- 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
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.
Thank you Ismail, it means both the resistors are 18k
Thank you guys, this is extremely helpful!
You are welcome!
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.
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/
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
I did not understand your question.
sorry is for T1 and T2 not c1 and c2
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.
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.