[a.mehran]

a long time.

This project can control the amount of current flow is maximum. We modified the circuit as circuit in Figure 1. Which this circuit, the maximum current is adjustable from 0 – 2A.
How it works
Figure 1 we add the current controller include transistors Q4,Q5,Q6 and Q7, and few others parts.
The potentiometer VR2 is controlled the maximum amount of current flow.
Normally, we adjust VR2 at Q5 work, so Q3,Q4,Q6 not also works.
When Q4 stop will make voltage at base of Q7 high up, makes Q7 works, LED L2 will glow to show that the normal operation.

Figure 1 the schematic diagram of 0-30V,0-2A adjustable voltage and current regulator.
If current flow too much than setting. The voltage drop across R4 will higher up until voltage at emitter compare with base is lower down. So makes Q5 not works. Then, the negative voltage to base of Q3 , makes Q3 works, but Q2 stop.
Thus,the transistor-Q1 not get the bias voltage makes works. And no current to load. While from Q5,will makes Q6 works then voltage at base of Q5 is positive up, so Q5 stop working. This circuit will be hold all time though we will remove the load.
Cancellation of holdem circuit. Do this by pressing the reset switch to short the bias of Q6 until stop working to circuit back to normal state. But still short circuit we press the reset switch will not work.
At the same time, when over load, until Q5 stop that will makes Q4 work. The LED L1 will glow to show we know that overload. While Q7 will stop working so LED L2 go out.
The LED L1,L2; we may use the 2 color in one. By will display is green for normal and red for overload.
Parts you will need
IC1_____________LM723___HIGH PRECISION VOLTAGE REGULATOR
Q1______________2N3055___100V, 15A, 115W, >2,5MHz NPN transistor
Q2______________BD140____80V 1.5A PNP Transistor
Q3,Q4,Q5,Q6,Q7__ BC327___45V 800mA PNP Transistor
D1,D2___________1N5402___3A 100V Silicon Diode
D3,D4___________1N4148
ZD1_____________ 30V 1W____Zener diode
ZD2 _____________ 13V 1W____Zener diode
ZD3_____________ 15V 1W___Zener diode
L1,L2____________ 2 color LED
R1,R8,R14,R18____ 10K___0.5W__Resistors tolerance: 1%
R2,R3,R6_________ 100K___0.5W__Resistors tolerance: 1%
R4_______________ 0.3 ohms 5W__Resistors tolerance: 5%
R5_______________ 47 ohms___0.5W__Resistors tolerance: 1%
R7,R9,R11_________ 4.7K___0.5W__Resistors tolerance: 1%
R10______________ 470 ohms___0.5W__Resistors tolerance: 1%
R12,R20__________ 3.3K____0.5W__Resistors tolerance: 1%
R13______________ 1K____0.5W__Resistors tolerance: 1%
R15______________ 27K____0.5W__Resistors tolerance: 1%
R16______________ 6.8K x 2 in parallel ____0.5W__Resistors tolerance: 1%
R17______________ 6.8K ____0.5W__Resistors tolerance: 1%
VR1_______________ 5KB___Potentiometer
VR2_______________ 3KB___Potentiometer
C1________________ 1000uF 50V___Electrolytic Capacitors
C2________________ 0.047uF 50V___Electrolytic Capacitors
C3________________ 100uF 50V___Electrolytic Capacitors
C4________________ 3.3uF 50V___Electrolytic Capacitors
S1________________ Push SW
T1________________ 24-0-24V 3A transformer
How to builds

Figure 2 the PCB layout and components layout.

We assemble all parts on PCB as Figure 2 correctly. Then, apply voltage to circuit and adjust VR2 until LED L2 glow, show that the circuit work normally. Next, measure the output voltage and adjust VR1 to output 0-30V as need.
Test short circuit at output of circuit. The LED L1 will glow. If it not glow,adjust VR2 until LED L1 glow. Then press the reset switch S1 the circuit will come to normal.