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Saturday, November 12, 2016

Putting a 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.  


  1. Hi Bill,

    Bravo! Very excellent solution and implementation. I was going to suggest the use of a Pelletier Junction for the heat sink but what you have is very adequate.

  2. There are different methods. From long experience insulating 2N3055's and LM regulators, look for heatsinking kits at your parts store. There are versions for most if not all package outlines, comprising a mica flake (mylar these days?) and a stepped insulating washer. Put the mica flake against the heatsink, a dab of paste then the package/device. Options for the holding screw: insulated from package or from heatsink. Insulated from package (least capactiance): put stepped washer on screw first, then its smaller diameter should fit the package hole/s. Screw into tapped hole or through with (obtrusive) nut on 'underside'. Attached to package: drill heatsink to outside diameter of washer's inner 'step', washer under heatsink, then screw through the lot. This allows for optional tag for extra connection (above or beneath heatsink). Examine mounting of similar devices in older equipment. This provides greater safety with far less 'live' bulk.
    Good luck!

    1. p.s. best to 'sleeve' the screw (a piece of small co-ax jacket, or of a straw?).


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