Dummy Project

A dummy load.

A board acquired at a junk sale at Aberdeen Amateur Radio Society in the late '80s.  350 resistors, each 18,000 Ohms and rated for 2W power connected in parallel.  The combined resistance is 51.5 Ohms, with a power handling capacity I'd estimate at around 100 Watts for a brief period, or maybe 50 Watts sustained.  The resistors could handle 350W in theory, but closely packed and without cooling thermal failure would be unlikely to take long...

The board also has a small circuit to allow connection of a DC voltmeter to measure the power dissipated in the load.


50 Ohms makes it a perfect match for the output of a radio transmitter, so it can be used for testing transmitters and related circuits under full load conditions at high power without transmitting (much of) a signal.

This has clearly been assembled by someone with care and attention to detail before being ditched before being completed (there is no evidence there was ever an input connector) so I like to think I'm putting their work to good use and completing their project for them.

I need to add a connector, and find a suitable (screened) enclosure before I can make use of it.  I also need to add a couple of resistors to bring it to an exact 50 Ohms - I suspect a couple of these are damaged or improperly connected so the resistance is just slightly high.

I suspect finding a suitable enclosure (given my complete lack of metalwork skills) will probably be the biggest challenge in bringing this into service.