Re: Self-resonant 555 astable conversion?

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

Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>

Hi Jolyon,

It should be possible but take system ground right there off the secondary 
ground wire too.  The "ground" may have like 10uH of inductance and 
resistance in it that could cause "sort of high" voltage spikes if the 
grounds were not all centrally located.  That single ground point thing...

I would put it in a small metal box and use those plug in prototyping 
boards so that circuit changes are easy which will be good when you have to 
add that phase correction circuit ;-))  Not that any wire leading from the 
box is an antenna and possible streamer strike too so add caps or whatever 
there.  But the environment inside the box should be very quiet really.  I 
like the heavy cast aluminum boxes from Digi-key and I use 1/4 inch copper 
tape around the seem.  Nothing gets past that!

Cheers,

         Terry


At 11:02 AM 5/17/2003 +0100, you wrote:
>Dear List,
>I am enquiring whether it is possible to connect up a 555 as a regular 
>astable  but with secondary of a small (ferrite bead?) current transformer 
>connected between the RC timing network and pins 2 and 6 of the IC
>the primary of the CT being connected into the ground wire of the TC 
>secondary.
>
>I have used 555 connected as a 50/50 mark-space astable to drive a small 
>(not very successful) magnifier, but with fixed frequency oscillator in an 
>SSTC any changes in the topload-to -ground capacitance will detune the 
>system; hence I am looking into a way of converting my setup into an SR-SSTC.
>
>Protection diodes would of course be connected between the junction of 
>pins 2 and 6 and the positive and negative rails to the astable, to limit 
>any excursions to 0.6v above and below the rail voltages.
>
>This way, the IC would start up as an RC oscillator, turning into a 
>feedback oscillator as soon as current started to flow in the secondary.
>
>Would it work and if so, would some form of phase-adjustment be needed to 
>get the feedback current into the correct phasing to drive the oscillator