Saturday, November 12, 2016

Putting a Larger Heatsink on the BITX40 Module

The fan that I installed yesterday was driving me nuts.  It was noisy, both acoustically and electrically.  And I would occasionally get my fingers in the blades.  Not good.  While it did seem to keep the IRF510 from getting too hot, I knew that a real heatsink would do better thermally.

But how was I going to attach the sink to the transistor?  That tab on the IRF510 goes to the collector, so if it touches a grounded heat sink, you get a short.   A nylon screw and some mylar between the transistor tab and the heat sink is one option.  But I didn't have a nylon screw.  So I decided to just keep the heat sink electrically insulated from the chassis.

This project required me to refresh my memory on how to tap a 4-40 hole.  I went back and watched the short video I made on the tribal knowledge that Pete had shared with me.  Out came the Tap and Die gear and the machine oil.  The process went very smoothly.

Here is what I did to get the heatsink in place:

1) After removing the original heatsink, I gently bent the leads on the IRF510 so that the transistors outer edge would be flush with the edge of the PC board.
2) I put a strip of thick tape (Gorilla Tape) along the lower side of the heat sink. This will keep the heat sink from shorting to the chassis.
3) I placed the heatsink where I wanted it, and carefully marked where the mounting screw (through the transistor's tab) should go.
4) Drill!  Tap!  (see video)
5) I applied some heat sink compound (or Desitin!) and then attached the transistor to the heatsink.
6) I put a few drops of glue between the heatsink and the board and the chassis, just to mechanically stabilize it a bit.
7) Bob's your uncle.

It seems to work great.  The MOSFET stays cool. even after long "old buzzard" transmissions.  And I notice no stability problems.  It was fun  to put to use some tribal knowledge and refresh a mechanical skill.  

73 Bill N2CQR


  1. I found that with some jiggling (for testing purposes only, of course) the heat-sink would short, for some reason, even though it's aluminum oxide. I went ahead and put in the usual mica separator & washer. No issue with it now.

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