Bearing the load

A 2 W carbon film resistor is hardly going to withstand our 100 W PEP of SSB, so it is obvious that the design of our dummy load must be a little more complex than a single resistor and a switch, despite what Figure 1 might suggest! In fact, it uses 20 resistors, each of value 1000 Ω (1 kΩ). How does this solve our problem?

Perhaps a little theory is in order here, but no more than is required by the Radio Amateurs’ Examination.

When two equal resistors of value r are combined in parallel (i.e. side by side), the total resistance, R_T, is given by:

Adding 1/r to 1/r gives 2/r, therefore:

So, by connecting two equal resistors in parallel, we get a combined resistance which is half the individual resistances. If we combine three in parallel, we get a third of the resistance, and so on.

Here, we are connecting 20 resistors of 1 kΩ in parallel, so we will produce an overall resistance of one-twentieth of the individual resistance, i.e. 1000/20 = 50 Ω, which is what we set out to achieve!

This is not the only advantage, though. Each resistor is capable of dissipating 2 W, so 20 of them will safely dissipate 40 W, for short periods at least. This power dissipation is approximately the same as 100 W PEP of normal speech, so the design should be capable of use in an ‘average’ amateur station. The power-handling ability of any dummy load can be improved by providing a ‘heat sink’ which helps to conduct the heat energy away from the resistors, thus lowering their temperature. One popular heat sink is a can of transformer oil, into which the resistors are immersed. This design uses a rather more mundane heat sink, but which is adequate for the job in hand.

Pouring the heat away

Two basic PCBs are needed, the details being given in Figure 2. Both measure 11 × 35 mm. Each has a set of 20 holes of diameter about 1 mm to take the ends of the resistors. Only one of them has two 3 mm holes which are used for mounting the completed load. Figure 3 shows how the heat sink is assembled. Solder one end of each resistor into the ‘earth’ PCB, and then mount this to the box using the two bolts as shown in the diagram. Between each rank of resistors is a ‘sandwich’ consisting of an aluminium strip and two pieces of ceramic wall tile, to act as a heat sink for the resistors. Heat sink compound is used to provide good thermal contact between the resistors and the tiles, and between the tiles and the aluminium strip. This is shown in the lower part of Figure 3. Thread the loose wires of the resistors through the holes in the second PCB, solder into place, and crop the protruding wires.

Switching

A changeover switch must be used so that the transceiver can be switched between the dummy load and the aerial without unscrewing connectors. For most purposes, an ordinary 10 A 230 V changeover switch will suffice, as found in many electrical shops and DIY stores.

Wire this into the circuit as shown in Figure 1 and Figure 3. Check your wiring. Put S1 in the ‘dummy load’ position. If you have a multimeter which includes an ohmmeter, measure the resistance across the socket, J1, before connecting it to the transceiver. It should be very near 50 Ω. With S1 in the ‘aerial’ position, there should be an infinite resistance across J1. Move your ohmmeter to read the resistance across J2. It should be infinite for both positions of S1. If all seems correct, close the box and your dummy load is ready for use!