Re: 556 sstc idea

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Justin Hays by way of Terry Fritz <twftesla-at-qwest-dot-net>" <pyrotrons2000-at-yahoo-dot-com>

>Hi, i just thought of something and wondered if its been done or 
> could  be or how hard it would be to do it.
>
> Since a 556 is a dual osccilator, is it possible to sync it right 
> so it would turn on one mosfet while the other is off?And then it 
> would go off and the second would turn on?
>
>
> How hard would this be? Is it possible?
>
> Im just curious,
> Matt

Hi Matt and All, it's very possible, they are used in this way all
over the place. Not exactly for MOSFET half-bridges, but, for time
delay. Where one output goes low, then after a preset time, the other
output goes high and vice versa. Where the two outputs are never both
hi at the same time.

It is this the so-called "break before make" operation that you want
in MOSFET bridge circuits, where the one (in a half-bridge) or two
(full-bridge) MOSFET's don't conduct at the same time. When they are
all on at the same time, the situation is referred to as
"shoot-through" or "cross-conduction", and usually results in gooey
MOSFET splattered all over the inside of a project box ; )

Just for the record, I will say that there's nothing you can do with
a 556 that can't be done with two 555's. Other than save board space.
The 556 is nothing more than two 555's, with their Vcc and GND pins
internally connected. Everything else is totally separate.

I can't remember the schematic offhand, but I think it results in a
time delay that is way too long for our SSTC use (introduction of
dead-time where no FET's are on).

I think it's better suited for photography and other stuff that runs
in the millisecond range, but I imagine a variation would get down
into our required microseconds.

Take care,

Justin Hays
KC5PNP
Email: justin-at-hvguy-dot-com
Website: www.hvguy-dot-com





 





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