Re: 556 problems

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

Original poster: "David Sharpe by way of Terry Fritz <twftesla-at-qwest-dot-net>" <sccr4us-at-erols-dot-com>

> Matt

Check Jan Wagner's site concerning 555/556 drivers for SSTC.

 http://www.hut.fi/~jwagner/tesla/SSTC/results-and-progress.htm

Make sure your reset line is hard tied to Vcc, or if you are gating
you will need a pull up resistor in the 1 - 10K ohm range to reset pin.
Pull reset low to disable output, and reset does have priority function
in 555 chips. Also, bypass the control voltage pin with a 0.01uF cap
to (-) of dc power.

The minimal part count comment concerns me some.  Watch your
frequency pots between DC and discharge, and discharge and trigger/
threshold pins.  A minimal resistance (1/10 of pot value) should be
placed in series with pot to prevent the pot wiper setting input
resistance to 0 to discharge (I've made same mistake and killed the
oscillator and the chip).  I am confident that there is a wiring problem.

If the circuit frequencies are less then 1Mhz
and the circuit works on a breadboard, the only reason it won't
work on a hard wired circuit is defective chips, wire placement,
insufficient bypass caps, or a wiring problems.  There is a physical
reason why the circuit isn't working.  I would physically build circuit
on both breadboard and circuit board.  Get breadboard working
and then physically ohm out each wire.  It will take about 2-3 hours,
you'll learn alot, and there will be something missing.

Regards
Dave Sharpe, TCBOR
Chesterfield, VA. USA

>
> Everything runs cool.No hot spots.Ive got several 556 timers and have tried
> replacing it.No help.Ive checked the board for solder bridges and still
> nothing.When you say bypass each end do you mean each unconnected pin?Does it
> have to be a ceramic?Wont a 1uf ceramic be very big?The transistors are good
> too.I used them right out from the solderless breadboard.
>
> Its so strange that every time i transfer the circuit to a Radio Shack board,
> it goes nuts.Always 9 volts out.
>
> As far as a circuit goes, MAXIM has a simple schematic for a 50%duty cycle.It
> uses only 2 parts.A timing cap and a pot for tuning.Im just doing this to
> both sides of a 556 timer and connecting the output of the staccato to the
> reset of the second timer.
>
> I have had the 555's blow up before in the past.I dont know why.Probably cuz
> it was an unregulated input.Now im using a regulator and it stays nice and
> cool.
>
> Again, maybe its Evil Radio Shack spirits!LOL!
>
> Matt