Hi guys, could you please help me with some doubts about PWM control? How can an astable multivibrator be adapted or used to generate PWM signals? What modifications are needed to control the duty cycle of the output waveform? Without affecting the frequency? I have been reading about the free-running frequency of an astable multivibrator and trying to implement the PWM control using a basic astable multivibrator as shown in the article Free Running Frequency of Astable Multivibrator. It's been clear how to calculate the frequency of an astable multivibrator. But what still I'm not sure about is the duty cycle control. Please help. Thank you.

 

Welcome to AAC!
The linked circuit will give a 50% duty cycle if its left half is electrically symmetrical with its right half. Does that give you a clue?

 

an ideal PWM circuit would have inputs that can be controlled independently and conveniently using single parameter per control. for example one voltage input (or potentiometer) to control frequency, another to control duty cycle, another to control amplitude...

astable multivibrator is not particularly handy to do this. there are several reasons why.
one can adjust component values to get fixed duty PWM but it does not lend itself for the most universal PWM circuit (practical for all applications)
- frequency and duty are not controlled independently
- multiple components may need to be adjusted
- not easy to add control signals etc.

but if you are ok to adjust duty cycle manually using potentiometer, this is very nice and simple circuit.



above circuit should have Pot value much larger than R2 and R3 resistors to get wider control range.
for example
if Pot = R2 you only get duty cycle range 33-66%
if Pot = 10x R2 you get about 8.5-91.5%
if Pot = 100x R2 you get 1-99%

so probably want to keep R2,R3 at some reasonable values (1k) and make potentiometer value 100k or 500k
then you can use calculation in the link you shared to determine capacitor values.

 

Welcome to AAC!
The linked circuit will give a 50% duty cycle if its left half is electrically symmetrical with its right half. Does that give you a clue?

thanks for the reply. I know that the circuit in the article I was referring to will have 50% duty cycle since, both timing circuits are symmetrical. And If I change the value of either of the timing resistors I can change the PWM. But it will also change the frequency. So what I want is duty cycle control that will not effect the output frequency.

 

did you read the article and look at schematic i posted? did you do the math or build it?

the way the potentiometer is connected, will always have total resistance same and thus period/frequency will be the same too.

R2=R3=R
C1=C2=C

d = duty cycle (potentiometer position in range 0-1) Potentiometer resistance is Pr

Ra= R + d*Pr
Rb=R+(1-d)*Pr

T1 = 0.69*C*Ra
T2= 0.69*C*Rb

T=T1 + T2 = 0.69*C*(R+d*Pr + (1-d)*Pr)
T=0.69*C*(R+Pr)
f=1/T=1/(0.69*C*(R+Pr))

note that d is no longer part of T or f
so adjusting potentiometer will have no effect on T or f but will change duty cycle as T1 and T2 can be adjusted by turning the potentiometer.

 

 

Note that you need to add a diode in series with each transistor's base if the supply voltage is more than 5V, to avoid reverse base-emitter voltage above the typical BJT max rating.
Otherwise the transistor could get zapped.