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Adjustable Current Limit for Dual Power Supply Schematic Circuit Diagram

Introduction to the Circuit

This current-limiting circuit, demonstrated as a part of a compact bench power supply, possesses versatile applications in conjunction with various dual-rail current sources. The primary focus of this circuit is to limit the current input to the dual voltage regulator (IC4 to IC7), safeguarding it against overload. While the output yields ±15 V and ±5 V using regulators 7815/7805 and 7915/7905, the current-limiting section, incorporating LM317 and LM337, requires a detailed explanation.

Current Limiting Functionality

The upper LM317 (IC1) in this circuit oversees the current-limiting task for the upper segment. The ingenious aspect lies in the configuration of resistors R1 and R3 between the output and the adjust input of the regulator. In a standard LM317 setup in current-limiting mode, a single resistor is utilized, maintaining a constant voltage of 1.25 V across it. Consequently, the current becomes limited to 1.25 V/R. To achieve a maximum current, such as 1 A, appropriate resistor values must be selected based on the formula.

Application and Adaptability

The circuit is suitable for driving a high-brightness 660 nm red LED, making it suitable for a DIY phototherapy unit. Due to its simplicity, the circuit can be directly soldered or assembled on a small stripboard. Its operational flexibility allows the use of various voltages, accommodating alkaline AA cells or AA-size NiMH rechargeable cells with a voltage of 1.2 V. The circuit consumes approximately 20 mA. Upon correct assembly, the red LED should illuminate brightly upon power application, providing noticeable benefits after regular use.

Adjustable Current Limit for Dual Power Supply Schematic Circuit Diagram

Challenges in Building an Adjustable Dual-Rail Current-Limited Supply

Building an adjustable dual-rail current-limited supply faces challenges due to the unavailability of stereo potentiometers with the required 1.2 Ω value. The problem is resolved by dividing the resistor into two: R1 for current sensing and R3 for an additional voltage drop, adjustable according to a secondary current. When the combined voltages reach 1.25 V, current limiting initiates, allowing smooth adjustment of the current limit through R3. This technique applies to both positive and negative branches.

Optimizing Current Adjustment and Limitation

Efficient current adjustment involves ensuring the secondary resistor (R3) has significantly greater resistance than the main resistor (R1). For instance, if R1 equals 1.2 Ω (yielding a 1 A maximum current) and R3 equals 120 Ω, the voltage drop is achieved with an extra 10 mA current. In the negative branch, LM337, along with R2 (1.2 Ω) and R5 (120 Ω), serves similar functions. LM317 (IC3) sets the overall current limit point by regulating the additional current.

Adjustment Range and Voltage Considerations

The resistance combination of R4 (22 Ω) and P1 (100 Ω) determines the additional current and output current range, adjustable from milliamps to 1 A. The input voltage depends on the desired output voltage and maximum output current. An input of at least ±22 V is required for well-regulated outputs at ±15 V and ±5 V. Single transformer winding use necessitates careful consideration of diodes, smoothing capacitors, and ripple effects. Electrolytic capacitors (C6 to C9) with 4.7 μF or 10 μF values can be utilized based on the application. Additional stability can be achieved by connecting electrolytic capacitors in parallel with C1, C2, C4, and C5.

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