Aluminum-Housed Resistor Hole Locations and Derating

The battle plan is to mount some resistors on those heatsinks to warm up the disinsector.

These seem to be the right hammer for the job:

Aluminum housed resistors

Aluminum housed resistors

The big one is rated 50 W @ 25 °C ambient. Use two, derated by 50%, times three air-cooled heatsinks for 150 W of low-temperature heating. The little one is 25 W @ 25 °C.

The derating curve is linear from 100% @ 25 °C down to 10% @ 250 °C, when mounted to a square foot of flat aluminum plate: -0.40% / °C.

Assuming a max heater ambient of  150 °F = 65 °C, you can use 84% of full power. Derating by 50% isn’t all that unreasonable.

The relevant hole locations:

  • 50 W: X=1.562 inch / 39.67 mm Y=0.844 inch / 21.44 mm
  • 25 W: X=0.719 inch / 18.26 mm Y=0.781 inch / 19.84 mm
  • 10 W: X=0.562 inch / 14.27 mm Y=0.625 inch / 15.88 mm

Divide those by 2.0 for from-the-center offsets, which may be more useful for manual CNC operations: zero at the resistor mounting center, then back-and-forth from there.

The mounting hole size for 25 & 50 W resistors: 0.125 inch / 3.18 mm diameter, just exactly what you want for a 4-40 mounting screw. Tap drill #43, clearance drill #32 (close fit) or #30 (loose fit).

The mounting hole size for 10 W resistors: 0.094 inch / 2.39 mm to fit a 2-56 screw. Tap drill #50 (better: #49 for 50% threads), clearance drill #43 (close) or #41 (loose).

The Vishay-Dale data sheet is there

  1. #1 by Erik Scott on 2010-12-13 - 11:23

    There’s just something inherently cool about a resistor that needs cooling fins. I’ve used these for a satellite dish defroster. It’s pretty neat seeing wisps of vapor coming up from the dish when it starts to snow.

    • #2 by Ed on 2010-12-13 - 12:49

      They look great, don’t they?

      That gold anodizing is the finishing touch…

  2. #3 by ozzy on 2011-01-28 - 15:44

    cool idea do you know where i might buy these resisters thanx oz

    • #4 by Ed on 2011-01-28 - 16:29

      The most recent batches came from Digikey and Mouser, but I’d expect any of the full-line distributors to carry at least one manufacturer’s version.

      Read the datasheets carefully, as not all manufacturers use the same specs!

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