In article ,
"Ian Field" wrote:

....
It still overheats (a new Sign' NE555 failed) even after replacing the 16V
zener with a 12V part and adding a second 68 Ohm resistor inseries as the
dropper.

It occurred to me that it didn't like driving the gate capacitance of an
IRF740, but it still does it with the MOSFET removed.

I've run the point of a scalpel between all the tracks on the stripboard to
eliminate any solder bridges I might have missed.

The only thing left I can think of is maybe I overdid the Vcc decoupling,
the 680u is a very low ESR from a VCR PSU and the 330n might only be making
matters worse.

Its well known that the 555 draws large current spikes during output
transitions, totally OTT decoupling could be whats cooking it!

The original 555 does run hot. That's why the CMOS version is popular.

No, it doesn't like directly driving big MOSFETs. When the MOSFET gate
is driven low, inductive ringing on the drain appears on the gate via
capacitive coupling. That ringing wears out the 555 chip by pushing the
output pin below zero volts.

A Schmitt trigger inverting MOSFET driver very roughly resembles a 555
chip having pins 2 and 6 tied together. It could be good enough for
blinking lights and regulated power inverters.
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