It seems that for the standard negative voltage regulators LM79xx, in the TO-220 package, the heat sink tab is connected to the unregulated negative supply instead of the ground. This presents a significant hazard, especially since the heat sink of a positive regulator LM78xx is grounded. If I'm building a dual-supply circuit, these two would be near each other, and then there is a danger of a short.
What is the point of this design? What's the advantage?
This has little to do with "advantage" - this is a side effect of the semiconductor design:
And alas ...
BTW: thinking about it, seems that the pin in the middle is usually the one that is connected to the substrate too. So, as a rule of thumb, for this type of packages (metal tab, odd number of pins) the pin in the middle is short to the tab. A connection to the middle pin can be replaced by a connection to the tab (and vice versa).
Regards
Would adding a non-conductive link between the substrate and the heatsink screw everything up too much?
It's just that it seems that exposing the negative rail so prominently is a big liability.
Yes (regarding the insulation layer).
There are issues of mechanical strength, thermal expansion coefficient mismatching, heat transfer etc.
On solution sometimes found is to isolate the whole component at the outside by a plastic coating. (I recall e.g. MOSFETs in an isolated package from International Rectifier.)
Leaving it to the user has its advantages: the user can decide whether to have the heat sink isolated from everything else and get the best cooling. Or isolate the tab vs. the heat sink (isolating heat transfer material and screw isolation). Thus it is not so bad - just surprising the first time.
Regards
P.S. Even 2-pin power diodes in TO-220 have one side of the diode short to the tab. And - as it is - often the side that is currently not the 'convenient' one.
It's a hell of a surprise - I was working with a breadboard, so the parts could move a little. The negative regulator leaned over, and its heatsink arced to the positive regulator. Killed the battery. Lucky 9V batteries don't have enough power to set anything on fire.
Thank you for giving me some background on the design constraints there! I guess I just have to be conscious about this forever.
A piece of mica was the usual way to isolate power transformers from the heat sink.
https://commons.wikimedia.org/wiki/File:Mikanit.jpg
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