Building 420 watt Dump Load

[snowcrow]

  I started work on a 420 watt dump load today. I had a jar full of (14) Dale 5 ohm 30 watt power resistors just kicking around the shop and decided to put them to good use. Find a thin plate of aluminum that use to have 4 power resistors mounted on it for keeping a camera warm. Pictures below:









  I did get it all wired up and tested it with a old 12 volt NAPA tractor battery.

I had my "Doc Watson" connected between battery and dump load which peaked at 300 watts, 10 volts @ 30 amps. It should work fine for the Air-X and one panel.  The Air-X has only once seen 250 watts, and nothing higher. The dump load should be able to handle 350 watts continues with no problems.

  I'll be post some more pictures when I get it completed and installed.

Blessings, Snow Crow

[DamonHD]

Kewl.

Do you happen to know if the case is isolated?

And if you stay well within the ratings, is bolting to a heatsink necessary?  (I know the answer is probably "HELL, YES", but just in case...)

Rgds

Damon

[snowcrow]

  The cases are isolated, and yes, it is a good idea to have them bolted to a heat sink!! The test at 30 amps tells me I need more heat sink on top of them with a small muffin fan to help dissipate the heat. They do get hot enough that you don't want to touch them for very long, and still have skin on your fingers!!!

Blessings, Snow Crow

[Chagrin]

The resistors should be rated with a wattage without a heatsink, so no, technically it's not required. When you do that, though, the temperatures will go above 200 degrees F. Nobody likes that.

[fungus]

'The resistors should be rated with a wattage without a heatsink'

Well .. they arent - the ones I have certainly need a heatsink without which they will go over 200C and burn out ..

[ghurd]

They better be heat sinked!  And a fan too!

At 14.2V that will be over 40W in a 30W resistor.  And they need derated past 25C/77F.

Looks like at 200F ambient they are rated at 70% of 30W, or 21W.

G-

[snowcrow]

  I've already mounted a bi-metal switch on to the back plate, that will turn the fan on at 90 degrees F and off at 70 degrees F. The fan will not be part of the dump load but, connected right to the battery bank, this will allow the fan to run until the resistors cool.

Blessings, Snow Crow

[snowcrow]

  Yes I agree ghurd, as heat increases, so does the resistance and the wattage capacity decreases. The cooler you can keep them, the better off you'll be!!

Blessings, Snow Crow