Isolation washers.

[sureshot]

Just a general question, with the grey silicone isolation washers, would you use them dry, or with thermal compound ? I know with mica washers you should use thermal compound. But watching Dave Jones on the eevblog on his adjustable current source he made, the power devise was mounted dry, but in that instance no isolation washer.

If you need to isolate are the grey silicone isolation washers ok to mount dry ? I have googled this with no helpful results. Thanks for reading, any advise appreciated.

 

[davenn]

generally they are used dry, heatsink compound can be used if they are used where more heat is being generated,
have never heard anyone saying adding compound would adversely affect their operation

 

[(*steve*)]

The intent of thermal compound is to fill up the air gaps between "flat" surfaces. The soft silicone "washers" perform essentially the same purpose, so additional heatsink compound *should* not be necessary.

If you're mounting without a washer, heatsink compound is good, soldering the entire device to the heatsink is better

 

[sureshot]

Thanks for the information on this guys. Yes sure an unisolated package I would use thermal compound, and the T0220 package needs isolation, so ok I'll just go with the silicon isolation washer dry.

In this case its a model to test this emitter follower PSU, the last of my scrap CPU heatsinks, mounting a TO3 package on there was a challenge. And as the base and emitter pins go through the channel between heat fins, I had to use self tappers in blind holes to mount it. The heatsink will be at the same potential as the collector junction, so have to be careful with that. But its only a test model, and I will be using thermal compound on the transistor. Thanks for the info and help much appreciated.