7812 volt regulator - Question about the tab bit being grounded

[nomorebetts]

Hi all,

I recently put this circuit on vero;  https://www.tube-town.net//diy/lov/lov-power-eng.html



And when the tab bit of the regulator is screwed to the enclosure and grounded it drops from 12.85 volts to 12.10 volts.

Any ideas why this is?  The tab part is ground yeah?  So why would it drop?

The regulator is from Tayda, datasheet:
http://www.taydaelectronics.com/datasheets/A-207.pdf

[guitarmageddon]

The fin is to dissipate heat. it's screwed to the enclosure to use the enclosure as a heat sink.
I'm not sure why the output drop.

[lead2203]

Hi all,

I recently put this circuit on vero;  https://www.tube-town.net//diy/lov/lov-power-eng.html



And when the tab bit of the regulator is screwed to the enclosure and grounded it drops from 12.85 volts to 12.10 volts.

Any ideas why this is?  The tab part is ground yeah?  So why would it drop?

The regulator is from Tayda, datasheet:
http://www.taydaelectronics.com/datasheets/A-207.pdf

You need to isolate it from ground if you want to use the zener ....as pin 2 is also connected to the tab.

[psychedelicfish]

As said above: pin 2 is connected to the tab.
Pin 2 would normally be connected to ground, which is why it is labeled that on the datasheet. In your schematic a diode is used from pin 2 to ground, the forward voltage drop of which raises pin 2 0.6V above ground, so that the output wrt. ground becomes 12.6V rather than 12V. When you ground the heatsink, you short out the diode, making pin 2 connected to ground, which makes the output 12V wrt. ground.

[nomorebetts]

As said above: pin 2 is connected to the tab.
Pin 2 would normally be connected to ground, which is why it is labeled that on the datasheet. In your schematic a diode is used from pin 2 to ground, the forward voltage drop of which raises pin 2 0.6V above ground, so that the output wrt. ground becomes 12.6V rather than 12V. When you ground the heatsink, you short out the diode, making pin 2 connected to ground, which makes the output 12V wrt. ground.

Ahhhhh of course.  That makes perfect sense now.  Doh!
Funny that in the TubeTown link there is a picture of it screwed to the enclosure.    I wonder if they noticed a voltage drop? 
Cheers for the answers guys!

[Mike Burgundy]

They probably used one of those small insulator pads designed specifically for this purpose. They don't conduct electricity but do conduct heat, and are almost invisible under the regulator.

[thelonious]

Like Mike said, you can use a insulator pad (usually mica or silicone, sometimes other stuff like aluminum oxide) and nylon bolt/nut for isolation. If it's under heavy load and gets really hot, non-conductive thermal compound can help, too.