insulation from heatsink help please

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I think I just had one amp fry because the TO220 made contact with the heatsink (its trashed so I'm not sure). I have an identical amp that I used to diagnose the first. When I bolted it all together and tested it, it started to dump current in a big way. I took it apart again but couldn't find a fault. I used all the best parts to rebuild and it seems ok now. I wonder if I had over tightened the fixing screws. I'm scared to death that it might all be down to "feel".

What failsafe precautions can I take to make sure it stays insulated?
 
Minion has a good idea and have seen this done before with good success.

You will need a fairly heavy aluminum bar to clamp the transistors to the heatsink. The bar cannot bend or deflect when fastened, or the thermal transfer of each transistor to the heat sink will not be equal.

I've had VERY good results using Bergquist Sil-Pads. Excellent thermal transfer and electrical insulating. No goop or mess. You can also trim them with a scissors.

Here's a link to one of the to-220 patterns on Digikey. http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=BER107-ND

-David
 
Hi,
ensure the burr around the heatsink hole is removed.
Ensure the To220 face is flat.
Use an insulating top hat washer between the bolt and the To220 tab.
Ensure that the heatsink to tab insulation is undamaged and in the correct position while tightening up.
 
"You will need a fairly heavy aluminum bar to clamp the transistors to the heatsink. The bar cannot bend or deflect when fastened, or the thermal transfer of each transistor to the heat sink will not be equal."

U-shaped channels can be used too. I've seen steel used in an amp, should work nice as it is harder than aluminium. This was also an U-channel. There isn't really much to be gained by trying to extract heat through the top of the transistors so it won't matter that it has worse thermal performance.
 
Pbassred said:
I think I just had one amp fry because the TO220 made contact with the heatsink (its trashed so I'm not sure). I have an identical amp that I used to diagnose the first. When I bolted it all together and tested it, it started to dump current in a big way. I took it apart again but couldn't find a fault. I used all the best parts to rebuild and it seems ok now. I wonder if I had over tightened the fixing screws. I'm scared to death that it might all be down to "feel".

What failsafe precautions can I take to make sure it stays insulated?


1/ Make sure there is absolutely no burr around the holes.
I always countersink my holes to be 100% sure.
2/ Tighten down the bolt holding the transistor until tight but not until it gets close to stripping the threads.
3/ Use flat metal washers to spread the load with a locking washer on top of the flat washer.

4/ Dont use too much thermal paste as it can hydraulic and after a while the pressure goes and can loosen the bolt holding the transistor ! Spread it thinly.
 
megajocke said:
I'd use those top hat bushings, metal screws and compression (bell?) washers to get higher mounting pressure and reliability.

MJL: Thanks for the tip! Saw a picture of someone who did that for TO-247 but didn't think they would be big enough for TO-264. :)


I use the IRFP240/9240 and they have insulated bushes in the transistors.
 
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