# DIY Heatsink for Prototype Circuits

Proper cooling is essential for reliable and long-lived electronic devices. Unfortunately, thermal considerations are often overlooked during the prototype design stage. This engineering brief presents a simple tip to cool your TO-220 packaged semiconductors using a paper binder as a heatsink. The proof is in the thermal images shown in Figures 1 and 2. Here we can see that the heatsink has greatly expanded the surface area of the semiconductor and is only slightly cooler than the 7805 regulator. The makeshift heatsink has cooled the TO-220 regulator by approximately 60 °F.

Figure 1: Thermal image of a TO-220 packaged regulator with a makeshift paper-binder heatsink. Observe that the heatsink is slightly cooler than the regulator.

CAUTION: The makeshift paperclip is NOT electrically isolated from the metal tab of the TO-220 device. Recall that the metal tab is typically connected to the center pin of a TO-220 device. Depending on the device, this could be ground potential or high voltage. Be sure to consult the device datasheet. Also follow all appropriate local, state, and federal guidelines when working on electrical equipment.

Figure 2: Thermal image of a 7805-regulator operating dangerously close to its thermal overload limit. This image was taken under the same electrical conditions as Figure 1. The regulator is significantly hotter.

## Description of the demonstration circuit

The demonstration circuit is shown in Figure 3. Here a 7805 regulator (TO-220 packaged) is mounted on a breadboard. The regulator is powered by a B&K Precision 1550 power supply set to 7 VDC as shown in Figure 4. Two parallel connected 10 Ω 2W resistors serve as the load.

The math for this circuit is relatively simple. The parallel resistors form a 5 Ω load. Given the regulators 5 VDC output driving this 5 Ω load, we calculate the current as 1 A. Recall that the 7805 is a linear regulator consequently, the input current will be the same as the output current. Given a 7 VDC input we calculate a total regulator power dissipation as (7 V – 5 V) x 1 A = 2 W. While 2 W may not seem like a great deal of power, it is considerable when we consider the 65 °C/W thermal resistance of a regulator. The results are clearly shown in Figure 2 with the regulator hotter than boiling water.

Tech Tip: The thought process associated with circuit prototyping is not always the same as production. With the prototype circuit we can take shortcuts that would be wildly inappropriate for production of a finished product. The heat generated by the 7805 regulator featured in this article is an example as are the undersized load resistors. While this works as a demo a finished product would including larger component and heatsink to keep the operating temperature low thereby ensuring long equipment life.

Figure 3: The test circuit consists of a 7805 regulator and two 10 Ω parallel connected resistors. The regulator is driven by a 7 VDC power supply (not shown).

## Heatsink operation verified with a thermal imager

Figure 4 shows the big picture of the equipment used in this demonstration. This includes the older Fluke Ti32 thermal imager. Note that Fluke suggests the TiS55+ as the modern replacement for the older Ti32.

Figure 4: Equipment used to verify the validity of the paper binder heatsink including the circuit under test and an older Fluke Ti32 thermal imager.

The thermal imager includes a manual focusing ring that allows the thermal camera to focus on small details. The resulting pictures are shown in Figures 1 (regulator with heatsink) and Figure 2 (without heatsink). Using the Fluke SmartView Classic software the images were adjusted for the same color scale.

The results show that the makeshift heatsink provides a benefit to the regulator. The temperature is approximately 60 °F cooler when the heatsink is used. Figure 1 is especially revealing as we see the heatsink temperature is high indicating. This indicates that the heatsink has made reasonable thermal contact with the TO-220 regulator and is radiating heat to the environment.

## Parting thoughts

The makeshift paper binder heatsink is easy to use and provides reasonable cooling as shown in the thermal images. It’s a good tool to have in your bag of tricks when prototyping circuits.
Be sure to test your knowledge of the 7805 regulator and thermal properties of electrical circuit by completing the questions as found at the end of this article. As always, please leave your comments and suggestions in the space below.

Best wishes,

APDahlen

Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (partially interwoven with military experience). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing educational articles about electronics and automation.

### Highlighted Experience

With a rich background in component-level repair, Dahlen offers deep dives into troubleshooting techniques and best practices. His content is especially valuable for technicians and engineers seeking to enhance their repair skills and understanding of electronic components. A collection of similar articles may be found at in the Definitive Guide to Breadboarding Circuits.

## Questions

1. What is thermal resistance?

2. Is thermal resistance increased or decreased with the addition of a heatsink?

3. Calculate the power dissipated in the 5 VDC regulator when the input voltage is increased to 12 VDC. Assume a constant 5 Ω load.

4. With regards to the previous question, is this a reasonable operating condition for a TO-220 7805 linear regulator?

5. Locate at least 3 unique heatsinks for the 7805 from the DigiKey offerings. For each heatsink describe how the heatsink is mounted to the 7805 regulator. Also, if applicable, describe how the heatsink is physically attached to the PCB or chassis.

6. True / False: The makeshift paper binder heatsink featured in this article is electrically insulated from the regulator. If not, what is the voltage on the heatsink?

7. What is the purpose of products such as thermal grease, mica insulators, and thermal pads? Describe the impact on thermal resistance.

8. Research the term “thermal sensitivity” as applied to a thermal imager. Describe the implications in Fahrenheit for the Fluke FLK-Ti55.

9. The thermal images in this brief are uncorrected. Research “emissivity correction” and describe how Figure 2 could be misleading. Hint: Compare the emissivity of the metal tab to the regulators black epoxy.

## Critical thinking questions

1. Suppose you are producing a product that includes a resistor dissipating a continuous 2 W. What power rating would you chose for your resistor? Also, what item(s) did you consider in your decision?

2. Locate a drop-in “switching” replacement for the 7805 regulator. Calculate the power dissipation for this energy efficient substitution.

3. Why do instruments such as the Fluke Ti32 and Ti55 include an audio record mode? Hint: Consider the intended use of the instrument to perform a thermographic inspection involving many individual temperature measurements. Also, a related question involves the ability of the instrument to detect QR coded asset tags.

4. What does altitude have to do with this thermal discussion?

5. Describe the impact of heatsink physical orientation. Hint: Convection

6. Why are some heatsinks black anodized while other are bare aluminum?

7. Suppose your circuit requires a 10 second burst of current followed by an extended low quiescent period. What type of heatsink is recommended to keep cost low while keeping the circuit cool? Hint: Thermal capacitance.