ET227 Transistor: Monster Heatsink Mounting

Back in the day, heatsinks like this sat atop Moah Powah Pentium CPUs:

ET227 transistor on heatsink
ET227 transistor on heatsink

I picked it because the hulking ET227 transistor fit neatly on its backside, it seemed capable of handling 30 to 50 W of power, and I have several of them in the Big Box o’ Heatsinks. No careful thermal analysis was involved…

Mounting it on the polycarbonate sheet inside the repurposed GX270 case involved drilling & tapping a pair of 6-32 holes in one side:

ET227 Heatsink - tapping
ET227 Heatsink – tapping

That’s not rigid tapping on a Sherline, it’s aligning a hand-turned tap in the spindle bore. Sorry.

And, yeah, you’re not supposed to leave the semiconductors mounted when you’re drilling the heatsink. I figure there’s nothing I can possibly do without using a hammer that will bother that transistor in the slightest. What, me worry?

The transistor collector runs at line voltage, which means the entire heatsink will pose a lethal shock hazard. I thought about isolating the collector and failed to come up with anything I’d trust to be both thermally conductive and electrically insulating over the long term; the screw heads must be isolated from the collector plate, too.

The screws stick out below the polycarbonate sheet, just above the grounded EMI shell lining the case, so I flattened them a bit:

ET227 Heatsink - mounting screws
ET227 Heatsink – mounting screws

The simple rectangular strip to the rear of the chassis mounting clips is just slightly thicker than the screw heads, so they can’t possibly contact the case:

Chassis Clips
Chassis Clips

It gets glued to the underside of the nearly invisible sheet:

ET227 heatsink - gluing screw shield
ET227 heatsink – gluing screw shield

With Kapton tape over the heads, Just In Case:

ET227 Heatsink - mounted
ET227 Heatsink – mounted

It makes a nice linear counterpoint to the jumble of AC interface wiring:

AC Interface Chassis
AC Interface Chassis

The insulating sheet on the case lid came from the bottom of the original GX270 system board, where I think it served much the same purpose. It’s surely not rated for AC line voltages, but the thought must count for something:

AC Interface Chassis
AC Interface Chassis

More of the parts are flying in formation…

10 thoughts on “ET227 Transistor: Monster Heatsink Mounting

  1. Beeuuutiful. I have several boxes of brand new heat sink stock similar to yours (~ 30 lbs or so) in profile and about a foot long each … probably never have a use for most of it, but I can never bring myself to get rid of things like this. Pretty easy to slice up and machine the ends to any size. Built 3-4 CD racks with them many years ago and then gave them away to a friend, odd but pretty cool looking. They are still in use by him today. Looks like you might be using Anchorlube for tapping? Don’t see it very often.

    1. about a foot long

      That must ring like a bell when you saw it!

      using Anchorlube for tapping?

      Got it in one! It’s not quite a lifetime supply, but … I do as little tapping as possible these days.

      Tapping a blind hole in aluminum: I must be crazy.

      1. >Got it in one! It’s not quite a lifetime supply, but … I do as little tapping as possible these days.

        Thought so, pretty unique color.

        >Tapping a blind hole in aluminum: I must be crazy.

        I’m not sure why, I do it all the time either by hand or with a tapping head. I have tapped hundreds of blind holes in aluminum without a problem. Now it was going to be a deep hole that would be a different situation.

        1. I do it all the time

          You’re a better man than I, Gunga Din.

          When I learned that a “deep hole” is anything over three diameters, I realized that pretty nearly everything I do involves a kinda-sorta deep hole. I just twist-drill ’em and hope for the best…

        2. For what it’s worth, I think forming taps and blind holes work well together, especially in aluminum. I did break one when I moved the table several thou, forgot I had, then engaged the tap head before engaging my brain. Oops.

          Haven’t used them much, lately–I only have forming taps for 10-32 and smaller, and what little quality shop time has been more wood and biggish steel than aluminum. (Locally, I can get more lumps of steel than aluminum. Those usually have to wait for a trek over the Cascades.)

  2. I did something very similar with a big SSR. A co-worker who claimed to know more about thermal than me said that unless you have a fan that kind of heat sink may be pretty ineffective. The fins are too close for good convection. It might keep things cooler to remove every other fin. Just a thought. Even if it was only as effective as a similarly sized chunk of solid aluminum, it will probably keep the transistor cool enough.

    1. unless you have a fan


      It runs at a silent & stately 800 RPM. The third lead doesn’t provide a tach signal and doesn’t respond to any PWM that I’ve tried, but maybe the default speed will suffice to blow heat out of those fins despite the cramped quarters.

    2. I have used forming taps a few times before, they seem to work nicely enough … as long as you don’t forget they require a slightly larger hole. I’m sure they are less likely to break due to the lack of cutting flutes but I’m sure someone has done it. IIRC, it is 10-32 that is most likely and common to break due to some non-magical cross section proportions.

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