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Old 16th April 2005, 11:36 AM   #1
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Default How to isulate screws for to-220?

Maybe a silly question, but I did the silly thing:
I used mica between to-220's and heatsing leaving screws unisulated. Luckily I realised the problem before running the circuit. So, any ideas? I don't have plasic screws...

thanks in advance
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Old 16th April 2005, 12:06 PM   #2
Mark Kravchenko --- www.kravchenko-audio.com
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Default Been there and done that

Use an insulateing washer between the screw and the top of the TO2220 tab. A better plastic would do. You can even cut out a little washer. But be carefull to pick a plastic that does not have to low a melting point like polyethelene. In a pinch you could use a double layer of mica that you have pierced with the proper size hole.

There are fancy bushings that fit in the hole on the tab and allow a screw to pass through all nicely insulated to.

http://www.wiltronics.com.au/catalogue/shop.php?cid=137

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Old 16th April 2005, 06:12 PM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
have a look at your To220 case to see if it is already insulated.
Some are insulated and look the same shape but have a different case style code.
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Old 17th April 2005, 10:53 AM   #4
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Quote:
Originally posted by AndrewT
Hi,
have a look at your To220 case to see if it is already insulated.
Some are insulated and look the same shape but have a different case style code.
No, it isn't. Middle pins of power transistors are shorted when not isulated.

thanks mwmkravchenko

I'll try this.

best regards
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Old 19th April 2005, 11:41 AM   #5
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You can use nylon machine screws, and nylon nuts if necessary. You can't apply as much torque to them, but they work ok.
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