[raverbashing]

Yeah, still, if they're too hot to touch I believe it's being misused in the circuit (either missing a heat sink, or too much voltage drop across, or the circuit needs something more powerful)

But of course, this happens a lot in experimental circuits.

[agoetz]

Not necessarily. The LM7805 has a maximum operating temperature of 125C, so a circuit can be operating nominally at a very high temperature.

[VLM]

Actually thats the junction temp so unless you have an infinitely thermally efficient package and heatsink, if you're boiling water the innards are way out of spec.

Also check out the temperature derating curves.

That series has a thermal shutdown ckt so "operating nominally" above boiling pt is kind of hard to define.

There are also non-regulator thermal issues, like most cases are not designed to operate continually at 125C internal temps, and the electrolytic caps nearby the reg have their lifetimes drop with some polynomial of temp above room temp so smoking hot reg isn't good for everything nearby even if isolated by itself it would be OK.

Finally the general idea is still true, that you can get a nice 2nd degree burn or think something is "really hot" at a temp well below rated.

There are plenty of high power transistors rated to 150C junction temp. Not impossible with a really good package and heatsink (low thermal resistance) and good assembly technique to be able to boil water on one of those. As a stunt in the lab everyone tries this sooner or later to see if their anti-thermal runaway ckt is good enough and their assembly technique is good enough.

[VLM]

"(either missing a heat sink, or too much voltage drop across, or the circuit needs something more powerful)"

Starting in the 80s Low Drop Out LDO regs started hitting the market, using mosfets as the pass transistor instead of bipolar trans (and a few other changes) means you can run with much lower voltage drops.

However, legendarily, the bipolar voltage drop automagically provided some oscillation dampening and the LDO regs are notorious for being unstable, almost as much of a PITA as using a switching reg, almost. So sometimes with reactive (usually inductive) load at certain currents and voltages, they turn into little RF oscillators. Sometimes the oscillation is too high of a freq to interest the overheating and other self protection ckts so they literally catch fire and so on.

These 80s stereotypes of LDO regs have been continually improved since the 80s so they're somewhat more stable now.

Part of the shocker of both the bad 7805s in the story and the oscillating 80s era LDOs is the 7805 series used to be legendarily bulletproof because they have a lot of self protection circuitry built in. Blowing one up is non-trivial compared to most other chips. Usually decent reverse current protection, some reverse polarity protection, overheating protection, short ckt protection, they're pretty touch chips, so its surprising when they aren't.

[raverbashing]

Ah very interesting story, I didn't know about those LDO problems.

I guess BJTs at that power level are slow, as opposed to mosfets, which are much faster, so there's that as well.