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Building my dump load.

I decided to build my dump load using light bulbs. While a wire resistor based dump load is a better option, its difficult to come up with a design that could be easily copied by anyone reading this article.

Update. There IS a easy way to make wire resistor based dump loads, using heater eletments. Once you've finished reading this article, check out Building Encapsulated Resistors.

For the housing I used an old dead PC power supply. The circuit board, fan and power connectors were removed, and I spray painted the rusty old case.

Looking inside. I made a wood floor from some scrap 5mm plywood. I also serviced the fan and re-fitted it, but facing the opposite direction so it blows air into the case instead of sucking the air out. Why? Two reasons. Turbulent air is better at dissipating heat, and as I expect it to get very hot in there, I wanted the fan itself to get the first cool air. If the plastic fan was sucking the hot air out of the case, it would get soft from the heat and possibly distort or seize.

To make a connector for the bulbs, I used terminal strips with the plastic removed. The bulbs get very hot, and will melt plastic.

Each bulb was fitted with short lengths of copper wire, and then all bulbs were soldered onto two tag strips. At about this time I read the instructions on the back of the light bulb packet. It said you need to handle the bulbs with tissue paper and keep them perfectly clean, as any finger marks or dirt will form a hot spot on the glass, causing it to crack and fail.

Uh Oh.

Fact is, with these 6 light bulbs side by side, there is over 300 watts in a tiny space, a lot of heat. The fan will be blowing dusty air over them, and I cant guarantee I wont get cockroach poo on them. They will fail.



In the past I've used vehicle spot lamps as dump loads. You can buy 100 watt spot lights, but they are expensive compared to domestic 12v bulbs, as used for room lighting and down lights.

These are the light bulbs I planned to use. Each packet was worth 50 cents and contained two bulbs The bulbs were 12v 50 watt halogen.

I bought 3 packets, giving 6 bulbs, a total of 300 watts.

One of the forum members suggested I use sealed down lights instead. These have a 2nd glass envelope protecting the hot light bulb, so are not affected by finger marks or bug poo. And the larger surface area means they will be easier to cool.

These are worth about $3 each, cheap enough.

This time I went for 4 bulbs, giving 200 watts total. Not as much as I planned, but I think 300 watts was a bit too much for the little fan to keep cool anyway.

I re-used the timber board by fitting three 3/16th bolts and nuts. The timber is spaced above the metal case by about 10mm, so there is no chance the the bolts shorting to the case.

Crimp terminals were used to give a good electrical connection. Again, I've removed the plastic. The wire was crimped and soldered to give a low resistance joint.

Next Page - Putting it all together.


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