150 Watt Dummy Load

[K5DTR]

I just recently finished my 150 Watt Dummy Load project.

Parts List :

1 - one quart paint can
30 - 1.5K Ohm 5 Watt metal oxide resitors
1 - Bulkhead mount SO-239 connector
2 - small copper sheets (mine were 1/16" thick and 3" x 4")
1 - quart of mineral oil

I drilled 30 holes in each copper sheet to accomodate the resistors. I cut appox. 1 inch off of each resistor lead to make the resistor bank more compact.

Soldering the resistors to the copper sheets was a small challenge due to the high thermal conductivity of the copper. Traditional soldering was ineffective. I had to braze it with a butane torch using solder as the filler, which proved to be highly effective. I then attached two eye terminals with a bolt and then brazed those on as well.

Once the resistor bank was finsished, I had to prep the paint can. (Note : I bought an empty, new paint can from a hardware store for about $2). I cut several strips of self vulcanizing tape to line the bottom. First I poured some Starbrite brand liquid electrical tape to coat the bottom, then immediatly lined the bottom with the self vulcanizing tape so it would adhere to the bottom. Once the liquid cured and set, I poured another layer of the liquid tape and set the resistor bank in place while the 2nd liquid layer was still wet so the resistor bank would be "anchored" to the bottom of the can and insulated as well to prevent arcing to the steel can. The second layer was then given adequate vetilation so it would cure and set (I left it over a weekend).

I then punched a hole in the middle of the lid to accomodate the bulkhead mounted SO-239 connector. I used some blue RTV gasket sealer to seal the connector to the can to prevent leaks. Once that was done and the wiring attached I filled the can with mineral oil (it's electrically inert). The mineral oil acts as a coolant and further insulates the load bank from arcing to the sides of the can.

I calculate that this load will safely handle 150 Watts of power continuously, and possibly handle higher power intermittantly.

I calculated that :

1.5K Ohm x 30 in paralell = 50 Ohms
30 x 5 Watts = 150 Watts

I am currently testing the load and will provide and update on how well it performs.

[K5DTR]

Last picture.

[K7JEM]

One thought I would have is to make sure that the resistors are truly non-inductive. Check the SWR at several different HF bands to ensure that it is really "flat".

The construction will probably not allow the device to be very good in the VHF range, even if the resistors will work there.

Joe

[K5DTR]

You are correct K7JEM. This load stinks at VHF. It looks great on HF and would be OK on 6 meters (1.2 SWR). Overall I am happy at its performace for HF, and it was fun to build.

Here is a Smith chart and a SWR graph of its performance using an Agilent analyzer. Start and stop freqs on both charts are 1MHz and 148 MHz.

[G3TXQ]

Nice job! A couple of things to try if there's a "next time around":

1) Reduce the resistor lead lengths - get the bodies right up against the copper board.

2) Avoid the long leads between the connector and the load. If you could solder the resistors to the can lid, you'd then only need one short wire from the centre of the connector to the one copper board.

Even then it's tough getting a good 50 Ohms well into the VHF region.

Finally, are you sure the resistors are non-inductive? They look suspiciously like some of the ceramic-bodied ones I have which are wire-wound.

73,
Steve G3TXQ

[AC0GR]

I agree with G3TXQ that those look suspiciously like ceramic wirewound resistors. His shortened lead length advice is excellent as well.

You might also want to poke a -very tiny- hole in the lid to prevent any build up of pressure due to heating. Hate to think about that lid popping off while full of hot oil.

[WA9SVD]

I would question WHY you felt the need to prepare or insulate the inside of the can? Most dummy loads of similar design concept, including the Cantenna, have the shield of the coax connected to the can lid, which when in place, places the entire can at "ground" (or rather coax shield) potential. That's partly why the Cantenna has an internal structure that provides a tapered support for the resistor, and also keeps the bottom connection to the (LARGE) resistor at the same potential as the paint can lid and the shield connection of the SO-239 connector.
With the lower portion of the resistor (or resistor bank in your case,) connected the the shield of the coax through the SO-239, there would have been no possibility of arcing or need to treat the inside of the can in any way.

[K5DTR]

Thanks for the feedback guys. I appreciate the comments.

The resistors are indeed metal oxide resistors, not wire wound. When I initially looked for high power resistors, I specifically avoided the wire wound resistors to avoid having any "extra" reactance.

WA9SVD, the can is at ground/shield potential. With the way that the resistor bank sits on the bottom of the can (see pictures #4 and #5 above), I had to insulate the bottom (and only the bottom) of the can. If I would have put the resistor bank in a horizontal orientation, I would not have had to insulate the bottom as I would have put the shield/ground connection there.

[W4HAY]

Nice job!

The problem with the internal insulation is that it greatly lowers the thermal conductivity from the oil to the can. This was the reason the Heath "Cantenna" was painted black -- to increase it's ability to radiate heat.

Also, if the resistor bank were suspended about halfway up the can, there'd be more of a "chimney effect" to enhance oil circulation past the resistors.

[K5DTR]

Good point W4HAY.

I only insulated the bottom of the can to "anchor" the load bank, and to prevent arcing since the load is vertically oriented with bare copper on each side and sitting on the bottom of the can. The sides of the can are still bare.

I couldn't think of any way to suspend the load in the middle of the can reliably with the way I wired it. I would have loved to be able to as it would promote better oil/thermal circulation like you said. Though that is partly why I left about 3/4" of lead on the resistors, to get more oil near the resistors themselves.

If I remember correctly, the Heathkit Cantenna was rated for 1kW, right? I don't plan on feeding my load any more than 200 W.