Electronics LM317 question

Hey all,

I have a question on the LM317. I'm putting one in an aluminum project box and would like to use the actual box as the heat sink. As I undertand it from the data sheet, the heat sink on the lm317 (TO-220 packaging) is connected to the output pin. So I'd effectively be applying my output voltage to the project box. why the hell do they tie the heat sink to the output pin and how can I insulate it so I dont have a voltage potential on the project box?

thanks!

 

What voltage are you trying to get, if you can get a fixed regulator, typically the tab is grounded. Or you could get a sil-pad and a nylon bolt and have the heatsink isolated from the case electrically. If you have an old PSU around you can probably pull a sil-pad and a insulating washer from that.

 

Thanks for the quick response.
what is this sil-pad you speak of? can they be purchased seperately?

EDIT: Found data sheets and multiple suppliers of this sil pad stuff. Problem sorted, thanks!

 

Just for reference later incase someone else has a similar problem, a sil-pad is a thermaly conductive piece of silicone that is used as an insulator to prevent a heatsink from becoming live. You either use a plastic/anodized nut&bolt or a regular nut&bolt with a washer/sleeve that prevents the tab on the case from touching the bolt.

 

Just to add to ConKbot's reply... Instead of a sil-pad you can also use a mica insulator. Although the mica insulators are not as good as sil-pads performance wise they are as common as dirt and can be found almost everywhere.

 

To conduct heat away from the silicon better. It's almost universal practice with power transistors.

 

I'm just curious.... Have you seen any that haven't had a pin tied to the case?

 

My ESR catalogue shows the vast majority of TO-220 as tab to collector, with just a couple of bipolar and several mosfet as isolated tab (but I've not checked to make sure).

I can't see any mention of hot tab on the TIP120/1/2 and TIP125/6/7 darlingtons in the Fairchild or ST datasheets but a CDIL TIP122 I have has got the tab connected. Fairchild's TIP31 datasheet shows no connection, but my ST TIP31 has a tab connection according to the multimeter...

So I'd still run the meter over them first, whatever the datasheet shows.

Connecting the silicon to the thick copper spreader is good thermal practice, not just to annoy Yellowsnow17.