Variable Resistance Load Bank

Emphysys Inc.

Relevant Skills: thermal simulation, designing for assembly, fan flow analysis, soldering

Objective

Design and produce a load bank capable of dissipating RF power at designated resistances. Load bank needed to simulate human impedance to allow for medical device testing without organic subjects.

Variables / Constraints

The load bank needed to be portable, provide several different resistance paths, and dissipate 300W of power without any one resistor exceeding 100°C. The assembly should include instructions that make mass production easy for the client.

Design Process

[1] I began by performing equivalent resistance calculations to develop a resistor layout that fit within the footprint of a chosen heat sink.

Top view showing chosen resistor layout

[2] I used thermal simulations to validate the resistor layout, determine heat sink material (aluminum), and select cooling fans. Fan dimensions and flow curves were incorporated in the analysis. I created models of each resistor in SolidWorks based on drawings and material descriptions.

Section view highlighting heat conduction from resistors to heat sink

Configuration of maximum energy dissipation per resistor

[3] Once a resistor layout was validated, I added mounting features to the heat sink and built out the assembly to include a Delrin clearance spacer, sheet metal standoff bracket, and acrylic safety shield. I soldered magnet wire to form connections between resistors.

Sheet metal standoff adapter plate

Heat sink with custom mounting features

Acrylic safety shield to isolate resistors

Delrin clearance spacer to keep resistors from contacting heat sink

[4] After assembly was completed, the load banks were tested under full load, which validated the thermal simulations and even allowed the fans to be swapped for lower power, quieter models.

Final assembly - isometric, top

Final assembly - isometric, bottom

Solution

Once the initial model was built, I created assembly instructions and tested the ease of use by constructing several more units. The end product satisfied the proposed requirements, even exceeding the cooling threshold by approximately 35°C.

Assembled load bank - white labels indicate connection points for different equivalent resistances

Top view highlighting magnet wire pathways through resistors

Back view of load bank showing additional resistance paths