TO-247 insulator from a TO-3 - diyAudio
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Old 12th April 2006, 07:35 AM   #1
JCM is offline JCM  United States
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Default TO-247 insulator from a TO-3

I could not get TO-247 mica insulators from Mouser or Digi-Key, and didn't want to use some kind of sil pad thing-a-ma-jig, so I looked at a TO-3 insulator. I had one that could easily be used, it was large. But it was the exception. Most TO-3 insulators, such as the ones I got recently, are cutting it close.

And yet I believe they will work. I bought 10, and modified them all. The last ones were best, and the first attempts were not quite good enough.

I used sharp scissors and a drill press with a 1/8" bit, drilled off-center from one of the lead holes. There is not much 'wiggle' room - if the insulators are too far to the right or left, the metal on the back of the transistor will be exposed to the heatsink surface directly.

I have cut them so that if the tops are parallel with the top of the transistor, they're aligned properly. The holes through which the screw must pass are just large enough to allow a #8 screw to pass. I used a small needle file to gently enlarge them, otherwise, the screw, when turned, engages them.

Started out with this :
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Old 12th April 2006, 07:45 AM   #2
JCM is offline JCM  United States
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The result :
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Old 12th April 2006, 07:53 AM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
a clever bit if re-engineering.

Are you able to confirm if the larger To264 will fit?

In view of the space still available is there room to rotate the To247 downwards and fit a second one in the upper gap.

What size is your No.8 thread?
I find that No.6 fits, but not No.8.

For drills I would use 3.25mm or 3.3mm or a No.30
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Old 12th April 2006, 08:10 AM   #4
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Quote:
Originally posted by JCM
The result :
As you noticed, no problems with cutting mica.
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Old 12th April 2006, 08:11 AM   #5
mzzj is offline mzzj  Finland
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Quote:
Originally posted by AndrewT


Are you able to confirm if the larger To264 will fit?
I can confirm that to264 doesnt fit on most of to3-insulators.

Finding good insulators for TO264 at reasonable price seems difficult. I have used 2pcs of to220 insulators, but thats not so great way to do it.
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Old 12th April 2006, 08:17 AM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
To247 pads and micas fit To264.

But you do need to centralise them accurately without a twist(rotation).
There is just enough land to see the pad hanging out all round the metal conductor .

I found that sticking the pad to the transistor made it easier to keep the registration while fitting.
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Old 12th April 2006, 08:23 AM   #7
JCM is offline JCM  United States
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I'm not sure about TO-264 - I would guess that wouldn't work - but not sure.

There is only room enough for one transistor per insulator - and that was cutting it close.

The screw was #8, 24 thread. Of course, a 32 thread would work also.

The 1/8" bit seemed to be the perfect size - all I had to do was file it a wee bit with a little needle file.

---------------

No, no problem cutting it at all.

I was surprised to be able to get away with drilling - but it is possible also.

I could almost swear Mouser and Digi-Key had the TO-264 - but not sure. They seemed to go out of their way NOT to have the TO-247, though.

I would happily buy a a TO-247 from Mouser or Digi-Key if I could find one there - I just don't want to use some sil-pad.
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Old 12th April 2006, 08:47 AM   #8
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JFTR:
there are 4" x 4" mica sheets, which are even very economical.
I've used those sheets to make MT200 style insulators for Sanken and Toshiba ring-emitter devices, as readymade insulators could not be had overhere or were pretty expensive.
Punching holes for screws/bolts is also quite easy.
Finding aluminium oxide insulators for MT200 is much more difficult.
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Old 12th April 2006, 10:52 AM   #9
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http://www.bokkiwokki.com/index.php?...angset&lang=GB
http://www.bokkiwokki.com/index.php?...roduct&id=8613
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Old 12th April 2006, 11:09 AM   #10
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http://www.allelectronics.com/cgi-bi...CA_SHEET,_2.55"#34;_X_1.77"#34;_.html

All Electronics has 2.55 x 1.77" sheets at 6/ $2. They are a surplus house so they might not have many, though. They have a dozen less than they did yesterday
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