Highfields Amateur Radio Club
Constructors Corner.

A 50+ Watt Dummy Load.
By Brian, MW0GKX.

Parts List:

Optional: (for longer use / higher power capability)


The dummy load will dissipate power of 50 Watts for about 1 minute without any further cooling, the copper clad boards make a good heatsink in themselves, but for safety it should have an enclosure to keep it from touching metal objects, fingers, the shack cat etc. The enclosure should be metal as plastic could melt or even ignite with the heat.

One option is to fit the assembled dummy load into an empty metal paint tin (make sure all traces of paint are removed) affixing it to the centre of the lid with the connecting socket and letting the load hang on the coax (or other support, see * in the mounting section). For extra power handling capabilities, filling the tin with mineral or transformer oil. If not filling with oil a few slots or holes in the tin will aid air flow and keep things cooler (don't decide to put the oil in later if you've put holes in the tin, it makes a heck of a mess!). Vegetable oil, motor oil, methylated spirits etc. are NOT suitable substitutes, the heat transfer and flash point characteristics are nowhere near good enough!

With air cooling, whatever enclosure you use, remember that heat "flows" upwards so holes in the bottom will let cool air in (use feet if holes are in the very bottom as standing it direct on the bench will block the holes) and holes in the top will let the heat out.

Another option is to use a metal project box, with holes or slots, again for air flow, which can be augmented with a computer type fan for extra power handling capabilities. Again affix the socket to the casing but use an "L" bracket on the GROUND ("B") board for support of the resistor pack.
Do NOT use a plastic "L" bracket.
Do NOT use a metal "L" bracket fixed to the board that is connected to the centre coax, unless you can be absolutely sure that:
1 It is totally insulated from the ground connection (remember the metal case is ground).
2 The insulation will hold up to the heat that will be produced.

If you add a fan then, because of the "forced" nature of the air flow and the position of the fan, up may not be the way out for the heat. If mounting a fan blowing from the side, align the "in" holes and fan with the resistors and make the "out" holes opposite. If top mounting the fan then holes in the bottom and feet to hold it off the bench would be the order of the day with the fan mounted to the top blowing the air out. These methods will give the best cooling effects.

Another way, though not so efficient at cooling, is to mount the fan at the back, blowing air into the box, making the "in" holes where the fan is and "out" holes over the resistors.

If not using a fan then the more holes in the bottom and top the better.


Please Note:
I can take no credit for the Dummy Load circuit design.
Similar circuits have been available for years in many periodicals, reference books etc.


Take the 2 pieces of copper clad board and clamp them together (if single sided then copper sides outwards). An easy way to "clamp" them is to put them together and wrap some tape around the edges, folding it over onto the faces of the boards.

Using the illustration as a guide, drill holes at each of the black spots with a drill bit that is the correct size for the leads of the resistors to pass through. If mounting in an equipment case then use a drill to suit the fixing bolt and drill the green spot, or nearer the edge (but not too close), depending on the size of your "L" bracket.

Seperate the two pieces of board, put 1 aside for now (we'll call this board "A"). With the other board (we'll call it "B") carefully enlarge the centre hole, marked in red in the guide illustration, so that the centre conductor of the coax and its' insulation (dilectric) will easily pass through, slightly larger is good. Once the hole has been enlarged take a 10 mm drill bit or similar and carefully turn it by hand in the centre hole, you are only trying to remove some of the copper around the hole. If the board is double sided copper repeat on the other side. Doing this will stop the hot copper damaging the dilectric when in use.

Now take board "A" and check, enlarging if necessary, that the centre conductor of the coax will fit the centre hole, just the conductor this time, no dilectric.

With both boards check for cleanliness around the resistor holes and the coax hole, as you will be soldering to a large piece of copper, which is a good heat sink, and any dirt will foil your attempts to get the solder to take.

Take the "A" board and fit the resistors to it, have the resistors hard up against the board for good heat transfer and to keep the leads as short as possible. Using a high wattage soldering iron (because of the heat sink effect of the copper clad board), solder the resistors in place, you can do this one at a time or in groups or even all at once, depending on how dexterous you are, being careful not to overheat the resistors.
TIP: If fitting and soldering a few at a time, start with the innermost ones, it will be easier.
Snip the soldered leads off close to the board.

The soldered side of the board should look similar to this, the grey is solder, the centre hole is empty and the optional mounting hole is green.

Now take the "B" board and fit the resistor leads through the coresponding holes, slide the "B" board all the way on until it is hard against the resistors and repeat the soldering and snipping process. When finished it should look like this.

Put the completed assembly aside for now and prepare the coax for connection to the boards. Start by stripping the outer sheath of the coax so that there will be enough centre conductor, with its dilectric, to pass through the middle with about 10 mm extra. Fold back the braid and strip about 5 mm of the dilectric. Tin the stripped end.

Push the tinned end through the large hole in the "B" board and align it with the hole in the "A" board and push home. Solder the end to the "A" board and snip off any excess.

With the screening braid we need to trim it so it can be soldered to the "B" board, a good place to solder it if using a project box enclosure would be the resistor immediately above the hole. If the dummy load will be hanging in a paint tin you could tease out the braid to 4 lots of equalish amount strands, twist, and solder 1 set of twisted strands to each of the 4 corners of the "B" board. If doing this try to keep the lengths of the screen strands the same as this will help support the weight evenly. (or see * in the mounting section).

Paint Tin:

Make a hole (or holes, dependant on the type of socket) in the centre of the lid of the paint can to fit SK1 and mount SK1 to the lid, ensuring a good electrical connection to the lid, (plug connection side out, but I didn't need to tell you that, did I?).

If using a socket that has mounting holes then you could use ring terminals for connecting the screening braid.

Measure about ½ way down the tin and transfer this measurement to the load/coax construction, measuring from the "A" board to the coax. Cut the coax at the measured length, strip enough outer sheath off so you can connect to the socket (or ring terminals) and also prepare the centre conductor. As with the "B" board screening braid connection, if using a socket with 2 or 4 mounting holes, prepare the braid equally to get a central hang and even weight spread *. (You may wish to practice on an offcut of coax to get the lengths correct).

Solder the centre conductor to the centre of SK1 and connect the screening braid (and support wires if used).

* You could, if you wish, use some stiff copper wire (like mains cable, the twin & earth that's used to wire the sockets in the house) for the support(s). Solder to the "B" board and fix to the socket mount(s). This may be easier for you and will certainly save the need to prepare the coax so carefully!

If not using the oil make holes in the lid to let the heated air out, and holes near the base of the tin to allow cool air in.

If using the oil then fill the tin to just over ¾ full. Make a small hole (1 mm is plenty) somewhere on the lid to allow air in and out while the oil expands and contracts due to the heating while in use. Failure to do this could mean that if the oil heats up sufficently the lid could "pop" off due to the pressure. This can be messy!

Place the compleated lid assembly into the tin and (espscially with the oil filled version) securly close the lid.

Your Dummy Load is now ready for use. Make sure you store and use it in an upright position. You could make a label, for asthetic reasons, on your computer and print it off, affixing it to the tin with glue, tape etc.

Project Box:

If using a fan it would be a good idea to check that it will fit if mounting internally (neater) with the resistor pack close but not touching. If it is going inside make sure you have some way to feed the power wires out (optional power socket perhaps?) and make sure that they are routed away from the resistor pack.

Prepare the box with the cooling holes or slots after having decided on the direction of air flow. Also locate where the resistor pack will be and drill a suitable hole for the "L" bracket. While locating the resistor pack, note the length to trim the coax back to.

Make the hole(s) to attach SK1 in the chosen end of the box and mount the socket ensuring a good electrical contact with the box.

Strip and prepare the coax for affixing to the socket.

Attach the "L" bracket to the resistor pack and, in whichever order best suits your chosen layout, assemble the fan and resistor pack into the box.

Solder the centre conductor to the centre of SK1 and connect the screening braid to the outer of the socket (a ring terminal could be handy for this).

Unless fitting a power switch and indicator for the fan the Dummy Load is now complete except for securing the cover.

If you wish you can make a label on your PC for the front panel, for asthetic reasons, and glue of tape it into place.

In Use:

The dummy load will dissipate 50 Watts of transmitted power for about 1 minute, longer if oil or fan cooled (or a bit more power for less time). If using it to make transmitter adjustments I suggest a 25% duty cycle (1 minute TX 4 minutes rest). Of course at lower powers the TX time can be longer. With the fan cooled version you can monitor the heat output and, when it gets too warm, cease TX and wait for cool down.

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