RE: 555 50% duty cycle

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

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

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