Question

Find the current and voltages values for C1, C2, C3, C4. Show all work

vortage reaolator Rv C4 C3 2. Dodes IN4

the voltage regulator is a LM7812

capacitor ,ceramic disc, 0.1uF, 50V, 20%
capacitor, radial, 4700uF, 25V, 20%
capacitor, radial, 1uF, 25V, 20%

Answer 1

From the datasheet of 1N4001 diode, forward voltage is 1.1 V. Screenshot of datasheet of 1N4001 is shown below

Ratings at 25 CJ ambient temperature unless otherwise specified Single phase, half wave, 60 Hz, resistive or inductive load For capacitive load, derate current by 20% 1N4001 1N4002 1N4003 1N4004 1N4005 1N4006 1N4007 UNITS 100 200 400 75 100 200 Maximum Recurrent Peak Reverse Voltage 50 Maximum RMS Voltage Maximum DC Blocking Voltage Maximum Average Forward Rectified Current .375"(9.5mm) Lead Length at Ta=75 ¢J Peak Forward Surge Current 8.3ms single half sine-wave superimposed on rated load JEDEC method Maximum Forward Voltage at 1.0A DC and 25 CJ Maximum Full Load Reverse Current Full Cycle Average at 75 CJ Ambient Maximum Reverse Current at TA 25 cJ At Rated DC Blocking Voltage T,-100 ¢J Typical Junction capacitance (Note 1 Typical Thermal Resistance (Note 2) R EKJA Typical Thermal resistance (NOTE 2) R EK JL Operating and Storage Temperature Range 600 800 1000 140 280420560700 400600 800 1000 35 50 1.0 30 1.1 30 5.0 Eg A Eg A Eg A 500 50 25 55 to +150 CJ

At any half cycle, two diodes are conducting at a time. So total voltage drop=1.1+1.1=2.2 V.

Therefore,. Voltage drop across C1 and C2 will be the difference between secondary winding voltage and total voltage drop across diodes

Voltage across C1 and C2= 28-2.2=25.8 pulsating DC.

Average value of full wave rectifier

2Vm 228 = 17.8V

So average voltage across C1 and C2 is 17.8V

Now,

LM7812 is a voltage regulator , that provide constant 12V at its output pin. Screenshot of LM7812 is shown below

Electrical Characteristics (LM7812) (V 19V, Io-500mA, 0°C STjs125 °C, unless othe e spec Parameter Output Voltage Line Regulation Svmbol Conditions MIN TYP MAXUNIT 12.5 240 120 240 120 12 10 0 11.50 Ti 25 °C V,-14.SV to 30V T,-25 °С Vi 16V to 22V T 25°C o-5mA to 1.5A, 25 °C lo 250mA to 750mA, 25 °C Vi 15V to 25V, f 120Hz F 10Hz to 100Hz TJ 25 °C T, 25 °C т,-25 °C V,-14.SV to 30V, lo 5mA to 1A, T 25 °C mV Vo Load Regulation mV AVo Ripple Rejection Output Noise Voltage Dropout Voltage Quiescent Current Quiescent Current Change dB 75 2.0 4.3 8.0 mA mA AlQ Т,-25 °С 0.5

So the voltage across C3 and C4 will be 12V because they are connected parallely to output pin of 7812.

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