# 555 timer multivibrator charge and discharge

With this kind of connection I expect 555 to run as a multivibrator.

As far as I understand the discharge of the capacitor should be via R2, while charge of the capacitor via R3 and D1. => discharge slope should not depend on R3.

However,

I've got 3 different discharge slopes, for 3 different values of R3

Moreover, when R3 = 0, it seems as not a multivibrator anymore with constant voltage across the capacitor equals supply voltage.

Could anyone please explain, why discharge slope depend on R3, and why when R3 = 0, capacitor holds constant charge and does not discharge at all.

• Do you understand how a 555 works internally? – Ignacio Vazquez-Abrams Jan 26 '14 at 20:42
• Personally, I don't think understanding a chip's internals is of crucial importance... I would rephrase the question: Do you understand how a 555 works? – Dzarda Jan 26 '14 at 20:48
• Understand the internals of something like a MCU is not usually feasible much less required. The 555 is so simple that understanding its internals will tell you everything you need to know about its operation. – Ignacio Vazquez-Abrams Jan 26 '14 at 21:00
• R3 seems a little on the low side. – Spehro Pefhany Jan 26 '14 at 21:04

From the wikipedia article, "Particularly with bipolar 555s, low values of $R_1$ must be avoided so that the output stays saturated near zero volts during discharge, as assumed by the above equation. Otherwise the output low time will be greater than calculated above."