4 channel dummy load

       This is a project born out of necessity. I need some sort of high power dummy load for testing amps that I build or repair. High power aluminum cased resistors are pretty expensive and hard to find here so there has to be a cheaper option.

       Inspired by Rod's Project 124, I decided to build myself another one.

       The first one I had from many years ago used 15x 56ohm 10W resistors in parallel mounted on blank copper clad boards with binding posts (pic somewhere below) to get a stereo 3.7ohm load but back then I didn't know any better and packed them all tightly which gave poor results regarding power dissipation. I also needed a 4 channel load so it was time to build a new one.

       Although Rod's project suggested using resistors in a series-parallel configuration, I decided to use an all parallel configuration to make assembly easier. I basically used ten 39ohm 10W resistors in parallel to get 3.9ohms total and then build 4 sets to make a 4 channel 100W/ch, 400W total dummy load which can probably take more by using better cooling. Assembly is done using aluminum angle bars and lots of screws. Take a look at the pics below to see how I built mine.

Here are the materials fresh from cutting with the bandsaw. Aluminum angle, blank PCB and perspex offcuts and 40pcs 39ohm 10W resistors.

Since I'll be drilling a lot of holes and will be doing it eight times, a template is necessary.

I forgot to take pics while drilling so here's the finished product, drilled and self tapping screws put in.

Using the blank PCBs as supports (copper cut in the middle to prevent shorting the load together)
You can see one finished with ten resistors mounted.

All four done.

The perspex/acrylic is cut and drilled to act as supports to keep the four channels together
but still insulate and allow a lot of airflow in between.

Another shot from the side.

My old dummy load on top of the new one.

Since I'll mostly be testing car amps, I used wires for the connections.
Solder tags are added so that I have somewhere to clip the oscilloscope probes to.

The phenolic offcuts weren't as strong as I had hoped so I made new supports from FR4 material.
Here it is being drilled on a jig to make things faster.

Another shot of the jig which is basically a wood plank with screws to hold the material in place while drilling.

All done and ready.

And installed.

I also added solder tags at the back for more places to attach clips to.

It can also be placed horizontally if needed.

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