Experiments with dummy loads

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

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  1. AA3EE

    AA3EE Ham Member QRZ Page

    You're fighting the package type. Flange mount devices don't really lend themselves to series/parallel combinations. The only way to really make this work is to look at the load with a VNA and add some type of compensation network to cancel the reactive components. You're better off buying a 50 Ohm resistor from a Bird load and building a proper mount for it. Instructions are in the old ARRL handbooks, and it basically mimics the Heath Cantenna. A little sheet metal work for a pretty good load.
  2. N2EY

    N2EY Ham Member QRZ Page

    How many watts can a resistor that small take with adequate heat sinking?

    The reason I ask is that I've got two 50 ohm flange mount resistors that claim to be 250 watts, but they're only 1" long by 3/8" wide (that's the flange size). I only need 100 watts but even with a big heat sink, will it just fry?

    73 de Jim, N2EY
  3. VK2TIL

    VK2TIL Ham Member QRZ Page

    There is a procedure for determining heatsink size by accounting for each heat obstacle in the chain between the internal "chip" of the device and free/forced air; it should be found in heatsink manufacturers' data.

    But a "back of envelope" calculation serves quite well.

    Say 100 watts is to be dissipated at room temperature (26C) and the heatsink should be no hotter than 76C; temp rise 50C.

    A heatsink rated at a minimum of 0.5C/watt is required.

    My home-made load uses a 250 watt Florida RF flange-mount resistor;

    Photo 1.JPG

    The two heatsinks have a combined rating of 0.23C/watt; it runs comfortably at 100W and I think its continuous maximum might be about 150W.

    These loads may be checked with a DC power supply if it can supply sufficient voltage & current; for instance, 100W into 50 ohms is 71v at 1.4A.

  4. K4VBB

    K4VBB XML Subscriber QRZ Page

    Well, I got my replacement RF Florida flange-mount resistors in. Soldered them back into the re-designed dummy load and tested. The re-design is a BIG improvement. Much less inductance than there was before, but it's not perfect. It's 1:1 at the low end of my RigExpert (1.8MHz) and rises steadily to 1.22 at the top of 10m (29.700MHz). I could likely do a little better if I redesigned the entire project from scratch and used different geometry to keep the wires even shorter, or possibly even use more shielded cable to make the connections. But this will serve my purposes for the moment.

    As far as heat goes, it doesn't take long at all for these things to heat up in air. However, in oil I can key the amp at 1K watts for a minute before I feel the can warming up. Probably a good idea to stick one of those "fish" thermometers on the outside. Again, I could likely go a lot longer with a bigger can (say a 1 gallon paint bucket), but again, the 1 quart paint bucket will suit my needs for the moment.

    There is some slight weeping of oil out of one of the screw holes at the top. At some point I'll take it apart again and do a better job of sealing. In the mean time I have a functional, high-power dummy load.
  5. AC9YY

    AC9YY Platinum Subscriber Platinum Subscriber QRZ Page

    Do you have pictures of your improved design?
  6. W2WDX

    W2WDX Premium Subscriber QRZ Page

    You really are not doing the calculations for heat dissipation. 1200+ watts for even a few microseconds is a great deal of energy to dissipate. You need to use materials able to transfer heat rapidly and be able to dissipate it quickly. Oil alone is not a good solution. Copper and lots of it is a good place to start, then you need a radiating surface and most likely forced air. Otherwise you'll have rapid temperature rise within the devices and the values are then going to be all over the place on your devices. Consider a failure mode as well. If devices start to fail due to heat stress from slow dissipation, you could end up toasting the amplifier.

    I built one with similar devices, used a single flat strip of copper to connect them (not a wire) and mounted them to a 1 thick piece of 6"x6" solid copper. I mounted the devices in a single plane, not wrapped around each other. This keeps inductance and stray capacitance low. I then used a CPU heatsink cooler with two fans. Nearly 1.15 SWR at 30Mhz and marginal temperature rise at the devices for two minutes at 2500W continuous. I also use a temperature controlled switch to activate the fans when a fairly low temp threshold (just a few degrees above my typical ambient) is reached. The fans pretty much kick in about a second of applying power. Needs 12V and it is very loud, but works.

    I'm using four 800watt flange-mount 200 ohm devices giving me 3200W total, derated down to 2500W.
    Last edited: Aug 31, 2021
    AC9YY likes this.
  7. AC9YY

    AC9YY Platinum Subscriber Platinum Subscriber QRZ Page

    Do you have some photos?
  8. K4VBB

    K4VBB XML Subscriber QRZ Page

    I have to seal up the unit a bit better. I'll likely get around to doing so sometime this week, and at that point I'll take/post some updated pictures. The mechanical design is no different. The design changes are electrical in nature, but I'll post them up soon. Pretty sure it's going to make a goopy mess when I take it apart.
  9. K4VBB

    K4VBB XML Subscriber QRZ Page

    You're right. And I have no plans to calculate the BTU/Hr rate of heat dissipation, conduction/convection/radiation rates into the dissipation plates and the volume of surrounding oil, and the subsequent convection rates into the surrounding air. And to be clear, I've only tested this at 1KW for 1 minute.

    The total capacity of the resistors is 2.25KW. At less than half of that output power (AL-80B), and for 1 minute (more time than I plan on using it in one sitting at that power level), the current design shows no measurable difference in performance (with my equipment, that is). Although it does get warm.

    I'll post here when your prediction comes true and I toast the amplifier. Until then I'll let it continue to serve my needs.
  10. KC3TEC

    KC3TEC Ham Member QRZ Page

    if its experimenting in heat dissipation many factors are needed for design.
    liquid immersion requires an electrically non conductive fluid such as a silicone based oil, but the downside of it is silicone oil alone is not a good medium for thermal transfer.
    ethanol and water based coolants offer excellent thermal transfer but are electrically conductive no to mention a little corrosive.
    hybrid inter-coolers ( oil inside inner bottle and water based in much larger outer envelope) handles phenomenal amount of power for protecting expensive processing chips but the question is would it be practical for a dummy load?
    most likely not!
    air cooling requires the use of heat sinks and the higher in wattage you go the increase in size (surface area) is exponential, good air movement greatly improves performance and using a fan would help with heat dissipation.
    but would likely cause interference.
    so lets think on this a little,
    adding a heat sink to a resister and immersing it in oil will it transfer heat to the oil faster? will it induce unwanted voltage and noise?
    the greatest thing about experimenting with design is that you learn so much with both success and failure
    and thank you for posting your progress

    I know a bit about inter-cooling as some of the production equipment relied on them.
    we designed a special cpu cooling unit at work and ( no air fans)fluid pump and inter-cooler tank were in the next room so absolutely silent in the computer room.
    extremely efficient temp control too!
    Last edited: Sep 1, 2021

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