- The negative voltage passing through the capacitor C2 turns off the transistor Q1 which causes the capacitor C1 to initiate charging through resistor R and Vcc, as well as through the base emitter of transistor Q2. This causes the transistor Q2 to acquire the momentary ON state.
- During the process, the capacitor C2 gradually discharges until it's completely empty, and then it starts charging from opposite direction through R2.
- As soon as the voltage in capacitor C2 is sufficient to turn ON transistor Q1, Q1 switches ON and forces capacitor C1 to initiate discharging.
- The above process keeps recycling causing a sustained and alternate switching of the transistors as long as the circuit in the powered state.
Design
R – Collector Resistor
The resistance R needs to be dimensioned so that it is able to limit the collector current Ic below the specified threshold.
The following formula represents it:
R = V/Ic ,
where V is the voltage across the resistor R.
Normally this could be expressed as, V = (Vcc – Vce) = (Vce – 0.3) however in cases where an emitter load such as an LED is utilized, the expression may be modified as:
V = (Vcc – Vce – Vled) , where Vled is the voltage drop across LED.
In most cases the maximum collector current Ic could be much higher than than the required for emitter load current. During such instances Ic could be tailored in such a way that it stays below the max current specification of emitter load.
Therefore,
- R = (Vcc – Vce – Vload) / Ic
R1 & R2 – Base Resistors
R1 & R2 must be selected to obtain the desired collector current during saturation state.
- Min. Base Current, Ibmin = Ic / β, where β is the hFE of the transistor
- Safe Base Current,Ib = 10x Ibmin= 3 x Ic / β
- R1, R2 = (Vcc – Vbe) / Ib
T1 & T2 – Time Period
- T2 = OFF Period of transistor Q1 = ON Period of Transistor Q2 = 0.693R2C2
- T1 = OFF Period of transistor Q2 = ON Period of Transistor Q1 = 0.693R1C1
From these expressions we can evaluate the value of C1 and C2.
It may be defined as the ratio of time Tc when the output is high to the total time period T of the cycle.
Thus here, Duty Cycle = Toff/(Toff + Ton) when the output is acquired from the collector of the transistor T.
Calculator
Florian Beldean says
Hello Sir.
I want to let you know that this is one of my very beginning circuit and your info have been very helpful. Thank you so much. I checked my theory with your calculator and I have been really close. I calculate min and maximum, both cases in same time and everything was exactly where supposed to be. But I’m not shore how your program chose best way because I realised that calculate close to minimum but with a small percentage over. Can you please tell tell me what is that percentage?
Thank you so much for sharing your knowledge with people like us
Swagatam says
Hello Florian, this software was actually not created by me, rather purchased from an external source so i do not have much idea regarding how the software was designed and how it works internally.
Muhammad Hamza says
Can you please provide the resistor calculation formula and pls correct me it is not
T = 0.69RC but how can we altered it with duty cycle
Swagatam says
There are already plenty of information available online, you an check them out.
Muhammad Hamza says
Actually I have a practical exam tomorrow 😔and I’ve also search online but couldn’t find something relevant like how can we altered duty cycles
Swagatam says
Please search “transistor astable multivibrator calculations” you will quickly find it in the first search result.
Yahka says
When I adjust T1 and T2, so T1 = 0.2 and T2=0.8, the calculator gives me two different values for C1 and C2. Good. However I’d prefer C1=C2, but T1 and T2 different. Can that be done? It is also said one can’t get a duty cycle of less than or equal to 50%. How come?
Swagatam says
If you want an output with unsymmetrical duty cycle then either the base resistors will be different or the base capacitors will be different. If you want the capacitors to remain identical then the base resistors will need to be altered appropriately.
If the software does not help, then you can use the formula for solving it manually….
Ismail says
Well-done sir, when I calculated it, my result for R1 & R2 was = 18833 Ohm, sir! Do I need to divide it or for each resistor? Thanks.
Swagatam says
Thank you Ismail, it means both the resistors are 18k
Pietro Foralosso says
Thank you guys, this is extremely helpful!
Swagatam says
You are welcome!
youngking says
sir I fill in T1 and T2 as you instructed but the answer I got are strange which I listed below. R =
585 Ω
R1, R2 = 18833 Ω
C1 = 0.8 μF
C2 = 0.8 μF 12 volt is my
Supply Voltage my question. can this capacitor and the resistor be found in the market.
Swagatam says
Youngking, these values are not critical, you can easily adjust the values little bit here and there as per your inconvenience. If you change the capacitor value, then you may also have to change the base resistor values proportionately and vice versa.
You can also use the values given in the last diagram from this article:
https://www.homemade-circuits.com/7-simple-inverter-circuits/
youngking says
sir I have done as u said and this is my answer. R = 585 Ω
R1, R2 = 18833 Ω
C1 = 0.8 μF
C2 = 0.8 μF 12 volt is my
Supply Voltage, Vcc : now my question can r1
and r2 be found because the value is much. sorry for too much question
Swagatam says
I did not understand your question.
Wolf says
Como podria obtener una frecuencia de 50hz con un suministro de 9 v
Youenn says
Wolf, remember F=1/(2PIxT)=6.28T, and T=T1+T2 if you want a duty cycle of 50% that means T1=T2 (F in Hz, T, T1 and T2 in s ) so, make the calculation by yourself, it is too much easy! Then, put T1 =T2 and fill this calculator with this value and the supply voltage you want, nothing more, just get the result!.. 🙂
(Thanks to Swagatam for this job! 🙂 )
youngking says
sorry is for T1 and T2 not c1 and c2
youngking says
sir i in insert 0.01 for c1 and c2 in this Transistor Astable Calculator software, now how do I know Base Emitter Voltage, Vbe :,Collector Voltage, Vce :,Amplification Factor hFE, β:,Required Collector Current, Ic : and Voltage Required for Emitter Load. pls kindly reply.
Swagatam says
Rest everything is already filled which you don’t have to change, I have already told you about this.
Fill only T1, T2, and the supply voltage, and click calculate.