4 Thermal Considerations

There are a number of considerations to take into account when designing such a resistive load with regards to its power and thermal ratings. The resistors on the resistor plate will be the hottest components when applying a load to a voltage source. The thermal limit of the components will set the primary limit of how much current the motorized resistive load can handle. The resistors selected for this design are rated for a maximum temperature of 155°C and a power rating of 50 W. The resistors will generally hit their temperature limit at a power level under their maximum power rating. The power rating generally applies when the resistor is sufficiently cooled via heatsinking, or active cooling. Thermally-optimized layout can generally help cool the resistors, but not as effectively as a heatsink or active cooling. If the resistor is not heatsinked properly or actively cooled, then the power rating is derated.

Given the series connection of the load resistors, a different number of resistors will form the load for a given voltage. For example, to load a 1-V voltage rail with 10 A, the motorized resistive load needs to ensure that the resistance between the load terminals is 0.1 ?. If the leads and arm are assumed to have a resistance of 0.05 ? then the voltage will be applied over a single 0.05-? resistor. The temperature and power rating can then be compared against the ratings of this resistors. In this example, the temperature rises to 100°C and stabilizes around that point without active cooling as shown in Figure 4-1. The power dissipated in the resistor is the product of the applied voltage over the resistor by the current through it as shown:

In this case the temperature and power rating of the resistor are not violated, but if the voltage were increased to 3.3 V, then the necessary resistance to apply 10 A would be 0.33 ?. On this design that translates to 5 resistors, if we continue assuming that the arm and leads are 0.05 ?. Now the power dissipated per resistor is less, and so higher currents may be applied, but if the load is held continuously then the temperature will rise due to the increased heating of adjacent resistors. The thermal image shown in Figure 4-2 demonstrates this effect, as the resistors continue to heat up after applying the load for less than 5 seconds and do not stabilize at an acceptable temperature. This example illustrates two main points: active cooling is required for higher current capability, and assuming only one resistor value is used for all resistors, then the power dissipated by each resistor is: