Multivibrators Theory

[307pavlos]

Hello, I'm in need of multivibrator Notes. Can anybody send me his notes or any websites to download or read the theory of multivibrators with both bjt transistors and lm555 chip.

thanks in advance!

 

[ericgibbs]

Hello, I'm in need of multivibrator Notes. Can anybody send me his notes or any websites to download or read the theory of multivibrators with both bjt transistors and lm555 chip.

thanks in advance!

Hi,
You should use Google.
https://www.google.co.uk/search?q=m...s=org.mozilla:en-GB:official&client=firefox-a

 

[307pavlos]

ok we have this astable multivibrator. My questions are :
1) How the inner RS Flip-Flop works inside the lm555
2)for what purpose is that Capacitor at pin5? How can i calculate its value for a circuit?
3)Pin 7 has a transistor which is connected at the output of the RS Flip-Flop. When is it working and when not?
4)When i want to have a DC%=50% what should i do?

thank you in advance!

 

[ericgibbs]

ok we have this astable multivibrator. My questions are :
1) How the inner RS Flip-Flop works inside the lm555
2)for what purpose is that Capacitor at pin5? How can i calculate its value for a circuit?
3)Pin 7 has a transistor which is connected at the output of the RS Flip-Flop. When is it working and when not?
4)When i want to have a DC%=50% what should i do?

thank you in advance!

hi,
All this information is in the 555 datasheet, this is the CMOS version.

 

[307pavlos]

thanks!!! Well while reading i had a trouble once again. An excercise question in an astable multivibrator asks to calculate the max and min current through Pin 7 while timing capacitor is discharging.

Well i thought what means max and min current while discharge and i think it means the moment the timing capacitor starts to discharge from 2/3*Vcc so i calculated Icmax=[(2/3)*Vcc]/R, wherein R is the resistance the cap. discharges through it. Then i calculated the minimum current of pin7 which is when the cap. has 1/3Vcc voltage, so Icmin=(1/3*Vcc)/R

Is it correct or i've just miserably? (Like Homer Simpson says )

 

[ericgibbs]

hi,
Look at this page from the d/s it tells you the discharge paths for the timing capacitor.

 

[307pavlos]

hi, hmmm i cant understand

 

[ericgibbs]

hi, hmmm i cant understand

hi,
Its during the tc(L) period while discharging.
Its at its maximum at the start of tc(L) and at a minimum at the end of tc(L).

The discharge current decreases exponentially over that period

You know the values of Ct and Rb so use the exp formula

 

[307pavlos]

oh i see!! thanks mate! i'll try it!!

 

[ericgibbs]

oh i see!! thanks mate! i'll try it!!

ho,
Looking here should help.
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capdis.html

 

[307pavlos]

hi, so i have 2 equations.
The first is when capacitor voltage starts to fall. Imax={[(2/3)*Vcc]/R3}*e^(-t/R3*C1)
and Imin={[(1/3)*Vcc]/R3}*e^(-t/R3*C1)

wherein t=tdc=ln2*R3*C1 ????

This is the circuit

 

[ericgibbs]

hi, so i have 2 equations.
The first is when capacitor voltage starts to fall. Imax={[(2/3)*Vcc]/R3}*e^(-t/R3*C1)
and Imin={[(1/3)*Vcc]/R3}*e^(-t/R3*C1)

wherein t=tdc=ln2*R3*C1 ????

This is the circuit

In the original circuit from the d/s, Rb is replaced by R2 and R3, which have series diodes.

D1/R2 and D2/R3 effect the charge/discharge periods of C1, this modification of the periods is often used to change the mark/space ratio of the the output pulse on pin3.

If you look at the d/s for the 555 it shows the equations for timing when these diodes are used.

 

[colin55]

max and min current through Pin 7 while timing capacitor is discharging.

The maximum current though the transistor connected to pin 7 is determined by current through the resistor between pin 7 and the supply rail plus the current through the resistor connected between pin 7 and the capacitor.
The voltage on the capacitor will be 2/3 of the voltage on the supply rail.

 

[307pavlos]

Hi,
sorry for my late reply, i tried to understand what you told me, but no success. I read the data sheet but i cant find any equations i could use here.
This circuit charges through R1, D1, R2 and discharges only through R3, D2, right? So what are the equations?

Colin55 do you have any notes on this? I really want to study this circuit not only for university (which is closed down now protesting....for nothing! ) but to exceed my knowledge in electronics.

At the moment i cant see how i can calculate a lot of things in a circuit like current, voltage, inner resistances etc. But i'm trying hard.

 

[colin55]

A non technical approach to understanding the 555:
It explains how the 555 works and these facts have never been covered before ANYWHERE.
<snip: spam link>

The questions you ask are very simple. Just use Ohms Law.

 

[307pavlos]

i read this before. it didnt help anyway i'll try to ask some friends of mine who they know electronics and see what i do! thanks