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Old 18th Jun 2015, 3:21 pm   #1
G6Tanuki
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Default TO-5 chassis mount heatsink?

I have a need to mount four TO-5 type transistors in good thermal-contact with a big flat-faced and large heatsink, but electrically insulated from it.

(The transistors being the thermal-sense elements of a low-source-impedance bias supply - being for a sort-of double-AN758 type linear amplifiier).

In the past it was possible to buy a cast alloy block with a very-accurately-bored hole for the TO-5 to be pressed into, which then had two smaller holes for bolts to attach the block to the bigger heatsink (with mica washer and red 'top hat' nylon insulators providing electrical insulation).

I could have sworn the ones I used 25 years ago were by Thermalloy, but looking at their website now the nearest I can find is this:

http://www.aavid.com/products/standard/1130bg

but I don't think it would necessarily have good-enough thermal conductivity to accurately (and rapidly) track the temperature of the main heatsink-block.

Any ideas? Or do I have to do a rework and use a BD135?

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Old 18th Jun 2015, 3:58 pm   #2
Top Cap
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Default Re: TO-5 chassis mount heatsink?

Farnell still do the small ones but 13 each!
http://uk.farnell.com/aavid-thermall...ink/dp/2161588

But the block ones may have been special for OEM's.
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Old 18th Jun 2015, 4:17 pm   #3
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Default Re: TO-5 chassis mount heatsink?

I don't think the ones you show would have good-enough thermal contact for my particular application. Looking at them, the amount of metal between the mounting-screw and the crimped bit the transistor pushes into looks sufficiently small that the transistor would probably track the forced-air-cooling temperature not the heatsink temperature.

The ones I used in the past came with the transistors already fitted to the blocks - and if there was an issue we replaced the block-and-transistor as one. Could well have been specially made (for a cost-plus contract you don't fret about small-batch costs).
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Old 18th Jun 2015, 4:38 pm   #4
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Default Re: TO-5 chassis mount heatsink?

Thermal glue or tape http://www.maplin.co.uk/p/thermal-adhesive-tape-a18hh ?
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Old 19th Jun 2015, 4:43 pm   #5
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Default Re: TO-5 chassis mount heatsink?

I think I may have some beefy individual fanned heatsinks for TO-5's, not really what you want though.What you describe isn't t too complex to make and a local engineering firm or college (as part of a course) may be able to knock you some up.

I noticed the Farnell parts have a delivery charge, which means they come from abroad and can't be returned; easy to miss for the unwary.

Hope you get sorted, Andy.
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Old 19th Jun 2015, 5:03 pm   #6
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Default Re: TO-5 chassis mount heatsink?

Part of the problem may be that the TO-5 encapsulation is now obsolescent. I'm not aware of any TO-5 types still in production, though obviously there are plenty of warehouse stocks.
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Old 19th Jun 2015, 5:08 pm   #7
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Default Re: TO-5 chassis mount heatsink?

Quote:
Part of the problem may be that the TO-5 encapsulation is now obsolescent.
Could you use small power types instead such as the BD136 in TO126 package?
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Old 20th Jun 2015, 12:08 am   #8
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Default Re: TO-5 chassis mount heatsink?

That sounds like an idea worth pursuing- the chip is bonded to the metal tab underneath with plastic all round to keep the effect of stray air down nicely. Some plastic sleeving on the legs might be useful, too.
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Old 20th Jun 2015, 1:08 am   #9
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Default Re: TO-5 chassis mount heatsink?

German manufacturer
fischerelektronik
and try to buy outside UK

http://www.fischerelektronik.de/en/l...8-and-similar/
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Old 20th Jun 2015, 7:41 pm   #10
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Default Re: TO-5 chassis mount heatsink?

I have some. About 1 inch diam with two fixing holes. They are red anodised with three rings turned into them.

The transistor goes in to the top and there is a insert that screws into the heatsink to hold it.

If they will do, you would be welcome to them.

Daniel.
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Old 21st Jun 2015, 12:46 am   #11
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Default Re: TO-5 chassis mount heatsink?

I have them in black as well.

I also have some of the ones shown in your post.

Daniel.
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Old 21st Jun 2015, 1:02 am   #12
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Default Re: TO-5 chassis mount heatsink?

Any transistor of the same material, similar power rating and gain as the original should be affected the same by temperature; even in an easier-to-deal-with package such as a TO-220 or TO-126 which can just be bolted straight to the heatsink with suitable insulators. Then you just have to deal with the leads being in the wrong order .....
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Old 21st Jun 2015, 7:47 am   #13
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Default Re: TO-5 chassis mount heatsink?

Low thermal resistance heatsinks for TO-5 devices just didn't work. Most of them gripped the transistor around the cylindrical part of the cap which is not well-connected thermally to the die. A few gripped them around the flange where the cap is cold-welded to the header, which is a little better.

One oscilloscope family used TO-5 packages for the Y-amplifier output stage. The transistors ran appreciable power and needed heatsinking as well as possible. They got mounted on Beryllium Oxide washers shaped like those plastic PCB mounting pads, sitting on top of an aluminium heatsink with three holes for the three wires. But BeO is best avoided.

If good thermal contact is needed for a TO-5, the place to heatsink is on the underside, right where a 4th lead would have been, because the die is right above there.

But for a bias-control sense device, there doesn't have to be much power dissipated, so the aim is to get the transistor at the same temperature as the heatsink and protect it from ambient.

I've just been looking in the attic because I remembered having some stud-mount TO-5 sinks which mount the transistor upside down with its legs sticking upwards through a hole in a cap-nut. They relied on anodised insulation. I've not found them, and I don't remember there being as many as four. I did find a new CV450, so I'll take that along to Norman's doo for the TV people.

For temperature sensing, I'd go with a tapped hole, a screw, a washer, a top-hat bush, a silicone rubber insulating washer, and I'd pick a common TO-220 transistor or diode.

David
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Old 21st Jun 2015, 10:53 am   #14
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Default Re: TO-5 chassis mount heatsink?

I used to service a lot of Neve's professional audio kit and there was a strange affinity for the heat-sinked TO5 approach- it struck me as an expensive and ineffective way to transfer heat from die to metalwork when plastic package power devices were available. I've checked for threaded cups, alumina washers etc but I obviously decided they were'nt mainstream enough to hang onto. Sorry.
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Old 25th Jun 2015, 9:05 am   #15
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Default Re: TO-5 chassis mount heatsink?

Thanks for all the suggestions - a solution has been found. A friend-of-a-friend is a modelmaker: I now have four rectangular copper blocks, each with TO5-sized holes very accurately reamed into one face, into which the transistors have been pressed (with some significant force).

Will probably be significantly better in terms of thermal transfer than aluminium!
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