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.
Step 1: You'll need...
-- Heatsink material: copper, aluminum or other sheet metal
-- Heat shrink tubing
-- Thermal paste compound (for CPU heatsinks)
-- A nibbler (or tin snips)
-- Files and sandpaper
Step 2: Cut out the Heatsink
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.
Step 3: Fine-tune the Shape
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.)
Step 4: Press, File and Sand until Flat
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.
Step 5: Assemble
-- 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.
Step 6: Use it!
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.