Astable Multivibrator – Duty Cycle From 0 To 100%

From AdvancedElectronicsManual.pdf

Advanced Electronics Manual.pdf

Astable multivibrator with duty cycle variable from 0 to 100%:
In some applications, it is needed to vary the duty cycle from about 0 to 100%. In that case the circuit is designed as shown in the circuit diagram. Here a potentiometer, RX, is used so that RA = R1+R2, RB = RX-R2+R3. A diode is now connected across a variable RB.
Thus a variable duty cycle is achieved. Therefore, the frequency of oscillation and duty cycle can be derived as follows.

The time for charging C from 1/3 to 2/3 Vcc, i.e, ON Time = 0.693 (RA + RB). C The time for discharging C from 2/3 to 1/3 Vcc, i.e. OFF Time = 0.693 RB. C To get the total oscillation period, just add the two:

Tosc = 0.693∙(RA+RB)∙C + 0.693∙(RB)∙C = 0.693 ∙ (RA + 2∙RB) ∙ C

fosc = 1/ Tosc = 1.44/(RA + RB).C = 1.44/(R1 + RX + R3).C
Min. Duty Cycle = R1/(R1 + RX + R3)
Max. Duty Cycle = (R1 + RX)/(R1 + RX + R3)

I had only a 5k trim pot and 1.5k/2k5 resistors, not 10k/1k so could only get about 20% to 66% duty cycles not 0-100% as this circuit claims:

Dong the calcs for my values:

Min. Duty Cycle = R1/(R1 + RX + R3)

Min. Duty Cycle = R1/(R1 + RX + R3) = 1500 / (1500+5000+2500 = 9000 ) =

15/90 = 1/6 = 17%

Max. Duty Cycle = (R1 + RX)/(R1 + RX + R3) = 1500 + 5000 / (1500+5000+2500 = 9000 ) = 6500 / 9000 = 0.72 = 72%

I added another 5k trim and got these results at max and minimum resistance:

This has changed both duty cycles from about 1/6 to 3/4:

Min. Duty Cycle = R1/(R1 + RX + R3)

Min. Duty Cycle = R1/(R1 + RX + R3) = 1500 / (1500+10000+2500 = 14000 ) =

1500/14000 = 15/140 = 10.7%

Max. Duty Cycle = (R1 + RX)/(R1 + RX + R3) = 1500 + 10000 / (1500+10000+2500 = 14000 ) = 11500 / 14000= 82%

The capacitor charge was read from pins 6/2.