How to isulate screws for to-220?

darkfenriz · 16th April 2005, 11:36 AM

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

mwmkravchenko · 16th April 2005, 12:06 PM

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

MArk

AndrewT · 16th April 2005, 06:12 PM

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.

darkfenriz · 17th April 2005, 10:53 AM

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

jbateman · 19th April 2005, 11:41 AM

You can use nylon machine screws, and nylon nuts if necessary. You can't apply as much torque to them, but they work ok.

16th April 2005, 11:36 AM	#1
darkfenriz diyAudio Member Join Date: Jun 2004 Location: Warsaw	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

16th April 2005, 12:06 PM	#2
mwmkravchenko Mark Kravchenko --- www.kravchenko-audio.com diyAudio Member Join Date: Jan 2002 Location: Perth Canada	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 MArk __________________ Mark

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16th April 2005, 06:12 PM	#3
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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.

19th April 2005, 11:41 AM	#5
jbateman diyAudio Member Join Date: Oct 2000 Location: The People's Republic of Maryland	You can use nylon machine screws, and nylon nuts if necessary. You can't apply as much torque to them, but they work ok.