The mounting kit for the LM317 (TO-22 package) contains a mica sheet for isolating the transistor from ground. Should this sheet go between the transistor and the heat sink or between the heat sink and the chassis?

rrman75

 

Since the tab is connected to Vout, then clearly the tab cannot be allowed to come into contact with chassis ground (or any other potential). If the heatsink is electrically connected to ground (or any other potential except Vout), then the mounting kit is used to isolate the tab from the heatsink. Note that the bolt used to mount the LM317 to the heatsink must not short the two. Use the small Teflon shoulder washer supplied in the kit to prevent that...

 

Note that both sides of the mica must be covered with a thin coating of thermal grease otherwise the thermal impedance will be much higher than expected.

 

The mounting kit for the LM317 (TO-22 package) contains a mica sheet for isolating the transistor from ground. Should this sheet go between the transistor and the heat sink or between the heat sink and the chassis?

rrman75

The LM317 is not a transistor it's a voltage regulator. And it's a floating regulator which means the tab must be fully isolated from the heatsink it if touches anything else.

 

Note that both sides of the mica must be covered with a thin coating of thermal grease otherwise the thermal impedance will be much higher than expected.

Key word THIN layer of thermal grease. Too thick actually does more harm than good.....

 

You can save yourself an immense amount of hassle if you use the LM317P, which is in a TO-220 package with an all-around plastic coating. The plastic must block heat transfer to some extent, and I don't know what the thermal characteristics are, but it's designed for the job, so I assume it's not too bad. No mica, no little Teflon pieces, no grease--I use them all the time.

http://www.mouser.com/Search/ProductDetail.aspx?qs=swDD%2bF%2bps7e6xZSe3iJsFg==

 

FWIW -- Pictured, is the heatsink arrangement for a parallel gang of 3 LM 317 adjustable regulators. This was to boost the output of the supply to 4 amps to drive Electronic flash units...

It has served me well the last 25 years, and continues to do so

 

You can save yourself an immense amount of hassle if you use the LM317P, which is in a TO-220 package with an all-around plastic coating. The plastic must block heat transfer to some extent, and I don't know what the thermal characteristics are, but it's designed for the job, so I assume it's not too bad. No mica, no little Teflon pieces, no grease--I use them all the time.

According to the data sheet the TO-220FP insulated case has a thermal resistance, junction-to-case, of 60°C/W as compared to 50°C/W for the uninsulated TO-220 package. But you still want to add a thin coating of thermal grease to minimize the thermal resistance between the case and heat-sink. Otherwise the thin layer of air in the tiny gaps between the case and heat-sink can greatly increase the temperature difference between the device and heat sink under high power dissipation conditions.

 

The mounting kit for the LM317 (TO-22 package) contains a mica sheet for isolating the transistor from ground. Should this sheet go between the transistor and the heat sink or between the heat sink and the chassis?

rrman75

About the heat conduction of mica sheet, you can see here.

 

According to the data sheet the TO-220FP insulated case has a thermal resistance, junction-to-case, of 60°C/W as compared to 50°C/W for the uninsulated TO-220 package.

That's the junction to ambient rating which doesn't apply for use with a heatsink. Junction to case numbers for TO-220 is typically in the 4 - 5 C/W ballpark.

Not sure about the plastic coated ones but I don't think it is a whole lot worse.