#dummyload

Syrup tin dummy load project for the intermediate exam.

I'm currently training for the intermediate at the Carmarthen Amateur Radio Society that meets at Cwmduad in West Wales. The Dummy Load project is a favourite of the incumbent tutor Trevor GW4RVA.

This design is based on the January 1983 Practical Wireless article by F.C. Judd G2BCX. I bet that this design has been tried and tested by hams all over the world!

The idea is to put 20 x 1K ohm resistors in parallel to give you 50 ohm. By having 20 resistors in parallel you also increase the power dissipation.

The resistors I used were 3W Carbon Film, twenty of them, which in theory should give me a 60W dummy load. The 'book' that we use for training says when making a dummy load that you shouldn't use wire wound resistors due to the inductance that builds up in the wound wire construction of the resistor. This is echoed in all the dummy load construction guides online. The rational is that RF travels along the surface of a material, and therefore carbon film resistors are much better suited for this application.

Soldering the resistors onto a thick wire isn't easy especially with today's lead free solder. Lead has a lower melting point temperature and would be much easier to work with. A thick wire is needed due to the high RF voltage and anticipated 100W power capability (it also has more surface area). In the end I used my gas soldering iron that is much hotter than any of my electric irons, that meant I was able to solder joints much quicker and reduce the risk of causing heat damage to the resistors while also reducing the risk of dry joints.

The article suggested bedding the SO239 and the LED with sealant to make the finished dummy load watertight and then filling the can with mineral oil to dissipate the heat. I used sand instead. I didn't fancy accidental oil spillages.

With the sand to dissipate the heat the dummy load should be good for up to 100 - 120W TX power.

You can't see it in the final photo where it is hooked up to the radio, but there is a LED on the top of the can which indicates the presence of RF, it glimmers dimly at 0.5W and has a nice comforting glow at 5W.

In the article it said that the finished load would work up to 30MHz, I've measured it with Allan's (GW4VPX) analyser and my finished project is perfectly usable up to 50MHz (6m). I suspect had I kept the leads shorter on the resistors that it would have probably given me an even better working range.

I'm glad that it is finished now. My next project will be to make a much smaller one in an Altoids tin suitable for VHF and UHF.

Years ago, when I started working on amps on a semi-regular basis, I realized very quickly that I was going to need a dummy load. My dad very graciously whipped one up for me out of some big-ass 75W 3 ohm wirewound resistors. It wasn't in an enclosure, but it did the trick for awhile. 

But, as time went on, I realized I needed a bit more. So, I went to Value Village and bought an old PC Modem/Serial switcher box (circ. 1988). I ordered up 5 Dale 4 ohm 50 watt panel mount power resistors. I heatsink compounded them onto a big hunk of aluminum, and bolted that to the lid of the serial switcher. A 4P3T rotary was used to switch between combos of resistors to give me:

2 ohms - 100W

4 ohms - 200W

8 ohms - 100W

That worked just great. Then I decided, why not add a line out? OK! So I added another jack and a pot and voila - lineout. I think it's a 10-to-1 voltage divider with the 100K pot a VR following. Works a treat for monitoring the device on the bench through a little Roland cube that doubles as a practice amp. Then today, I decided I wanted even MORE.

MORE! I'm greedy and lusty and not yet out of panel space, so I add another jack and a switch to act as a speaker/pass through connected to the switch which is labeled "LOAD" and "Speaker".

The pass through presents a danger now though: what if the switch is in Speaker mode, and no speaker is plugged in? Dang! That ain't good. But hold on! We can attach the load line to the switch fin on the speaker jack! Hallelujah! So when the switch is in SPEAKER position, but no jack is plugged in, the dummyload line is connected to the ampline via the switch fin. If a jack is inserted, it opens the connection on the switch fin, disconnecting the loadline and allowing the FROM AMP to pass to the SPEAKER THRU jack. Clever eh?

I got an old Peavy speaker in a cab for free, and wired that up to live under my work bench. Now I can test on the load, monitor on the Roland cube, and then test on the Peavy speaker for "real world" results. Mind you it's a 30W Peavy keyboard amp speaker, but still, better than hauling out a giant cab just to test a head for a few minutes.

Isn't that great? But I'm not done yet! I had one more brainstorm idea tonight. I'm going to add a panel mount BNC jack in parallel with the FROM AMP input to allow me to hook that to my 2nd scope channel. It will live there so Ch 2 will always be output monitor, and Ch1 will be available for tracing or watching something else. Now THAT'S not bad for around $20! I may also add dome 3V or 5V zener diodes to the line out to help save me from blowing up the frontend of my Roland someday...

Looks pretty ghetto, but works great. I'm thinking about building a new one soon, but with bigger resistors, better spacing, better cooling, and a 16 ohm option. One of these days Laura...

If you're interested in building your own dummyload, have a look at Atkin's Amps PDF file showing several different configurations to meet your needs: http://www.aikenamps.com/DummyLoad.pdf

You could also go up to bigger resistors, but then the $$$ factor really goes up. Though, for about $120 CAD at the time of writing, you can get yourself 10x 2 ohm 100 watt resistors! If you doubled up my design that would give you:

2 ohms - 400W

4 ohms - 800W

8 ohms - 400W

16 ohms - 800W