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700 watt homebrew Dummy Load

Discussion in 'Home Brew' started by Radio Tech, Mar 8, 2010.

  1. Radio Tech

    Radio Tech Radio Operator

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    Today I needed another dummy load for the other repair bench.
    Went to lowes and picked up a 1 quart paint can.
    For this project you will need:

    1-1 quart paint can
    1 SO-239
    2-1 inch by 4 inch copper circuit boards
    14 680 ohm 5 watt carbon resistors
    4 4-40x12 machine screws and 5 nuts
    1 small terminal ring
    1 quart mineral oil
    A few inches of stiff hook up wire
    silicon sealer


    Mount SO-239 in top of paint can.

    Tape the 2 circuit boards together and drill 14 even spaced holes to pass the resistor leads.

    Install the 14 resistors in one side of circuit board. Copper side out. Leave about 1/8 of an inch between board and resistor. Cut leads and bend over. Solder all leads

    Install other side same as previous.

    At this time with an ohm meter check from one plate to the other. See how close to 50 ohms you are. If you need add or subtract resistors. With 14 installed mine was 50.7 ohms. This will depend on how close the tolerance of your resistors are.



    Install the small ring terminal to one of the SO-239 mounting bolts pointing out. Bend it so it points away from lid.

    Next cut 2 1 1/2 inches of stiff hook up wire. Solder one to the center of the SO-239 and the other to the terminal ring . Bend wire out and then down so it is the same OD as your resistor board.

    Next mount resistor board as close to the lid as you can by soldering the two wire to it.

    Insert in can and check SWR with a few watts. All should be good as this point.

    Remove lid and seal the bottom of the SO-239 and the 4 mounting screws.

    Take a break. You deserve it. Got to let silicone dry anyway.

    Come back and fill can with mineral oil leaving about 3/4 inch from top.
    Re-install lid and press in good

    If you have 14 resistors installed you now have a dummy load that will handle 70 watts dry for short periods. I see know reason why it will not handle several 700 watts in oil. I put 130 in mine and all was good.
    I will do the math later to figure out watt it will handle later unless someone else wants a stab at it.

    Total cost of my project. $10.00

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    #1 Radio Tech, Mar 8, 2010
    Last edited: Mar 8, 2010
    1 person likes this.

  2. Robb

    Robb Yup

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    Very cool!
    Is the lid of the can grounded well enough; or do you solder the lid to the can on a place or two?
     
  3. Radio Tech

    Radio Tech Radio Operator

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    This can was coated on the inside so I removed the coating were the lid fits.
    Actually you can run it with the lid off. The rf is converted to heat so it will not radiate very good. You could do this in a plastic container and it would work. The reason for the paint can is because the oil will get hot and plastic could melt and a glass container could shatter due to heat.

    I built several "salt water" dummy loads and had one to shatter.

    And the can does help shield the rf. One the lid is hammered on it will make a good connection.

    Also want to add:
    For those of you that wish to add power monitoring capabilities, you have to add two binding posts by drilling two additional holes, and mounting the posts. One red and one black. Use two 1N4148 signal diodes wired in series from the center conductor to the red binding post. You can now check power in voltage.
     
  4. Radio Tech

    Radio Tech Radio Operator

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    I was reading on another forum that a resistor of 2 watts was good for 20 watts in oil. So with that this puppy should handle 700 watts.
     
  5. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    I did the same thing but on a larger scale. My dry dummy load consists of ten Globar 500 ohm resistors of either 100 or 200 watts each I forget. A couple gallons of silicone load oil I got when we burned up a 5 Kw load at work and a 16 qt. stock pot and it will handle all I would ever want to dump into it.

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  6. Radio Tech

    Radio Tech Radio Operator

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    Now that is cool. I bet you will never over load that one!.

    I think the next one I want to try is 40 2k ohm at 5 watt resistors.
    That would be 200 watts dry. So it should handle a kw with no problem
     
  7. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    I just re-read your original post. Be very carefull when using that load. I see you have everything sealed tight. :eek: Question:What happens when the oil heats up? :whistle:

    The lid on mine is clamped tight but offers a release of hot air, or in the worst case oil, in the event of overheating. Yes, if I tip it over it will spill some oil but accidently tipping a few gallons of oil in a pot that size is pretty remote. It's still better than having a catastrophic failure of the containment vessel however. I like deep fried foods but I don't want to become one.

    You could do as I did and drill a small hole in the top and insert a small rubber grommet. Take a small taper headed machine screw and insert it thru the grommet and on the underside of the cover slip a washer and small compression spring over the machine screw along with another washer and nut. This allows you to reasonably seal the can but offers a pressure relief. The spring holds the head of the screw down tight into the grommet making a good seal but as the pressure increases it will allow the screw to pop up and vent the pressure. You do not want to tighten the nut too tight or it will not vent. Just tighten enough to provide a snug fit into the grommet.
     
  8. Radio Tech

    Radio Tech Radio Operator

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    Good Point I failed to mention.!!!!
    I had some air plane fuel tubing let over from my RC planes. It is like 1/8th OD. I drilled a hole in the lid and bent the tube into a j. I took a little candle wax and put a blob over the tube that sits in oil. This way if it gets knocked over it will not spill out. But if the oil gets too hot it will (should) melt the wax and relieve itself.

    If you do not think that will please please let me know.
    I am up for any improvements.
     
  9. Radio Tech

    Radio Tech Radio Operator

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    And here is the finished product.

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    #9 Radio Tech, Mar 8, 2010
    Last edited: Mar 9, 2010
  10. W5LZ

    W5LZ Crotchety Old Bastard

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    Radio Tech,
    Another one of those 'R/C' surplus part thingys is an 'auto-bailer' (R/C boats). Same basic idea as your fuel line, just -much- more sensitive. Which may not be any improvement when you think about it.
    The old 'Cantenna' used the weight of a small bolt through a hole to keep a seal, and you're right, if tipped over it leaked. There's another thing to think about and that's water displaces oil. What's that got to do with anything? Well, if you should ever place your dummy load outside in the weather so that it can get rained on, you are taking a chance of that rain seeping into that dummy load, displacing the oil up out of that vent. And just depending on how often you use that dummy load, and how long it's been rained on, you may find that it's characteristics have drastically changed. I know that from extensive, and superior thinking. I would never pull such a 'dumb-axed' stunt my self! ...
    - 'Doc
     
  11. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    LOL, Doc, I had a five gallon can of used engine oil "disappear" into thin air and be replaced by five gallons of rainwater. :censored: It seems someone (OK it was ME) left the can of oil under the roof overhang of the shed. All the water that ran off the roof found it's way into the oil can and forced the oil out thru the vent hole that was open. Amazing that it all happened thru that small vent hole but it was there for quite a while. :headbang
     
  12. Radio Tech

    Radio Tech Radio Operator

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    Doc, that would work too. Just never got into r/c boating.But have a bunch of r/c planes. 13 that is.

    Funny about the water getting in the oil. I change my own oil and it always disappears. I leave the container out side and the next thing I know, its all water? LOL
     
  13. Beetle

    Beetle Sr. Member

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    Radio Tech -

    I know about "impedance". What's "impendence"?
     
  14. Radio Tech

    Radio Tech Radio Operator

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    You never heard of that????
    Well I am quite surprised!

    LM$O!!

    Dang spell check!
     
  15. loosecannon

    loosecannon Sr. Member

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    thats what the south was fighting for.

    "impendence" from us yankees. LOL
    LC
     
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