100W Dummy Load

100 watt dummy load project

This was a project to make a dummy load or, as the finished thing is more correctly labelled, a fifty ohm resistive load.

The project began with a heat sink. I have done without a permanent dummy load, making any necessary resistive loads as 100W dummy loadand when I’ve needed them. But having a heat sink to hand it seemed only right to make something that needed to dissipate heat!

I wanted to make the load as small as possible, so that meant finding a suitable box that would match the heat sink.
The heat sink’s footprint is 70mm by 37.5mm so a Maplin’s two-piece aluminium case type AB12 is just about right (76x51x25mm)



These resistors are impressive beasts, rated at 100 watts each. The BI Technologies TO-247 resistors are rated at 100W with a heatsink (and a measly 3W in free air) so two in parallel – together with adequate heat dissipation – should really provide fifty ohms for a 200W transmitter. (It happens that I don’t own a transmitter that can manage more than half of that, anyway!) Resistors, back-to-back

As the heat sink I used is rated at 4.1C/W the completed load will handle 100W for a good half a minute (maybe a little longer) without undue stress on the components – It’s warm after thirty seconds at 100W but not too hot to touch!

But the bottom line is this: It benefits from being able to withstand well over the 100W target for the project.Two fifty ohm resistors in parallel

The resistors are made in TO-247 packages with a metal plate on one side to aid conduction to a heat sink. I used two 100 ohm resistors back to back (with their leads twisted and soldered) to give the notional fifty ohms which, when checked with a multimeter, measured 49.9 ohms.

The only real downside to using these resistors is their being a fiver each (Well, a bit less, so that’s alright then!) I got mine from Farnell (http://uk.farnell.com, part number 1114421 where they are described as RESISTOR, 100 OHM 1% 100W TO-247)



There aren’t many parts in the dummy load – two resistors completes the component count and all the rest is hardware and conSketch of resistor with dimensionsnectors.

Parts used in the dummy load project
1. 2 x 100 ohm resistors
2. Heat sink with fins
3. Connectors

RCA (Phono)

CircuitCircuit diagram

The circuit consists however many sockets you choose – and that probably depends on the connectors you have in your shack – and a resistance of fifty ohms.

The resistance consists two 100R resistors in parallel.


To keep the input connector layout as uncluttered as possible, I put the two smaller connectors – the BNC and the Phono – on one end of the box
and the SO-239 on its own at the other end.

The top of the box was drilled to take the two fixing screws for the heat sink and then two 4mm holes were drilled to accept the leads from the resistors. The two leads from a TO-247 package are 12mm apart.

Drilling detailsThe other half of the case – the plain, U-shaped piece – is undrilled except for the two screw holes that are already present when you buy the box. The screws are also supplied with the AB12 enclosure.







Horizontal rule

Drilled case - base Two views of the drilled case

Drilled case - inside view

Horizontal rule

A couple of relevant links

Farnell logo               Maplin logo

The parts in pictures

Dummy load parts