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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?

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  • \$\begingroup\$ 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). \$\endgroup\$ – Andy aka Mar 25 '14 at 10:53
  • \$\begingroup\$ 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% \$\endgroup\$ – Mechatronic_gar Mar 25 '14 at 11:22
  • \$\begingroup\$ Lot of 555 timer and duty cycle questions this week. I assume there's an assignment out there somewhere! \$\endgroup\$ – Scott Seidman Mar 25 '14 at 12:40
  • \$\begingroup\$ Yep i have an assignment! And this site help a lot \$\endgroup\$ – Mechatronic_gar Mar 25 '14 at 12:59
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Try using this website and adjusting the numbers in the following 2 scenarios: -

enter image description here

For the left hand scenario I've entered 10k for R1 and 10k for R2 and got an output high time of 0.1386 seconds. Note the output low time of 0.0693 seconds i.e. 66.6% duty

For the right hand scenario I've frigged the results by making R1 = 19k and R2 = 1k (this is my guesstimate of what R2 will be when parallel with a diode. Note the high-time remains at 0.1386 seconds but the low time is 0.00693 seconds.

Does this help you a little bit in simulating the effect of a diode across R2? Yeah I know it's a bodge but unless you invest in free software from Linear technology called LTSpice I know of no other way without a formula. Here's the link to LTSpice

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  • \$\begingroup\$ The site you gave doesnt consider the diode at R2. That means whatever i put values for R1,R2 and C i will get a duty cycle of 50% minimum i need dudty cycle of 40% so that my span becomes 60% (40% to 100%) \$\endgroup\$ – Mechatronic_gar Mar 25 '14 at 13:03
  • \$\begingroup\$ it gives you on time and off times which allows you to trial R2 at two different values so you can get a feel for the effect when the slide bridges R2. That's what I've tried to demonstrate plus LTSpice is a better option realistically. \$\endgroup\$ – Andy aka Mar 25 '14 at 14:41

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