Dear friends, in this article, I have discussed in detail and provided the maximum information on how to make a lab power supply using the high-precision voltage regulator LM723 IC and two 2N3055 transistors, along with the schematic circuit diagram.
Since different electronic devices and circuits operate at different voltages, a variable voltage regulator is essential for any lab or electronics workshop as a stable, accurate, and powerful power supply. It is necessary for both testing and repairing sensitive equipment.
Unlike a simple adapter, a lab power supply provides precise control over both voltage and current output. In this article, I demonstrated how to build a 5-ampere (5A) high-precision power supply that is adjustable from 0V to 30V using the LM723C voltage regulator IC and two 2N3055 power transistors.
When reviewing the schematic circuit diagram during the design phase, don't lose interest by thinking it's too complicated. To build your skills in electronics, try to approach everything with a simple mindset and carefully observe each detail; you'll find that it becomes much easier. I recommend reading and thoroughly studying this article.
What is a Lab Power Supply?
A Lab Power Supply is a device specifically designed for experimental work, circuit testing, and repairing sensitive electronic equipment. It is far more functional and controlled than a simple wall adapter or battery.
A. Definition and Function of a Lab Power Supply
A Lab Power Supply converts an AC input into a stable and filtered DC output, which the user can precisely adjust within a specified range (e.g., 0V to 30V).
It offers two primary advantages:
- Stability (Regulation): It maintains the output voltage at the desired level, even when the load or the input voltage changes.
- Protection (Safety): It provides essential safeguards like over-voltage and, critically, current limiting. If a short circuit occurs during testing, the power supply quickly limits the current flow, protecting both the circuit under test and the power supply itself.
B. Which DC power source is most popular?
While many types of DC sources are used in electronics, Linear Regulation-based lab power supplies remain the most popular choice among professionals and hobbyists.
The reasons for this popularity include:
- Low Noise: Linear supplies typically generate very little electrical noise (ripple), making them ideal for testing sensitive analog and RF (Radio Frequency) circuits.
- Fast Response: They can quickly respond to changes in load conditions, ensuring the voltage remains stable and regulated.
In our project, we are building a reliable and popular DC source using the linear regulation capabilities of the LM723 IC.
The Complete 0V-30V 5A Variable Power Supply Schematic Circuit Diagram
Before understanding the functioning of the main components of the circuit, carefully look at the complete schematic diagram below. This diagram (Figure 1) clearly illustrates the basic rectification, voltage regulation (using LM723), and high current boosting (using 2N3055) sections of the lab power supply.
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| Figure 1: DIY Lab Power Supply Circuit Diagram (0-30V, 5A) using LM723 IC |
Component list and its value: Resistor, Capacitor, & Transistor
| Res | Value | Watt | Res | Value | Watt |
|---|---|---|---|---|---|
| R1,2,3,4,5 | 0.1Ω | 5 | R6 | 75Ω | 1/2 |
| R7,11 | 10kΩ | 1/4 | R8,10 | 100kΩ | 1/4 |
| R9 | 3.3KΩ | 1/4 | R13 | 100kΩ | 1/4 |
| CAP | Value | Volt | CAP | Value | Volt |
| C1 | 4700µF | 50 | C2 | 0.1 | 100 |
| C3 | 220µF | 50 | C4 | 470PF | 50 |
| Trans | Value | - | Trans | Value | - |
| Q1, Q2 | 2N3055 | - | Q3, Q4 | 2N4037 | - |
U1 - LM723CN IC
D1, D2, D3, And D4 - 1N4007
T1: Transformer 35V 8Amp (Note: For better safety with 50V caps, a 28V AC transformer is recommended.)
Circuit Analysis and Working Principle
I have analyzed the fundamental roles of the LM723 IC and the 2N3055 power transistors. Now, let me analyze the complete schematic diagram (Figure 1) to understand how each component collectively creates a stable 0V–30V, 5A variable power supply. I will break down the circuit analysis into four main segments: Input and Filtering, Current Boosting and Power Path, Regulation and Control, and Current Limiting.
A. Input and Filtering Section
This section is responsible for deriving the necessary DC voltage from the AC mains input.
- Transformer (T1): It steps down the AC mains voltage to a suitable lower AC voltage as required by the circuit. A 28V AC transformer (8A) is generally safer for the 50V filter capacitor (C1).
- Rectifier Bridge (D1, D2, D3, D4): Full-wave rectification is performed using four diodes. This converts the AC input voltage into a pulsating DC voltage.
- Main Filter Capacitor (C1 - 4700µF): This large capacitor smooths the pulsating DC voltage coming from the rectifier. It reduces the voltage ripple, generating a stable DC input voltage ($V_{IN}$) that goes directly to Pin-12 of the LM723 and the collectors of the pass transistors.
B. Current Boosting and Power Path
This part ensures the delivery of the high current required by the load, utilizing the control signal from the LM723.
- Pass Transistors (Q1, Q2 - 2N3055): These two transistors are the output power stage of the circuit, functioning as series regulators. They are driven by the control output of the LM723, enabling the delivery of current up to 5A through the load.
- Current Distribution (R1, R2, R3, R4, R5 - 0.1Ω):
- R1 and R2: These resistors ($0.1\Omega$ connected in parallel) are linked to the collectors of Q1 and Q2 ($V_{IN}$) to limit current spikes and ensure proper load sharing between the transistors.
- R3, R4, and R5: These three $0.1\Omega$ resistors are connected to the emitters of Q1 and Q2, playing a crucial role in current sensing and maintaining output line stability.
- Q2 Base-Emitter Resistor ($75\Omega$): The $75\Omega$ resistor connected between the base and emitter of Q2 is used for proper biasing and stabilization of the Q2 transistor.
C. Regulation and Control Section
This section forms the LM723 control loop, responsible for adjusting and stabilizing the output voltage.
1. Voltage Adjustment Mechanism
- Reference Voltage ($V_{REF}$): Pin-6 ($V_{REF}$) of the LM723 provides a highly stable internal reference voltage.
- Potentiometer (VR1): VR1 is connected between $V_{REF}$ and Ground. I set a specific voltage on Pin-4 (Inverting Input) of the LM723 using the center pin of VR1 via the $R11 (5K\Omega)$ resistor. This voltage is directly controlled by the reference voltage.
- Pin-5 Biasing (Non-Inverting Input): Pin-5 is connected to Ground via R12 ($100K\Omega$) and to $V_{REF}$ (Pin-6) via R13 ($10K\Omega$). This arrangement provides a fixed, stable biasing voltage to Pin-5.
- Feedback and Regulation: The output voltage of the circuit is fed back to Pin-4 via a resistor network. The LM723 compares this feedback voltage with the fixed voltage on Pin-5 and controls the pass transistors (via Pin-10) to ensure the output voltage matches the value set by VR1.
2. Achieving Zero Volt Output ($0V$ Control)
Typically, the LM723 design is not suitable for voltage regulation below $2V$. However, a special arrangement is used in this circuit to enable the voltage to start from $0V$.
- PNP Transistors (Q3, Q4): A specialized drive circuit is built using two PNP transistors (Q3 and Q4), which are driven by Pin-11 ($V_{C}$) of the LM723. The main role of this PNP drive stage is to level-shift the control signal from the LM723 and properly drive the bases of the pass transistors (Q1, Q2), especially when the output voltage is set near zero (0V).
- LM723 Pin-11 ($V_{C}$): Pin-11 of the LM723 is connected via R9 to the base of Q3 and the collector of Q4. The emitters of both Q3 and Q4 are connected directly to the main $V_{IN}$.
D. Current Limiting Mechanism
While the LM723 IC includes internal current-limiting pins (Pin-2 and Pin-3), this specific high-current design utilizes the external resistor network (R3, R4, R5) to monitor the current indirectly. Since the output current needs to be precisely limited to 5A, the combined low-value resistors ($R_{SENSE}$) in the emitter path of Q1 and Q2 act as the current sensing mechanism for the external control loop.
- The Role of External Sensing: In this high-power circuit, the current limiting functionality is implemented through the combination of the LM723's internal reference voltage and the control loop, which monitors the voltage drop across the emitter resistors (R3, R4, R5). When the current exceeds the safe limit, the control loop reduces the drive to the pass transistors (Q1, Q2).
- Protection: This is a crucial safety feature. If the load draws too much current (e.g., due to a short circuit), the current is capped at the maximum set value (5A), protecting both the supply and the load.
E. Stability and Compensation
- Compensation Capacitor (C4 - 470PF): Connected between Pin-13 (Frequency Compensation) and Pin-4 (Inverting Input), the C4 capacitor stabilizes the regulation loop. It helps prevent unwanted oscillation in the circuit.
- Ground Connections: LM723 Pins 7 and 9 are connected to Ground (GND).
- Output Sockets: The red banana socket serves as the Vout (positive output), and the black banana socket serves as the GND (negative Vout).
Core Components: LM723 and 2N3055
Your powerful lab power supply is based on two main pillars: the LM723 Voltage Regulator IC for high-precision control, and the 2N3055 Power Transistors for boosting the output current. The combined use of these two components allows us to achieve both the 0V–30V range and the high load current capability of up to 5A.
A. LM723CN: Precision Control IC
The LM723 / LM723C is a classic and highly reliable voltage regulator IC, primarily designed for series regulator applications. It serves as the brain of our circuit.
1. Key Functionality:
- Voltage Reference: The LM723 contains a stable internal voltage reference, which acts as the benchmark for regulating the output voltage.
- Regulation and Control: It uses the inverting and non-inverting input pins to monitor the output voltage, compare it with the desired value, and generate a precise control signal.
- Output Range: This IC allows the output voltage to be adjusted from 2 volts to 37 volts.
- Current Capability: The LM723 IC can inherently supply an output current of up to 150 mA on its own.
2. Pinout Used in the Circuit:
In this 0V–30V design, several key pins of the LM723 are utilized. The maximum input voltage capability of the IC is approximately 40V.
- Pin 12 ($V_{+}$): The filtered DC input voltage from the external adapter is supplied directly to this pin.
- Pin 7 ($V_{-}$): Connection for the ground or negative terminal.
- Pin 6 ($V_{REF}$): The stable internal reference voltage output.
- Pin 4 ($V_{INVERTING}$) and Pin 5 ($V_{NON-INVERTING}$): These pins are the core of the voltage adjustment and feedback mechanism.
(Note: While pins 1, 2, and 3 are typically used for internal current limiting, this high-current design relies on the external components for monitoring.)
B. 2N3055 Transistor: High Current Boosting
The LM723 IC's internal 150 mA current capability is insufficient for our required 5A load. To meet this demand, the powerful 2N3055 transistors are employed.
1. Role and Function:
- Pass Transistor: The 2N3055 acts as an External Pass Transistor. It takes the small control signal from the LM723 and amplifies it to supply the large required load current (up to 5A). The datasheet confirms that adding external transistors makes output currents in excess of 10A possible.
- Thermal Management (Heatsink): At high load currents, the 2N3055 transistors dissipate significant heat. To protect the circuit from overheating, it is essential to use an adequately sized Heatsink with the transistors. Using two 2N3055 transistors, as shown in the schematic, helps to distribute the current load and reduce the heat generated by a single component.
Conclusion
The DIY LM723 and 2N3055 Variable Lab Power Supply project successfully demonstrates how to combine precision control (LM723 IC) with high-current boosting (2N3055 transistors) to create an essential tool for any electronics workspace.
By understanding the functions of the three key segments—Input/Filtering, Regulation/Control, and Current Boosting—and utilizing the special $0V$ circuitry, we achieve a stable, accurate, and powerful $0V$ to $30V$, $5A$ output. Most importantly, the inclusion of the Current Limiting feature ensures that both your power supply and the sensitive circuits you are testing remain safe from short circuits and overload conditions.
Building this circuit is a highly rewarding exercise that significantly enhances your practical knowledge of linear regulation and high-power electronics. Happy building, and always prioritize safety in your lab work!
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