Re: Self-resonant 555 astable conversion?

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

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.