Re: Self-resonant 555 astable conversion?

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

Original poster: "Jolyon Vater Cox by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>

I have posted the  diagram of my proposed  setup is at
http://hot-streamer-dot-com/temp/555%20sr%20astable.bmp.

Looking at the diagram again I am concerned about phasing - should the
current transformer should be in one the primary leads from the H-bridge to
the step-up transformer, rather than the secondary- this after all, is the
position of the current transformer in half-wave converters used in compact
flourescent  lamps and electronic halogen transformers -but what is
implication of primary current  feedback on the TC frequency distribution/
bandpass characteristic?


With the current transformer in the secondary lead- isn't there a phase
discrepancy between the output of the current transformer and the primary
voltage that would need to be corrected in some way e.g by connecting a
capacitor in parallel with the secondary of the current transformer,
perhaps?

What appeared simple at first is now beginning to look rather more
complicated...!

Jolyon

Jolyon  ----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Monday, May 19, 2003 2:45 PM
Subject: Re: Self-resonant 555 astable conversion?


 > Original poster: "Stephen Conner by way of Terry Fritz
<teslalist-at-qwest-dot-net>" <steve-at-scopeboy-dot-com>
 >
 > At 05:36 17/05/03 -0600, you wrote:
 > >Original poster: "Jolyon Vater Cox by way of Terry Fritz
 > ><teslalist-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>
 > >
 > >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.
 >
 > It should be possible, I did something similar for zero-current turnoff in
 > my OLTC driver circuit. I used a two transistor circuit to amplify the
 > collector-emitter volt drop in the IGBT, and coupled the resulting square
 > wave into 555 pin 2 using a small capacitor and 1k resistor (the resistor
 > isolated pin 2 from the timing capacitor which would otherwise just shunt
 > the pulses to ground) In this way the turn-off was locked to the nearest
 > negative half-cycle.
 >
 > If you were to connect a secondary current sensor (probably a capacitor
 > would be better than a CT) to the amplifier input, and have the 555
running
 > at 50% duty cycle at a little below the lowest frequency of interest, it
 > would probably lock on.
 >
 > However I think that better results could be had using a SMPS controller
 > chip like the UC3525 that has a sync input pin. Normally used for locking
 > multiple SMPSs together to prevent annoying audible beat tones but could
 > perhaps be used for feedback. I know the TL494 is popular but I don't
think
 > it has one of these sync pins.
 >
 > http://www.scopeboy-dot-com/t3fnq.html
 >
 > Steve C.
 >
 >
 >
 >
 >