- 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