7. Design of BJT, Op. Amp., IC Regulated Power Supplies 7.1 Zener diode and Zener Regulator Design 7.2 Zener regulated Power Supply design 7.3 BJT regulated Power Supply design 7.4 Operational Amplifier Regulated Power Supplies 7.5 IC Regulator Design

7.1 Zener diode and Zener regulator design Regulated power supply = output dc is constant (stable) at different loads or at varying ac supply conditions = battery source characteristics Zener diode

Zener diode design conditions VSmin to VSmax Whether input voltage is VSmin or VSmax , the output voltage will be constant at Zener breakdown voltage VZ Constant output voltage is the regulated output voltage and the circuit is Zener regulator circuit. Zener current will becomes less IZmin at VSmin and it will increase to IZmax at VSmax Minimum Zener current IZmin should not less than 10% IZmax to maintain constant VZ Maximum Zener current IZmax should not more than PZ/VZ not to burn the Zener diode

Zener regulator design equation 1. When the load draws more current (ILmax), Zener current will becomes less (IZmin) and the supply voltage will becomes smaller (VSmin) then: 2. When the load draws less current (ILmin), Zener current will becomes more (IZmax) and the supply voltage will increase (VSmax) then:

3. Then equating the two equations, we have: Zener rating design equation Zener series resistor Ri rating design equation

Summary of Design Equations Zener Regulator Design IZmax Design Ri

Design Example Draw the Zener regulator circuit The load current ranges from 100mA to 200mA and the input source voltage ranges from 14V to 20V. Regulated output voltage is 10V Find the range of RL Design the required power rating of the Zener PZ Design the series resistor Ri and it’s power rating

7.2 Zener regulated Power Supply design Zener regulated Power Supply design (full-wave) DV Full-wave Rectifier with capacitor filter Zener regulator Zener regulated Power Supply (full-wave) Note that Ri is connected between VSmax and VZ, Because the Zener voltage is constant DV will appears across Ri fP = 2fS

Zener regulated Power Supply design (bridge) fP = 2fS Bridge Rectifier with capacitor filter Zener regulator Zener regulated Power Supply (bridge) Zener regulated Power Supply design (half-wave) fP = fS Half-wave Rectifier with capacitor filter Zener regulator Zener regulated Power Supply (half-wave)

7.3 BJT regulated Power Supply design BJT regulated Power Supply design (full-wave) In BJT regulated power supply, current IL is divided by b to get Zener output current now shown as IB

Now the capacitor discharge resistance is not only Ri //( VS / IL ) at the BJT collector where (VS/IL)<< Ri due to high collector current. Both VS and IL have (max) and (min). Designer should consider smallest REQ to satisfy the required VS range. Therefore VSmin / ILmax is taken as worst case REQ Note that REQ is across the capacitor and VS. Therefore VSmax is used to find C, whereas Ri is across (VSmax -VZ) which is used to find C due to Ri

Summary of Design Equations Regulated Power Supplies Zener Regulated Power Supplies BJT Regulated Power Supplies Design IZmax Design Ri Design IZmax Design Ri

Design Example Draw and Design a full-wave Zener regulated power supply using a 8:1+1 center-tapped transformer and an 8V, 1W Zener diode that will provide a constant 8V to a load varying from 200 to 500W. The input to the transformer is 120V,60Hz.,and ignore transformer losses and the diode drop. Determine:(a)IZmax and IZmin (b) R i and VSmin (c)Size of capacitor needed (d) Percent regulation if RZ=2W

Design Example Draw and Design a full-wave BJT regulated power supply using a 3:1 center-tapped transformer and an 12V Zener diode .The load current varies from 400 to 500mA. The input to the transformer is 120V,60Hz., Take DV = 30% and b = 100. Determine: (a) IZmax (b) R i (c) Size of capacitor needed (d) Percent regulation if RZ=2W

7.4 Operational Amplifier Regulated Power Supplies (detail of Op. Amp. in Chapter 9) +Vcc -Vcc Inverting Input (-) output Non Inverting Input (+) + - ii = 0 v+ = v- R0= 0 Properties of Op. Amp. (1) ii = 0 (2) vi = 0

(a) Small IL less than 25mA Variable R1

Example: Draw a 0 to 12V regulated output voltage Op. Amp. power supply. The load current is IL< 25mA PZ =0.5W and VZ=12V. Design the values of Rv, Ri, RL(min) if supply VCC=16V.

(b) Large IL using BJT output Variable R1

Example: Draw a 0 to 12V regulated output voltage Op. Amp. power supply. The load current is IL= 100mA PZ =0.5W and VZ=12V. Design the values of Rv, Ri, RL (min) and (min) if supply VCC=16V. Find actual Io if =50 for the present BJT .

(c) Zener voltage multiplier BJT output Variable R1

Example: Draw a 0 to 12V regulated output voltage Op. Amp. power supply. The load current is IL= 100mA PZ =0.5W and VZ=9V. Design the values of Rv, Ri, RL (min) and (min) if supply VCC=16V. Find actual Io if =50 for the present BJT .

(d) Output current limited Op. Amp voltage regulator Limited current IL=IC1 Variable R1

Example: Draw a 0 to 12V regulated output voltage Op. Amp. power supply. The load current is limited to IL= 500mA PZ =0.5W and VZ=12V. Design the values of Rv, Ri, RE1 if supply VCC=16V. Find IC2(max) if 1=100.

7.5.1 IC Regulator Design (Fixed Voltage ) Fixed Positive Voltage Regulator IC 1 Voltage regulator IC 2 3 7812 Vo=+12V +Vi Regulated power supply +12V Unregulated power supply C2 RL C1 is a ripple filter capacitor (C1 > 100mF) C2 is a high frequency filter (C2 < 0.1mF) IC Part Number Output Voltage Min input Voltage Max input Voltage 7805 +5V +7V +25V 7806 +6V +8V 7808 +10.5V 7810 +10V +12.5V +28V 7812 +12V +14.5V +30V 7815 +15V +17.5V 7818 +18V +21V +33V 7824 +24V +27V +38V

Fixed Negative Voltage Regulator IC 1 Voltage regulator IC 2 3 7912 Vo=-12V -Vi Regulated power supply -12V Unregulated power supply C2 RL C1 is a ripple filter capacitor (C1 > 100mF) C2 is a high frequency filter (C2 < 0.1mF) IC Part Number Output Voltage Min input Voltage Max input Voltage 7905 -5V -7V -25 7906 -6V -8V 7908 -10.5V 7910 -10V -12.5V -28 7912 -12V -14.5V -30 7915 -15V -17.5V 7918 -18V -21V -33 7924 -24V -27V -38

7.5.2 IC Regulator Design (Adjustable voltage) The IC regulator LM317 (LM318 for negative voltage) can be used to produce any regulated output voltage between 1.2V to 37V. Typical IC values are: Vref = 1.25V and Iadj = 100mA. Adjustable Voltage regulator IC 1.2V < Vo< 37V -Vi VIN VOUT ADJ LM317 R1 R2 Iadj = 100mA Vref = 1.25V Current through R1 and R2 can be taken for the design as 100Iadj=10mA Therefore R1 design will be R1 = 1.25V/10mA = 0.125kW=125W

Summary of Design Equations IC Regulators 1 Fixed Voltage IC 2 3 7805 Vo=-5V -Vi Variable Voltage IC VOUT ADJ LM317 ILmax VSmin C1 R1 R2 ILmax VSmin C1 IR1=10mA Iadj = 100mA Vref = 1.25V Min input Voltage Max input Voltage -7V -25

Example: Draw and design a -12V IC regulated bridge power supply using a MC7912 voltage regulator. Minimum and maximum rated voltages of the IC are -14.5V and -30V respectively. Select the transformer turns ratio “n” , and the capacitor value using an input of 115V at 60 Hz and an output current of 100mA to 500mA. 1 Voltage regulator IC 2 3 7912 Vo=-12V -Vi

Example: Draw and deign the LM317 IC regulator circuit values of transformer turn ratio “n”, C1, R1, and R2 if the maximum load current IL is 100mA, and if a ripple of peak-to-peak voltage of DV=2V is present at the input of the regulator. The output voltage of the variable IC regulator is 14V dc. Take 220V ac supply at 50Hz. and Vsmin = 15V VIN VOUT ADJ LM317 R1 R2 Iadj = 100mA Vref = 1.25V Current through R1 and R2 can be taken for the design as 100Iadj=10mA Therefore R1 design will be R1 = 1.25V/10mA = 0.125kW=125W