On Mon, 16 May 2011 15:51:15 +0100, "Ian Field"
wrote:


"John Fields" wrote in message
news
On Fri, 13 May 2011 11:50:14 +0200, "petrus bitbyter"
wrote:


"John Fields" schreef in bericht
...
On Thu, 12 May 2011 16:23:57 -0500, John Fields
wrote:

On Thu, 12 May 2011 13:45:28 -0700, Jim Thompson
wrote:

On Thu, 12 May 2011 21:16:15 +0100, "Ian Field"
wrote:


"Jim Thompson"
wrote
in
message ...

Post your schematic or a link.


Its at the same location as my other PC with no internet.

Vcc is clamped by a 16V zener and has 680u + 334 mylar all fed by 68
Ohm
from VDD.

With CR= 3n3 & 8k2 its a standard astable except the CR is driven by
pin
3
instead of pin 7 + pullup resistor.

There is the customary 10n from pin 5 to GND.


How do you post to SED ?:-)

...Jim Thompson
---
Indeed.

He could easily post an LTspice *.asc netlist to any of the text
groups or, using abse, post the same netlist as an attachment, which
will execute if the association is made between .asc and the LTspice
executable.

Failing that he could attach even a scanned, hand-drawn sketch to
abse.

---
Or, God forbid, an ASCIImatic.

--
JF

Well, it's not that bad. Much better anyway then the hand drawn flimsies
some other persons tend to provide
___
+-----+------------+----|___|-----
| | | 68
+-----------------)-----)-------+ |
| |8 |4 | |
| .-----------------. | |
| | V r | | |
| 7| c e | | |
| -|dis c s | | |
| | | | |
.-. | | | |
| | | | | |
| |8k2 | | | |
'-' | | | |
| 6| | | |
+-----------|thr | | |
| | | | |
| | |3| |
| | 555 out|-+-- |
| | | |
| | | |
| 5| | |
| +----|cnt | |
| | | | |
| | | | +----+-------+
| | | | | | |
| | 2| | | | |
+------)----|trg G | - | | +
| | | N | / --- ###
| | | D | Z16V - --- ---
--- --- '-----------------' ^ |330n |680u
---3n3 ---10n |1 | | |
| | | | | |
+------+-------------+---------------+----+-------+---
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de


petrus bitbyter

---
Good God, man, have you no decency?!!


.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][680µ]
. | | |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.

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!

---
According to Maxim:

http://datasheets.maxim-ic.com/en/ds...55-ICM7556.pdf


the current in the spike peaks at about 400mA and zeroes out about
200ns layer, so that's about 200mA average over the 200ns period.

Then, since:

W = qV = (It)V = 2e-1A * 2e-7s * 1.2e1V = 4.8e-7 joules,

that's about 4.8e-7J per spike.

If you're switching at 100kHz, then, you'll have 200,000 spikes per
second, which amounts to a total of

P = Wn = 4.8e-7J * 2e5 = 9.6e-2W = 96 milliwatts.

According to Signetics, the Absolute Maximum power dissipation is
600mW, so it doesn't seem like one-sixth of that should cause a
problem.

Why don't you measure the current spikes and see what's happening?

--
JF