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AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2


Square Waves applied to CR circuits

If the time constant is very long compared with the pulse duration, C can only partially charge and discharge in the time available and so we have another different set of waveforms for VC and VR

In practice we compare the time constant of each CR combination with the pulse duration of the applied square wave as follows:

a. If the time constant is one-tenth of the pulse duration (or less) we call the combination a short CR circuit.
b. If the time constant is ten times the pulse duration (or more) we call the combination a long CR circuit.
c. If the time constant is between these two extremes, i.e. between one-tenth and ten times the pulse duration, the combination is called a medium CR circuit.

Note that a circuit can be stated to be a short, medium or long CR circuit only in its relationship to a given input. A circuit which acts as a short CR to one input may well be a long CR to another.

Short CR Circuits

Let us assume that a symmetrical square wave of voltage, of amplitude 100V and pulse duration 1,000 us, is applied to a CR circuit where C = 0.01 uF and R = 2,000 W (Fig 7a).

    Time constant CR=0.01 x 10-6 x 2,000=20us.

This is less than one-tenth of the pulse duration so that the combination is a short CR to this particular input.

The resultant waveforms of VC and VR are shown in Fig 7b and are derived as follows:-

    a. A to B. The input rises from zero to +100V. Because C cannot change its charge instantaneously VC remains at zero volts and VR rises instantly to +100V.

    b. B to C. C commences to charge and VC rises to + l00V in a time of 5CR (100 us). VR falls to zero in the same time, maintaining the relationship V = VC + VR

    c. C to D. The input falls by 100V from + 100V to zero and, since C is fully charged to +100V and cannot change its charge instantaneously, VR must also fall by 100V from zero to -100V.

    d. D to E. C discharges exponentially and VC falls to zero in a time of 5CR (100 us). VR rises from -100V to zero in the same time.

    e. E to F. The cycle then repeats as from a.


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