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Homebrew Dummy Load

There are probably thousands of descriptions of these on the Net so I have almost nothing to add here. But I figured I'd put mine here anyway, maybe someone gets an additional idea. Besides, mine looks kind a small and sexy, at least to me. :)

I didn't want to make a powerful one, just being able to dissipate around 5W for my QRP experiments was fine with me. It needs to go in the metal container, plywood will not do here. In my opinion the best one is the kind with a lid you can take off and put back on again. I opted to use a tin of DAP Plastic Wood I found in the junk shelf. It measured 2.5" outer diameter by 2" in height. The stuff in it had dried up rock-hard. After a little clean-up with a screwdriver and nail polish remover the tin looked like new inside again.

Since a little perfectionism is built into me, I wanted it to get as close to 50 Ohm as possible. But I didn't have 10 500 ohm resistors. Finally the case was settled by using ten 1/2 watt resistors in parallel but 5 from the 470 Ohm camp and the other 5 from 560 Ohm. These are standard values and should theoretically get me 51 Ohms.

A hole was drilled in the center of the lid to receive the back of a BNC connector, this being a QRP device, and it was fastened to the lid. Made a little circle where the grounded ends of the resistors would go and divided that to 10 more or less equal sectors. After starting to solder the resistors I soon realized that the lid was a very good heat sink. Luckily I had a big 100W Weller which now saw service. The point where the resistor legs meet in the center I rested on the central lug of the BNC. Here I had to be careful not to melt the insulating teflon. Finally, to give my handiwork a tad better look I soldered the BNC to the lid from the outside.

This is what the lid looks like from the side. Luckily this particular tin is made from relatively thick sheet metal so pushing on the male BNC connector doesn't flex top of the lid.

Completed project, resting on top of a flux box.

It didn't take long before I had it hooked up to a Tx with about 4.5 watts out. It came as a bit of surprise to learn that my resistors got hot. The total dissipation of 10 half-watt resistors should have been 5 watts!

In real dummy loads they use transformer oil but me being a frugal soul I am I knew I had to substitute. First I tried regular olive oil, (hehehe, insert a little laugh here) and filled the can with it. That left a lot to be desired. First it didn't cool all that well and second, it smelled. Kind of gave "cooking your electronics" a whole new meaning!. A few days later I bought a pint of ordinary motor oil and that turned exactly out to be what the doctor ordered. No smell and far better cooling properties. As a test I pumped 10 watts continuously into it and it only got warm to the touch.

 

Happy testing, 73 de Brian.