Father´s Day

Thursday, April 8, 2010

Ramp generator circuit-using 555 timer IC


We know that if a capacitor is charged from a voltage source through a resistor, an exponential waveform is produced while charging of a capaci tor from a constant current source produces a ramp. This is the idea behind the circuit. The circuit of a ramp generator using timer 555 is shown in figure. Here the resistor of previ ous circuits is replaced by a PNP transistor that produces a constant charging current.





Charging current produced by PNP constant current source is
i= Vcc-V/ RE
where V= R/ (R1 + R2) * VCC + VBE
When a trigger starts the monostable multivibrator timer 555 as shown in figure, the PNP current source forces a constant charging into the capacitor C. The voltage across the capacitor is, therefore, a ramp as illustrated in the figure. The slope of the ramp is given as


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