DIY Heatsink for Small Transistors

Here's a little mini-instructable:

Want to squeeze a bit more current through those inexpensive TO-92 package transistors? Then add a small metal heatsink.

I made this for a PWM DC motor driver, as some 2N2222 bi-polar transistors were handy. It worked OK, but the 2N2222 was getting very hot (too hot to touch.)

This will work with any TO-92 device--but the device must have a flat part to contact the sink (as TO-92 cases do.)

It's not entirely crazy; commercial heatsinks are available for this package. And the 2N2222 specs include two power dissipation ratings, Tamb <= 25 C (500-800mW) and Tcase <= 25 C (1.2-1.8 mW) (being ambient air temperature and case temperature.) Keep the case at 25 C or below, and the current rating more than doubles.

Materials:

-- Heatsink material: copper, aluminum or other sheet metal
-- Heat shrink tubing
-- Thermal paste compound (for CPU heatsinks)

Tools:

-- A nibbler (or tin snips)
-- Files and sandpaper

The nibbler tool is a great way to cut shapes from any sheet metal material, even steel.

The heatsink should be large enough to soak up excess heat, but can be almost any shape. However, it must have a "tab," slightly wider and taller than the transistor.

It can contain a hole for attaching to a circuit board, if that's desired.

In order that it doesn't fall right off, the heatsink should be shaped by adding a few notches or a narrower "throat" at the top of the tab.

This prevents the "tab" from sliding out of the heat shrink tubing, and off the transistor.

Note: to be honest, tapering the "throat" at the top of the tab seems to work better....the drawing illustrates this alternate method (which I used on the prototype.)

The heat sink must be flat. The best way to achieve this is not to deform the metal during the process.

However, my aluminum was a nabbed from an old camping plate, and the tin snips deformed it somewhat. So, in order to flatten it for good contact with the transistor, a few more steps:

-- Press it. I used the end of a file handle. But using a good vice, perhaps between two pieces of flat steel would work better.

-- File it. Where the material resisted, filing took out the high spots.

-- Sand it. After filing, a smoother surface is need for full contact.

-- First, cut a small piece of heat shrink tubing. It should be slightly longer than the tab.

-- Test fit all the parts.

-- Apply a small amount of the thermal paste to the transistor (flat side.)

-- Slip the heat shrink tubing over the metal tab, then carefully slide the transistor in, being sure the flat side contacts the heatsink.

-- Shrink the tubing to complete. A heat gun, electric stove or other heat source will do.

During initial use, the heat generated by the transistor will only shrink the assembly more, making a more solid unit.

Ok, now the transistor / heatsink is ready for use.

I've been running mine for hours at about 2.75 watts, which is about 65% over the wattage specs for a 2N2222. So far, so good.

Note: While this certainly helps, the TO-92 package wasn't designed for use with a heatsink, so you cannot get the type of efficiency as you would from an integrated sink.

Maybe the right thing to do was to use a TO-220 package transistor, but this was fun and a learning experience, too.