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Adjustable 1.2–12V, 1A Power Supply with Low-Voltage Indicator

This adjustable power supply provides a DC output from 1.2V to 12V with a maximum current of 1A. It includes a low-voltage indicator LED circuit using three diodes and one LED, which allows you to monitor the output voltage, for example while charging a battery. The circuit also features built-in current protection through the LM317 regulator, which activates when the output current exceeds about 0.85A, warning you with the LED indicator about low voltage conditions.

Power Supply Requirements

  • Transformer: Select carefully; some 10VA transformers may not deliver the specified current reliably.
  • Input: AC voltage suitable for the LM317 to regulate to 1.2–12V DC.
  • Output current: up to 1A, with built-in current limiting and protection.

Key Components

  • LM317 Adjustable Voltage Regulator: Provides regulated output voltage from 1.2V to 12V.
  • Three Diodes + LED: Form the low-voltage indicator circuit, showing output voltage status or overload condition.
  • Filter Capacitors: Smooth the DC output and reduce ripple.
  • Resistors: Set the output voltage range and current limiting thresholds.
  • Transformer: Steps down mains AC voltage to a suitable level for regulation.

Circuit Operation

  • The transformer steps down the AC mains voltage, which is then rectified and filtered to produce DC for the LM317 regulator.
  • The LM317 provides adjustable output voltage from 1.2V to 12V using a resistor/potentiometer network.
  • The low-voltage indicator LED, in combination with three diodes, lights up when the output voltage drops below a certain level, for example during high load or battery charging.
  • If the output current exceeds about 0.85A, the LM317’s internal current limiting reduces the voltage, and the LED indicator warns of the overload.
  • The design effectively prevents mains noise from appearing on the DC output, providing a clean and stable voltage.

Key Features

  • Adjustable 1.2–12V DC output
  • Maximum current: 1A with current limiting protection
  • Low-voltage indicator LED shows output status
  • Protects connected batteries or loads from overcurrent
  • Clean DC output with low ripple and noise
  • Simple and reliable design for battery charging and general-purpose use

Construction Notes

  • Select a transformer capable of supplying the required current; verify the actual VA rating under load.
  • Use adequate heatsinking for the LM317 to prevent thermal shutdown under full current.
  • Ensure correct diode and LED orientation for proper low-voltage indication.
  • Use sufficient filter capacitance to maintain smooth DC voltage.
  • Test the circuit gradually with increasing load to confirm the LED indicator and current limiting work as expected.

Power supply 1-12V schematic