555 mark space

Posted on Feb 6, 2014

On astable multivibrators, the duty cycle is usually fixed unless there`s a potentiometer in place, but even still with a diode in parallel with R2, you will only get a duty cycle less than 50%. Also, to ask a question in a new thread, youcan click here. I`ve only been dealing with electronics for about 2 years now. The other people on this forum are much more helpful. it becomes necessary to insert a diode in parallel with

555 mark space
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RB, cathode toward the timing capacitor. Another diode is desirable, but not mandatory (this one in series with RB), cathode away from the timing capacitor. Now the charge path becomes RA, through the parallel diode into C. Discharge is through the series diode and RB to the discharge transistor. This scheme will afford a duty cycle range from less than 5% to greater than 95%. It should be noted that for reliable operation a minimum value of 3kW for RB is recommended to assure that oscillation begins. That circuit is quite common. The circuit without a diode, but using the output at pin 3 is susceptible to changes in frequency and duty cycle with changes in voltage. It is a tiny bit simpler circuit than the conventional circuit with a diode, but I prefer not to use it because of its poor stability.

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