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Best type of insulator pad for a to-220 LM317 power supply?

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george graves:
I'm thinking of building a power supply using a lm-317.  The tab is connected to the output pin.  So I'm assuming it would be a good idea to insulate that from the heat sink that will be sticking out of the case?

I see there are silicon( or "silica") pads, and "Mica" pads.  What performs better?  What should I use?

Mica is generally better under significant heat,

Silpads are the easiest to use and the most repeatable.  Mica is good but suffers from poor thickness and flatness control, and its performance depends on how well you apply the thermal grease.  I would generally use a sil-pad unless I was really pushing the edge.  In that case I would probably go straight to an aluminum heat spreader insulated from the main heat sink by kapton film.

Silicone is easier, it doesn't require separate thermal interface grease, whilst mica does.

When I built an LM317 PSU, I soldered the heat sink directly to a piece of copper bar which was fixed to the metal case, for insulation I used plain old Sellotape which worked perfectly.


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