Consider the voltage regulated DC power Supply shown below. R=100 Ohm and RL= 30

Question

Consider the voltage regulated DC power Supply shown below. R=100 Ohm and RL= 300 Ohm. The voltage across the capacitor turns out to have an average DC value of 28 volts plus a ripplt of plusminus 4 V vc= V0 + VR(t) where V0 =32 V and VR(t) swings between -4 to +4 V Find the DC Thevenin equivalent circuit as seen by the Zener diode (focus on the dashed box on the right: switch Zener diode with the load). Show that If there is a ripple of plusminus 4 V in vc show that the ripple in the VTh is plusminus 3 V. Specifically, what are max, min and DC value of VTh On the next page, plot 3 load lines corresponding to above values of VTh. Use Thevenin voltage and Thevenin resistance for each load line. What is the value for slope of these lines. Determine graphically, the corresponding ripple in the Zener diode voltage and give numerical Show (using graph and geometry) that Output ripple/Output ipplt=rz/rz+Rth

Explanation / Answer

1) replace the zener diode with an open circuit , then , using voltage divider , you will get :

Vth (dc) = [RL / (RL+ R)] VC(dc)

=> Vth = (300/400) * 28 = 21 V

Rth = RL // R = 100(300) / 400 = 75 ohms

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2) again :

Vth(ac) = [RL / (RL+ R)] VC(ac)

so Vth(ripple) = (3/4) * (vC ripple)

=> Vth ripple = (3/4) * 4 = 3 V

hence Vth ripple swings between 3 , - 3 Volts

therefore :

Vth(min) = Vth(dc) - 3 = 21 - 3 = 18 V

Vth(dc) = 21 V

Vth(max) = 21 + 3 = 24 V

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