On Mon, 16 May 2011 15:51:15 +0100, "Ian Field" wrote:
....
.Vcc---------------------------+
. |
. +---------------------+ |
. | | [68r]
. [8k2] +---------+ | |
. | | 555 | | |
. +-----6-|TH OUT|-3-+---|---OUT
. | |__ | |
. +----2-O|TR Vcc|-8-----+
. | | _| |
. [3n3] +-5-|VC R|O-4-+--+-+-----+
. | | | GND | |K | |+
. | [10n] +----+----+ [Z16V][330n][680?
. | | |1 | | |
.GND-+---+--------+---------+----+-----+


Close enough.

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.

From??

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.

It's a little MOSFET, what series gate resistor did you use?

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!

More likely the capacitive load?

Tried a pnp + npn follower for the gate drive? Pointless taking a
gate too far past 10V as you only have to suck the charge out of the
gate again to turn the MOSFET off.

555 has what 200mA output rating? 12V and 200mA gives what series
gate resistor value to stay in spec? Allow for the output drop
driving that 200mA when working out resistor.

Grant.