[abuhafss]

Hi

I need to attach a few TO-220 devices on a common (large) heatsink, using mica insulators and thermal compound.

Since the mica insulator is sandwiched between then device and the heatsink, I am little confused where should the thermal compound be applied:

1) between the heatsink and the insulator
2) between the device and the insulator
3) both

 

[davenn]

both sides

and don't forget the insulator insert for the hole in the tab where the screw goes through
else the screw will short the tab to the heatsink

Dave

 

[Gryd3]

I can't vouch for the accuracy, but I found this :

It makes sense to me, as the heatsink compound is there to fill any tiny air gaps between the device and heatsink.

 

[abuhafss]

Thanks, both of you.

 

[Daniel Robertson]

Do make sure that the hole in the heatsink is slightly larger than the hole in the mica washer.

Daniel.

 

[abuhafss]

Do make sure that the hole in the heatsink is slightly larger than the hole in the mica washer.

Could you please elaborate?

 

[Gryd3]

Could you please elaborate?

Take a look at the picture. Specifically where the component tab and mica insulator is.
Make sure the hole in the mica insulator is smaller than the hole on the component, or you could accidentally short he heatsink to the component on the inside of the mica insulator hole.

 

[abuhafss]

Take a look at the picture. Specifically where the component tab and mica insulator is.
Make sure the hole in the mica insulator is smaller than the hole on the component, or you could accidentally short he heatsink to the component on the inside of the mica insulator hole.

Oh sorry, I mistaken "mica washer" for "mica insulator.

 

[Gryd3]

Well. you still need a washer, like in the picture to prevent the side of the screw from touching as well as the head from touching the component tab

 

[hevans1944]

Or just use a 4-40 x 3/8 nylon screw, 4-40 nylon nut, and a #4 flat nylon washer. No need for the stepped nylon washer (although one probably came with the mica insulator) with all-nylon hardware.

 

[Daniel Robertson]

Sorry, yes the mica insulator. I built some 300 watt amps many years ago. The heatsinks were pre-drilled. The problem was that the holes in the heatsinks were slightly smaller than those in the insulator. The result was that after a while the rail fuses started to blow. The problem was arcing between the transistor and the heatsink. I drilled out the holes in the heatsink just slightly larger than the insulator holes, and as far as I am aware the amps are still running.