Bench Power Supply Adjustable 1.25V-15V and Fix 5V

Bench Power Supply Adjustable 1.25V-15V and Fix 5V

This is bench Power Supply give you three types of voltage output that are adjustable 1.25V-15V / 1A positive voltage, adjustable 1.25V-15V / 1A negative voltage and Fix 5V 3A positive voltage.

The regulator circuit in this bench Power Supply consists of a LM-317 integrated circuit. This IC is specially designed to perform the regulation function. Figure 6 shows a simplified circuit of how the LM-317 IC works.

Transistors Q1 and Q2 form a circuit known as a differential amplifier. Transistor Q1 base is connected to a stable 1.5V reference voltage. The base of Q2 is connected to the regulator output circuit through a voltage divider network. The collector of transistor Q2 is connected to a current source. This basically is a PNP transistor biased to draw about 1mA of current. Transistor Q2 sees the current source as a very high resistor of about 1 meg ohms. Thus, the gain of transistor Q2 is extremely high.

Transistor Q5 is called the pass transistor. It controls the current reaching the output. Transistors Q3 and Q4 are emitter followers. Their function is to raise the impedance of the pass transistor. Note that transistors Q2, Q3, Q4 and Q5 and resistor R1 form a close loop. Also, note that the feedback to the base of Q2 is negative, that is, when the base of Q2 goes positive, the output at emitter Q5 goes negative. Now if the 2V output voltage goes down because of current drain at the output, the base of Q2 will drop forcing the collector voltage of Q2 to go higher. This will bring the output voltage back to 2V. This is the basis of all negative feedback regulators.

The 5 Volt Power Supply
In the previous discussion of the variable voltage regulators, the ICs can handle about 1A of current. In the design of the 5V supply, we need 3A of current. To meet this current requirement, we must add an external pass transistor capable of delivering 3A.

Components List:

0.18 Ohm 3W : 1
2.7 Ohm 1/2W (red-violet-gold-gold) : 2
180 Ohm 1/4W (brown-gray-brown-gold) : 2
2K Ohm Potentiometer : 2

10mF (10000 uF) Electrolytic Capacitor : 5
2,200mF (2200 uF) Electrolytic Capacitor : 2
4,700mF (4700 uF)Electrolytic Capaticor : 1

Diode 1N4001 : 4
Diode 1N5400 : 4
Transistor MPSA70 : 1
Transistor 2N6124 : 1
IC LM-317 : 1
IC LM-337 : 1
IC LM-7805 : 1

Fuse 1.25A
Rocker Switch
Heat Sink

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Tags: adjustable power supply bench power supply lab power supply multi purpose power supply

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