Main Radar Home

 Radar theory Home

AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

Contact the Editor

AP 3302 Pt. 3

Section 2


Square Waves applied to CR circuits

discharged. The applied voltage V is then zero, Vc is zero and, since there is no current flowing through R, Vr is also zero.

When we switch to the charge position, C cannot change its charge - and hence its voltage - instantaneously, so that the whole of the applied voltage V appears across R. Thus Vr rises immediately to the voltage V of the supply, in this case + 60V; Vc at this instant is zero (Fig 3).

Subsequently Vc begins to rise and will continue to rise exponentially until C is fully charged to +60 V in a time of 5CR seconds. Since at all times Vc+Vr = V, Vr falls exponentially towards zero as Vc rises.

At the instant when C is fully charged to the voltage of the supply (+60V), and Vr is consequently zero, suppose we switch back to the discharge position. This causes the applied voltage V to fall immediately from + 60V to zero. Remembering that C cannot change its charge instantaneously Vc will initially be unaffected by this and remain charged to + 60V. But what of Vr? Since V = Vc+Vr, and the applied voltage V is zero, we have at this instant:

    0= Vc + Vr
    therefor Vr = -Vc = -60V

Thus at the instant of switching to the discharge position the voltage Vr across R falls immediately to -60V. (The negative sign means that point Pin the circuit of Fig 2 is negative with respect to earth).


Previous page

To top of this page

Next Page

Constructed by Dick Barrett

(To e-mail me remove "ban_spam_" from my address)

ęCopyright 2000 - 2002 Dick Barrett

The right of Dick Barrett to be identified as author of this work has been asserted by him in accordance with the Copyright, Designs and Patents Act 1988.