Re: 555 50% duty cycle

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

Original poster: "rheidlebaugh by way of Terry Fritz <twftesla-at-qwest-dot-net>" <rheidlebaugh-at-zialink-dot-com>

A consideration often overlooked by car-coil experimenters, is the fact that
the coil ,like the TC, is a resonant device requiring a .22u capacitor to
develop max voltage. Brute force can over come this, but at a high power
loss. Even the lamp dimmer circuits use a .2u cap in series for best results
    Robert  H 

> From: "Tesla list" <tesla-at-pupman-dot-com>
> Date: Sat, 22 Dec 2001 11:08:42 -0700
> To: tesla-at-pupman-dot-com
> Subject: RE: 555 50% duty cycle
> Resent-From: tesla-at-pupman-dot-com
> Resent-Date: Sat, 22 Dec 2001 11:11:25 -0700
> 
> Original poster: "Pete Komen by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <pkomen-at-zianet-dot-com>
> 
> The TTL 555's low output is very close to ground and the high output is
> about 1.5 volts less than the + supply voltage.  The threshold (pin 6) is at
> 2/3 of V+, and the trigger is at 1/3 of V+, thus using the output to charge
> and discharge the timing capacitor puts the charge and discharge at
> different parts of the RC timing curve.  Would this give exactly 50% duty
> cycle?  The transistor buffered output may be better because the voltage
> drops across the transistors would be more nearly equal.  Pin 5 could be
> connected to a potentiometer to adjust pulse width.
> 
> Doubling the frequency and using a counter or an edge triggered flip-flop
> has the disadvantage of putting the TTL 555 over the recommended operating
> frequency (the 555 should run up to 1MHz but is recommended only to 500KHz -
> CMOS to 3MHz).
> 
> Another method I have seen is to use two diodes in the standard astable
> configuration:  (I'll try ASCII art here)
> Pin                         pin
> R1     7                R2        2,6
> V+--^^^^^^^^----|----|<--------^^^^^^^^^-----|------||------GND
> |------------>|--------------|
> 
> If R1 and R2 are the same and the diodes are the same, you get 50% duty
> cycle. (very high frequency and the diodes have to be FAST)
> 
> I have one wired as above with pots to drive a transistor to drive an
> ignition coil, but haven't tested it yet. (much lower frequency)
> 
> How precise does the 50% have to be?
> 
> Regards,
> 
> Pete Komen
> 
> -----Original Message-----
> From: Tesla list [mailto:tesla-at-pupman-dot-com]
> Sent: Saturday, December 22, 2001 9:56 AM
> To: tesla-at-pupman-dot-com
> Subject: Re: 555 50% duty cycle
> 
> Original poster: "Jan Florian Wagner by way of Terry Fritz
> <twftesla-at-qwest-dot-net>" <jwagner-at-cc.hut.fi>
> 
> Hi,
> 
>> Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <Fucian-at-aol-dot-com>
>> 
>> Hi, how do i configure the 555 to run in 50% Duty cycle at around
> 480khtz?Can
>> I use the 555 timer calcs?I just need to know the values of the resistors
> and
>> capacitors.
> 
> With the standard setup - one cap and two resistors: very small timing
> cap <220pF, and discharge resistor >330 Ohm so the 555 doesn't blow
> up. Precise 50% isn't possible this way, though.
> 
> Alternative - one cap and one resistor: feed the output of the 555
> through a timing resistor to the timing cap. Done. Could work with
> CMOS no probs, TTL probably needs two transistors as buffers like in
> the left part of
> http://www.hut.fi/~jwagner/tesla/SSTC/halfbridge-555-drive.gif
> 
> cheers,
> - Jan
> 
> --
> *************************************************
> high voltage at http://www.hut.fi/~jwagner/tesla
> 
> 
> 
> 
>