# What is the minimum current that your power supply can supply before the zener exceeds its maximum power rating?

Why does the battery act like an unregulated power supply?

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(1) A full wave bridge rectifier power supply is powered from the secondary of a transformer which has a peak secondary voltage of 21V. The primary of the transformer is connected to a 50Hz, 230VRMS power supply. A 3400µF filter capacitor is used. A current of 1.3 Amp is drawn from the supply.

(i) Sketch a schematic diagram of the setup.

(ii) Assume that each diode conducts for one-twelfth (1/12th) of a cycle. What is the mean diode current while each is conducting?

(iii) Calculate the mean dc output voltage of the power supply.

(2) A circuit consists of a 12V battery, a LED with a forward voltage drop of 5.0V and a 330Ω current limiting resistor. Calculate the current in this circuit.

(3) A circuit requires a constant voltage of about 6 volts to be supplied to it at a constant current of 120mA. A 0.5 W, 6.0-volt Zener is available. The unregulated source to be used is a battery having an open circuit voltage of 14 volt and an output resistance of 2Ω.

(i) Sketch a schematic of your regulated power supply circuit.

(ii) Why does the battery act like an unregulated power supply?

(iii) Assuming that at the output current of 120mA the Zener is dissipating 0.25W, find a suitable value of Rz to place in series with your zener.

(iv) What is the maximum current that can be supplied by your regulated power supply?

(v) What is the minimum current that your power supply can supply before the zener exceeds its maximum power rating?

(vi) What voltage will be measured at the battery terminals when the design current of 120 mA is being drawn from the power supply?

(4) The circuit below shows a half-wave power supply feeding a LM7805 five-volt 3-terminal regulator. The LM7805 has a dropout voltage of 2 volts and a PSRR of 78 dB. The 3-terminal regulator supplies 1.5A at 5 volts to some load. The secondary of the transformer has an rms voltage of 10V and the primary is connected to a 230Vrms 50Hz mains power supply.

(i) Draw a labeled plot showing how the voltage across the filter capacitor changes with time. Make sure you label the peak-to-peak ripple voltage and the mean dc voltage on your plot.

(ii) What is the minimum voltage at the input (i.e.Vin) of the LM7805 3-terminal regulator if the 5V voltage supply is to stay in regulation?

(iii) What is the minimum capacitance of the filter capacitor required in the half-wave power supply (Cfilter) if the 5V power supply is to stay in regulation at all times?

(iv) What is the average voltage appearing at the input of the 3-terminal regulator assuming you use the filter capacitor you calculated in (iii) above i.e. what is the average value of Vin?

(v) What average power is dissipated in the 3-terminal regulator?

(vi) What will be the typical size of the ripple voltage at the output of the three terminal regulator assuming you use the filter capacitor you calculated in (iii) above?

(5) A pn junction of a silicon diode has a voltage of 0.68volts across it at room temperature. If the leakage (minority carrier) current of the pn junction is    at room temperature and N for the pn junction is 1.3, what will be the current through the diode?

(6) A sine wave with an amplitude of 19 volts is fed into the below diode limiting circuit. Assuming that the output is connected to a high impedance (so that there is no appreciable voltage drop across the current limiting resistor due to current going through the output load), draw what the output voltage waveform looks like and label the values of the voltages on the output waveform. All diodes have a forward voltage drop of 0.7 volts.

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