555 Astable Mode Explained - With diode to allows duty cycle of 50 percent or less.

Last 555 circuit I put here (see my other circuits) had a Duty cycle > 50% because it had no diode in parallel to R2, so the 10uF capacitor was charging through R1 (up 100k resistor) and R2 (down 100k resistor), and discharging through R2 only. To a 50% duty cycle the 10uF capacitor must charge and discharge through same resistance. To make possible a Duty cycle of 50% or less we can add a bypass diode in parallel to R2. This way the capacitor charges through R1 only and discharges through R2 only. This configuration allow us to control exacly the duty cycle. While capacitor is charging the cycle is in HIGH period of the cycle, while discharging it is in LOW state ------- TRY ------- - Blue grafic shows the output cycle. - Green grafics shows capacitor charging and discharging. 1) increasing or decreasing value of R1 (up 100k) or R2 (down 100k). Increasing R1 in relation to R2 will increase the time it stays HIGH because the 10uF capacitor charges slower than before. This increases the duty cycle. Increasing R2 in relation to R1 will increase the time it stays LOW because the 10uF capacitor discharges slower than before. This decreases the duty cycle. If value of R1 is equal to R2 the capacitor charges and discharges through the same resistance and at the same rate, creating a 50% duty cycle. 2) Increase or decrease value of the 10uF capacitor. Increasing it will increase the total cycle time (T high + T low), decreasing the Frquency F = 1/T Formulas to Astable Mode with bypass diode in parallel to R2: T = 0.7 × (R1 + R2) × C1 (Total cycle time) T high = 0.7 × R1 × C1 T low = 0.7 × R2 × C1 *Time is in seconds Sorry for the English, I'm Brazilian. Leave your comment. Thanks

published 8 years ago