PHYS1630: Thermal Control Systems
Lab #2: Heatsinks (2018F)

Introduction

In this lab you will measure the thermal resistance of a heatsink and component mounting as well as predict temperature of components cooled by a heatsink

Pre-Lab (Do this before your assigned lab period)

Hand-in the following (worth 10% of the total lab mark):

Calculate the expected temperature of both the heatsink and resistor mounted on a heatsink. The resistor (A Stackpole KAL50FB25R0) is mounted onto a heatsink of known thermal resistance (Wakefield 394-2AB) using thermal grease (Wakefield type 120). Assume an ambient temperature of 20C, convection cooling only (i.e. no airflow), and a power dissipation of one-half the maximum of the resistor. You will, as part of this prelab, be required to inspect the datasheets for each element.

The prelab assignment is worth 10% of the total lab mark and is due at the beginning of the lab period. Late marks are not assigned if the prelab is not received at the beginning of the lab: you lose 10% of the total lab marks immediately with no recourse if it is not received upon entering the lab (extensions will NOT be given to "print it out" in the lab ... be prepared with the hardcopy already printed).

Equipment

Heatsink with Resistor
The heatsink with resistor mounted on top. For this experiment connect a jumper lead to each resistor terminal and run to a 25V/1A power supply.

Heatsink with Resistor and Fan
The same heatsink/resistor combination with fan cooling to decrease thermal resistance.

IR Thermometer
The IR Thermometer used to measure temperatures in this experiment. Pressing the "Emissivity" button causes the "Emissivity Select" indicator to appear. Press the UP and DOWN buttons when this indicator appears to increase/decrease the emissivity to correct for various materials.

Heatsink Setup
The heatsink/resistor cvombination as configured for this experiment. One power supply provides current for the resistor and the other is set to 5V to run the cooling fan. During stabilization periods (i.e. while waiting for the elements to warm) be sure to close the plexi doors on the enclosure to inhibit air currents.

The Experiment

FIRST, calibrate the emissivity of the devices (the resistor and the heatsink) for the IR thermometer at room temp. The black anodized heatsink is known to be 0.95 however the emissivity of the gold anodized resistor is unknown (although it is likely between 0.60 and 0.95). Before applying power to the resistor, measure the temp of the heatsink with the IR thermometer and then that of the resistor. Record these two readings. Now adjust the emissivity of the thermometer and take another reading of the resistor (it should be closer). Again, record the temperature reading of the resistor. Adjust the emissivity lower, methodically (recording the temperature reading and emissivity values each time) until the temperature reading for the resistor is identical to that for the heatsink recorded earlier and note the final number for the resistor which you will now use for the rest of the lab.

SECOND, place the heatsink flush against a piece of insulating material (e.g. wood or a textbook), oriented as shown in the above photos (with the fins facing downward). Apply a current of 1A to the resistor (which will then dissipate power). Record the current and voltage. Allow the temperatures of the resistor and heatsink to stabilize (this takes over ten minutes) then record both those as well (making certain the correct emissivity is set when taking each reading as the heatsink and resistor will have different values, determined in the first step).

THIRD, support the heatsink on the edges (which each have threemounting holes) at least 5cm from the bench to allow air to flow underneath naturally, allow the temperatures to stabilize then measure them both again.

FOURTH, add a small computer-type fan to force air past the heatsink, allow the temperatures to stabilize then measure them both again.

Calculate the thermal resistance of the resistor-to-heatsink and the heatsink-to-air in each case above. The total thermal resistance is the sum of these two. These values will be required in future labs in this course.

Lab Report

For this experiment, an abbreviated lab report is required (word processed, never hand-written) with the same format as PHTN1300. Answer each question in the form "4a., 4b., 4c. ..." with each new question (#4, #5, etc) beginning on a new page. Do NOT answer an entire question (e.g. question 4) as a single paragraph without identification of sub-parts ('a', 'b', etc). Submit the lab report in a bound folder NOT simply a pile of loose, stapled papers nor a thick binder!

Each student must submit a unique lab report - no portion other than the results must be shared between lab group members.

  1. Calculate the thermal resistance of the complete system (i.e. resistor to air) each of the three conditions listed in the lab based on observations. We expect these to be significantly different. Show all calculations and values used.
  2. Calculate the thermal resistance of the resistor-to-heatsink for each of the three conditions listed in the lab (since you observed the resistor and heatsink temp for each case). We expect these to be similar. Show all calculations and values used.
  3. Calculate the thermal resistance of the heatsink-to-air for each of the three conditions listed in the lab. Show all calculations and values used.
  4. According to the datasheet for the resistor, it can operate at an elevated temperature but at reduced power. Assuming the resistor will only ever operate at one-half the maximum rated power, what is the minimum heatsink thermal resistance that may be used? Show all calculations.
  5. Another possibility (aside from operating at reduced power) is operation at elevated ambient temperatures. Assuming a "properly" mounted heatsink with ample air space, what is the maximum ambient temperature for operation at one-half the maximum power dissipation? Show all calculations.