Operation principle of 555 estable multi-vibrator model

Published:2011/7/30 3:06:00 Author:Zoey | Keyword: 555, Astable Multi-vibrator, Model | From:SeekIC

As shown in the picture 3-1, an astable multi-vibrator can be formed after connecting a 555, three resistances and a capacity accessory, the trigger (pin 2) is connected on loop C. When galvanized voltage VDD, 555 will be reset and pin 3 will havehigh level “1”, the discharge tube VT1 will cease to discharge, and C will get charged via Ra and Rb. Voltage on C increases along with time, when it gets 2/3 VDD threshold value level, the upper comparator A1 will turn over, RS trigger willset and output a low level”0”, at that time, the discharge tube VT1 will be saturated and conducted, electric charge on C will discharge through Rb to VT1. When C discharges to 1/3 VDD, the upper comparator A2 will turn over, RS trigger willreset and output a high level”1”, this process will be repeated many times. By analyzing the results, we can conclude that the change rule of voltage on the two terminals of C can be formulated as follow: Uc(t)=VDD(1-e-t(RA+RB)C) +1/3 VDDE-t(RA+RB)C If we replace (RA+RB) with t1, the formula can be simplified as follow: Uc(t)=VDD(1-2/3•e-t/t1) Following formula can be concluded: T=0.693(RA+2RB)C D=t1/T=(RA+RB)/(RA+2RB) If RA>>RB, then D≈50%， the output oscillation wave is square wave. finally we can draw following conclusions: (1) The oscillation period is not relevant to VDD, but is determined by charge and discharge time constant, that is, the value of RA, RB, and C (2) Duty cycle of the oscillation is not relevant to C , but is relevant to the ratio of RA and RB



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