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Old 03-07-2010, 06:24 PM
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KC4UMO KC4UMO is offline
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Default 700 watt homebrew dummy load

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 700 watts in oil.
I will do the math later to figure out watt it will handle later unless someone else wants a stab at it.
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