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Re: SSTC-556 timer IC malfunction



Original poster: "Jan Florian Wagner by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jwagner-at-cc.hut.fi>


> Hi, i just recently used a staccato design with the 556CMOS IC.I was doing 
> pretty good for being me.Once i added a MOV across the primary, the staccato 
> just failed immediatley.WHY?

don't change a running system! ;o) (ok why it actually failed, I can't
tell... just bad luck maybe... :)

> The funny thing is that it still is pulsing.At 
> the wrong frequency of coarse but it wont respond to POT adjustment.

ah ok. Well, shielding the drive circuit won't help the 556 any more. It
probably is half dead already. So why not plug in a new one, AND
use shielding? check if that helps. 

> get a sizzle out of an ignition coil.What could be happening?Also
everytime i 
> try using a totem pole transistor arangement with a 2N3904 and 2N3906, the 
> output voltage drops to about 7volts.WHY?

Did you hook up the totem pole right? I.e. NPN is above and PNP
is below? Emitters together? 
(More correctly, "complementary emitter follower". A "totem pole"
is something different. But I, and others, always say totem pole, 
when we really do mean a compl. emitter follower... ah why bother...)

Output voltage won't be exactly 0V and 12V. You may end up with 0.5V and
11.5V, which is due to V_base_emitter of the transistors (depends on
transistor).

How and where did you measure 7V, by the way? An oscilloscope would be
best in this case, because cheap digital multimetres don't seem to work
well at AC above a few kHz, even if the maybe built-in frequency
counter/freq display does work

If you have a resistor to the mosfet gate, and you measured the 7V max
peaks directly at the gate, this would mean that the gate resistor is too
large. So place a second resistor in parallel with it to get resistance
down. (or exchange with one smaller but higher wattage resistor)
Did this help?

cheers,
 - Jan

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