This is 5V switching regulator circuit using transistor, decreases the size voltage or Step down voltage converter circuit. Make voltage output there is the size voltage a little more input at from circuit picture will decrease volt 6-18V from be left 5V. It give current get 100mA. in working in switching forms The strategy works as follows.
As soon as we enter the voltage to the transistor circuit Q1 and Q2 are connected as the astable multivibrator, the circuit will generate the frequency output at collector of Q2 before send to base pin of Q3. Q3, which will act like the on-off switch, Q4 works by applying voltage to determine the collector pin, But must pass before R6 base of Q4.
When base pin of Q4 has low voltage will cause Q4 begin working , C3 will charge through coil- L1. The ouput voltage will increase up continuously, but when have voltage more than 5 volts will results to Q5 working since have a voltage bias at pin base in way of ZD1. The transistor Q1 is stopped, the pin base is equivalent to ground there. The circuit shutdown frequency out, Q4 will stop working.
Because the voltage at the base pin of Q4 through R5 output so not have voltage., Q5 so stop working. Then Q1 is working again. And the behavior, it would go like the first one. Cause the loop to do this. However, there are very high, thus making has output 5 volts to appear at it.
Resistors size ¼W +5%
R1, R4, R7: 4.7K
R2, R3: 47K
R6: 100 Ohms
C1: 0.0015uF 50V Polyester
C2: 0.01uF 50V, Polyester
C3: 470uF 16V, Electrolytic
Q1, Q2, Q3, Q5: BC548, 45V 100mA NPN Transistor
Q4: BC337, 45V 800mA PNP Transistor
D1: 1N4001, 1A 50V Diode
ZD1: 5.1V 500mW, Zener diode
L1-transformer toroidal core diameter 2.5-3.0 cm.
Copper wire size 0.4 mm.
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