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need to electrically isolate LM317 TO-220 heat sink

need to electrically isolate LM317 TO-220 heat sink

(OP)
I need to electrically isolate the heat sink for a LM317.  I don't want the heat sink to have the voltage on it (potential short hazard for my application).  

I believe that I can get a thermally conductive electrically isolating tape or pad for between the tab on the LM317 and the heat sink, but screwing it together with a metallic screw will defeat the purpose of the pad.  So I could use a nylon screw but it may melt.  Is there a better way?

Tracy Blackford
Comm and GPS Systems Engineer

RE: need to electrically isolate LM317 TO-220 heat sink

You are absolutely right - do not use a nylon screw. You can't tighten it without starting creep and after some time, you will have lost contact pressure. Some of those nylon screws also break (stress/fatigue/thermal cycling) after a few years. There are clips that press against the plastic body of the component. Those can usually be bought together with the heat sinks. Much better.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: need to electrically isolate LM317 TO-220 heat sink

You are supposed to use a heat sink that has the correct hole size to accept a nylon shoulder washer.  Then you use a steel screw, lockwasher and a nut. (I prefer Kepnuts)

The shoulder sits on the back of the HS.  The reduced part of the washer drops into the back of the HS hole and provides the centering. The screw goes thru the TO220 touching it.  It then passes thru the center of the shoulder washer - hence not touching the HS.  Out the back of the washer.  You run your nut onto the screw. It can't touch the HS because it's on the nylon.

Of course you have an insulating pad under the TO220's tab.

Keith Cress
kcress - http://www.flaminsystems.com

RE: need to electrically isolate LM317 TO-220 heat sink

You need to use a thermal pad and a shoulder washer. Nylon sholder washers are common, but, as you mention, there is the potential heat problem. For TO-220, the screw size is typically a #4-40 (english.

When the heat is higher, you need to use a phenolic sholder washer, or something like a Polyphenylene Sulfide shoulder washers (PPS plastic).

http://www.aavidthermalloy.com/products/standard/access/washers.shtml

TO-220 size PPS are available from US distributors like Mouser. I don't recall which one on the link is the correct one, but it's one of the two in stock at Mouser cause I currently using it in a design.

When dealing with a hot part, the common grey silpad type insulator may not be suitable. In which case, you use a silpad capable of the temperature. Old fashion mica meets the temperature, but the thermal resistance of mica and grease will only make the part run hotter still, and possibly beyond the parts ratings. Bergquist K10 material which is Kapton based has one of the highest temp rated insulators, has an excellent thermal conductivity, and is available at Digikey. Bergquist also makes other materials that handle high heat that may be better for your application (cost). K10 is what I can think of off-the-top-of-my head, but its more expensive and it's coating easily scratched if not handled carefully.

RE: need to electrically isolate LM317 TO-220 heat sink

STMicroelectronics has an isolated package lm317p.
 

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