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555 Oscillator With 50% Duty Cycle

2015-01-29 14:21  
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To generate 50% active factor (duty cycle) using 555 IC, we can use two diodes to separate the charging and the discharging path. With no diodes, we can also generate 50% duty cycle using 555 IC by configuring the circuit as shown in the schematic diagram.

oscillator 555 50 duty cycle circuit schematic diagram

The high state period is similar to regular configuration, t1 = 0.693 RA C, and the low state period is equal to t2=[(RA.RB/(RA RB)].C.Ln[(RB-2RA)/(2RB-RA)]. Finally, we can get the total period of t1 t2, or the frequency of 1/(t1 t2). Please be aware that the circuit will work only if RB is smaller than 1/2 RA. Higher value of RB cannot bring the? level of pin 2 down to 1/3 VCC and trigger the lower comparator. [Circuit's schematic diagram source: National Semiconductor Application Notes]


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