555 timer equations

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Fluzer

Joined: 14 Sep 2008
Posts: 1

14 Sep 2008 23:48 555 timer equations

Hi everybody.
I have registered hoping to find the answer to my doubts. But far from learning the meaning of life, I only need one simple (or not xD) thing.
I'm studying 555 timer, I know its equations and its two modes, astable and monostable, but the answer is. How can I deduct its equations? Where do they come from? I cannot find the answer. I'd thank you could lend me a hand xD
The equations I am talking about are:

Monostable
Th = 1,1 Ra*C

Astable
t1 = ln2 *(Ra + Rb)*C
t2 = ln2*(Rb)*C

analayout

Joined: 20 Feb 2007
Posts: 96
Helped: 7

18 Sep 2008 6:26 555 equations

Hi,

You can just look at the block diagram of 555 timer
then you will get an idea.

you can get BD from internet

Regards,

Analayout.

m.mokhtar

Joined: 10 Apr 2005
Posts: 24
Helped: 2

18 Sep 2008 21:13 555 timer equation

Hi,
if you know the equations of carging, and discharging a capacitor you will get the equations you mentioned.

the upper and lower limits of the capacitor voltages are 2/3 Vcc, and 1/3 Vcc (from NE555 datasheet).

the equation of discharging a capacitor is Vc(t) =2/3 Vcc * exp-t/Rc substitute Tlow for t and 1/3 Vcc for Vc(t) this will determine Tlow.
As for T high the following equation applies

Vc(t) = (Vc final _ Vc initial) (1- exp-t/rc) + Vc initial

substitute Vc final =Vcc ( the capacitor will try to charge to a final valur of Vcc but the comparator will allow it to charge t to 2/3 Vcc only and will change the output state ), Vc initial = 1/3 Vcc, and t=Thigh
If you do so you will get the equqtions you mentioned.
If I can help you more let me know
regards,
M.Mokhtar

SWINI

Joined: 19 Mar 2007
Posts: 53
Helped: 2

21 Sep 2008 9:41 equations for 555 timer

for monostable vibrator mode
charging tome constant = ln2 R * C
and ideally discharging time constant is zero
as C charges gradually
Vc increases hence when trigger input is applied the output changes from active low quasi stable state to stable state
hence
duration of QSS = ln2 RC

Added after 1 minutes:

for astable vibrator circuit M.Mokhtar sir has already mentioned

regards teja

Kral

Joined: 28 Mar 2005
Posts: 1182
Helped: 187

21 Sep 2008 17:46 555 astable equation

The following might help:
cng.ateneo.edu/cng/wyu/works/papers/555.pdf
Regards,
Kral

sujojk

Joined: 22 Jun 2008
Posts: 14
Helped: 1

22 Sep 2008 8:07 555 circuit equations

The derivation is given in text book "Linear Integrated Circuits" by D. Roy Choudhary. This is an Indian text book. I don't know whether it is available there.

Hope you find that,

SUjO