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Electronics => Beginners => Topic started by: george graves on February 17, 2012, 08:36:27 am

Title: Best type of insulator pad for a to-220 LM317 power supply?
Post by: george graves on February 17, 2012, 08:36:27 am
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?
Title: Re: Best type of insulator pad for a to-220 LM317 power supply?
Post by: Rerouter on February 17, 2012, 08:43:13 am
Mica is generally better under significant heat,
Title: Re: Best type of insulator pad for a to-220 LM317 power supply?
Post by: ejeffrey on February 17, 2012, 08:59:17 am
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.
Title: Re: Best type of insulator pad for a to-220 LM317 power supply?
Post by: LukeW on February 17, 2012, 09:42:36 am
Silicone is easier, it doesn't require separate thermal interface grease, whilst mica does.
Title: Re: Best type of insulator pad for a to-220 LM317 power supply?
Post by: Zero999 on February 18, 2012, 03:32:14 pm
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.
Title: Re: Best type of insulator pad for a to-220 LM317 power supply?
Post by: Mechatrommer on February 18, 2012, 09:55:24 pm
Quote
plain old Sellotape
if you talking about that transparent plastic shiny sellotape, extreme heat will screw it up, electrical tape is better heat resistant. if the op dont have a screw and nut to stick to the heatsink, my way is silicone that people used to seal aquarium (in tube) in between any insulation used. or the "elephant" glue, not sure what its called, but usually it has elephant picture on the bottle and if you put one drop between your fingers and put together, the fingers will stick each other within seconds.
Title: Re: Best type of insulator pad for a to-220 LM317 power supply?
Post by: Rerouter on February 18, 2012, 10:19:49 pm
elephant glue sounds like superglue, and really, you never want to heat superglue over 75 degrees, parts of it vapourises, and it burns your eyes and nose even hours later :/
Title: Re: Best type of insulator pad for a to-220 LM317 power supply?
Post by: Zero999 on February 19, 2012, 02:42:14 am
Quote
plain old Sellotape
if you talking about that transparent plastic shiny sellotape, extreme heat will screw it up, electrical tape is better heat resistant.
I never had a problem, partly because the LM317 was already soldered to a piece of copper bar which spread the heat more evenly. Oh and electrical tape will be worse because it's normally made of PVC which has a lower melting point than cellophane or polypropylene which is what Sellotape is made from.