# amp mosfet contact to heat sink?



## pwnt by pat (Mar 13, 2006)

I'm working on some old Fultron Big Belle amps that have pads between the main power mosfets and heat sink. Do the backs of the mosfets need to be isolated or am I okay removing the pads and putting the mosfets in direct contact with the heatsink?


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## TrickyRicky (Apr 5, 2009)

Did it have that yellow film? Or did it had any contact before? Put it back like it was and check it with a multimeter to see if it had any contact before.

If it did you might be able to get away with out it, just aslong as you use the right thermal compound and make sure its clean (no burrs or debri).


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## dragonrage (Feb 14, 2007)

Transistors should always be isolated from the heatsink. Mica is often used for that.


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## spooney (Nov 5, 2010)

Depending on the package type of the particular semiconductor you might not need an insulator behind it. Some parts are fully encapsulated and don't need to be isolated from the heatsink. Most are not and have a metal backing that if it comes into contact with the heatsink will cause various failures in the amp. If there was an insulator of some sort originally though you most certainly need to replace it. Mica insulators,Kapton/Polymide tape or film,and silicone insulators all work great. Just pick the one that best suits your needs. There is a store on ebay that sells the kapton film really cheap and you get a ton of it. I use it in just about every repair I do although it is a pain in the butt to cut to size at times. Some good kapton tape/film and some heatsink compound and you are good to go. Make sure the heatsink and the transistors are free of any debris and or burrs. Any little metal shaving that pokes through that film can cause a short and destroy the mosfets.


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## aho77 (Jun 8, 2009)

you will have to put heat sink compound under it or the mica the transistor should never touch the heat sink this will cause a short in the transistor and you will blow them and taking the amp apart some times lets little peaces of metal to get behind the transistor when reinstalling the board and this also will blow them


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## TrickyRicky (Apr 5, 2009)

In my case the transistors are T03's metal case and are screwed down to the heatsink and the heatsink to the chassis, explain to me why mine havent blown up.


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## minbari (Mar 3, 2011)

aho77 said:


> you will have to put heat sink compound under it or the mica the transistor should never touch the heat sink this will cause a short in the transistor and you will blow them and taking the amp apart some times lets little peaces of metal to get behind the transistor when reinstalling the board and this also will blow them


that is not 100% true. only tab hot devices need to be insulated this way. if the tab is ground, then you can mount it directly.


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