel like working a little at home.

    later I upload pictures and a spectrum analysis report of the working dummy load.


    Resistor datasheet
    KU3X, KA9JLM, KD0CAC and 1 other person like this.
  2. VE3CGA

    VE3CGA XML Subscriber QRZ Page

    they work great. Cant detect any swr on mine from 160m to 70cm. I used a fairly large heatsink 6"x3" with large wide spaced fins
    Used it when testing the power supply I made for the IC9100. Fed my FT757 into it full key down for 3 minutes at a time. It got fairly warm but the FT757 got really warm, power supply only slightly warm. Ended up having a fan blow on both the FT757 and the dummy load
    If I were to use it again for long tests I would put install a fan on the heat sink. The white screen is just the patio table not part of the dummy load
    KU3X, KD0CAC and CX3CP like this.
  3. CX3CP

    CX3CP Ham Member QRZ Page

    Hi Bob, very nice DL homebrew project. I see that you used folded tinplate to shield the resistor. I'm going to use PCB copper clad cuts and solder them together with tin to form the cover, similar to what you did. It is true, this type of resistor has good linearity in a large bandwidth. The main purpose of using this DL is to test linear amplifiers. So I think this weekend I will finish the DL project or at least I hope so. :rolleyes:

  4. AA7QQ

    AA7QQ Ham Member QRZ Page

    That CPU heat sink just is not designed to handle the heat. For very short keydowns, you might get away with 400W.
    If you lived in the USA, I would send you a heat sink that could handle it for postage. I have about 40 old induction motor drives with excellent heatsinks.
    Now, if you only plan to use it for your rig at 100-200W, just keep the fan on & it should be good.
    Remember, if you let the smoke out of that resistor, you may let the smoke out of your rig or amp.

    CX3CP likes this.
  5. VE3CGA

    VE3CGA XML Subscriber QRZ Page

    thanks Alberto. Not sure why I had that sheet of brass laying around but it came in handy - might have been shim stock to offset a rc plane engine
    Impressive test equipment you have there.
    is that a fluke freq counter I see on the bottom shelf on the right, hard to tell but looks like one I use
    CX3CP likes this.
  6. VE3TMT

    VE3TMT XML Subscriber QRZ Page

    I just bought one of these to make my own dummy load. Can anyone suggest a good source for a heat sink?

    CX3CP likes this.
  7. CX3CP

    CX3CP Ham Member QRZ Page

    Hi Ed, many thanks for the offer you make me, but unfortunately I don't live in US, It's true, surely a heat sink of that size can not handle hundreds of watts of heat dissipation. however, the tests that I thought to do with this load are for short time period, 5 to 10 seconds in CW or depending on the power to be injected. not more, is just enough to see the tested equipment working a little. I will try to don't fry the DL and/or the amp. but It would not be a bad idea to attach the DL to a pan and try to make some fried eggs with bacon, while testing a linear amplifier.

    it could be the first HF frying pan ;) so as not to waste heat while cooks for the lunch or dinner.
  8. CX3CP

    CX3CP Ham Member QRZ Page

    Is not Fluke, is an Tektronix CMC251 1.3 Ghz,
  9. KU3X

    KU3X Ham Member QRZ Page

    Looking forward to seeing the return loss over the operating range.
    CX3CP likes this.
  10. AA7QQ

    AA7QQ Ham Member QRZ Page

    A frying pan with water in it can help greatly.
    Here's a hint. Search around for inverter motor drive rebuilders in your area. They end up with many units that are beyond repair. From them, you can get a good heatsink. At work, we scrap about 40 per year that have blown or gotten rained on.

    I work for the company that "Flies" entertainers all over the world with winches & wires. Sometimes, we push units beyond their limits for a short duration & then just scrap the inverter drive immediately.

    Another idea... Find a car audio amplifier shop & ask if they have any burned car amplifiers. You could get a decent heat sink from them.


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