# 555 timer duty cycle

In order to increase the duty cycle span of a 555 timer above 50%, I should connect a diode. The equation of duty cycle without diode is $$D = \frac{R_1+R_2}{R_1+2R_2}$$ but when I add the diode what is the exact equation of the duty cycle?

• There isn't an equation as such; when the capacitor is discharged the diode acts as a near-short-circuit to R2. This means R2 takes on two different values in the equation; one for charging (R2) and one for discharging (~0.6V). – Andy aka Mar 25 '14 at 10:53
• So when I add the diode how can I choose the values of R1,R2 and C to obtain a duty cycle span of 60% – Mechatronic_gar Mar 25 '14 at 11:22
• Lot of 555 timer and duty cycle questions this week. I assume there's an assignment out there somewhere! – Scott Seidman Mar 25 '14 at 12:40
• Yep i have an assignment! And this site help a lot – Mechatronic_gar Mar 25 '14 at 12:59