Various type of power supply circuit from basic regulated power supply to advanced digital control power supply unit is already posted in bestengineeringprojects.com. Now here one more power supply circuit is posted and can be used to provide up to 300mA and voltage between 6V to 13V.
Circuit Description of Variable Switching Power Supply
The input AC is stepped down using a step down transformer T1 to 18V AC which is further rectified by a bridge rectifier. The rectified output is filtered by 1000 capacitor C1. The zener diode ZD1 is used to form the reference voltage and output voltage is selected by potentiometer VR1.
Transistor T1 forms emitter follower, and voltage at emitter of T1 is equal to the voltage at the varying point of VR1. Transistor T2 is used to sense the output voltage.
Timer IC (IC1) is here configured as monostable multivibrator (timer circuit) for the time period of 0.2ms. The collector of PNP transistor T3 is connected to pin 2 of IC1 and is used to trigger. When the circuit is switch on, the voltage at T1 is equal to voltage at centre point of VR1. If voltage at centre point of VR1 is 8 volt, the emitter of T2 is also at 8 Volt and base is at zero. When transistor T2 is in off state no voltage is developed across R4 and transistor T3 continues in the off state. When transistor T2 conduct the voltage at collector is developed and pass through resistor R4. This turn transistor T3 on and voltage at pin 2 of IC1 becomes more then Vcc/2. This led output at pin 3 of IC1 goes high and transistor T5 start conducting and goes into saturation, while capacitor C3 get charge when the voltage across it reaches 5.6V.Aafter 0.2 ms the output of IC1 goes low and the capacitor C3 discharges through load.
When the voltage at pin 3 of IC1 decrease from 5.6V, transistor T2 goes off which further turn off transistor T3. This produce a negative trigger at pin 2 of IC1. Its output goes high and further turn on the transistor T4. This process is repeated continuously. The power loss is very low because transistor T4 only operates in saturation and cut-off mode.
When output load is increased, the capacitor C3 gets charged due to leakage current. To avoid this problem resistor R10 is used. To increase the current capacity of the circuit use high power transistor instead of SK100 (T4) or add one SK100 to its parallel.
Here IC1 is used as comparator and also provides some delay to output change. If the output ripple is high, decrease the value of C2 or omit it altogether.
BEP LAB NOTE: In BEP LAB we use AC128 instead of SK100 transistor and heat-sink to drive a stereo set.
PARTS LIST OF VARIABLE SWITCHING POWER SUPPLY
Resistor (all ¼-watt, ± 5% Carbon)
R1, R3 = 470 Ω
R2 = 330 Ω
R4,R5 = 100 KΩ
R6 = 82 KΩ
R7 = 1Ω, 1W
R8 = 4.7 KΩ
R9, R10 = 1 KΩ
VR1 = 10 KΩ Lin.
C1 = 1000 µF/20V
C2 = 2200 pF
C3 = 1000 µF/15V
C4 = 0.1 µF
IC1 = NE555
T1, T2, T5 = BC148
T3 = BC558
T4 = SK100
ZD1 = 12.V zener diode
X1 = 230VAC primary to 18V AC, 500 mA secondary transformer