AS431 Adjustable Shunt Regulator for Power Supply and Voltage Reference

ElecSuppliers Part Number GuideAug 22, 2026

AS431 is an adjustable shunt regulator designed for voltage regulation, voltage reference and power management applications. It provides a nominal 2.5 V reference and allows the regulated voltage to be set with external resistors.

The device is widely used in switching power supplies and other circuits that require a stable and adjustable reference voltage. Its three-terminal configuration provides a simple way to build feedback and shunt-regulation circuits without using a more complex regulator IC.

AS431 Key Specifications

AS431 features a nominal 2.5 V reference voltage and supports an adjustable output range of approximately 2.5 V to 36 V.

The device can sink up to 100 mA of current, making it suitable for a variety of feedback and regulation circuits. Depending on the specific version, different accuracy grades and package options are available.

The operating temperature range can extend from approximately -40°C to +125°C, making AS431 suitable for commercial, industrial and power-management applications.

When purchasing the component, the complete manufacturer part number should be checked because different AS431 versions may have different tolerances, packages and pin configurations.

AS431 Adjustable Voltage Reference

The main advantage of AS431 is its adjustable reference function.

A conventional Zener diode generally provides a fixed voltage characteristic, while AS431 allows the designer to establish the required regulated voltage using an external resistor divider.

This makes it possible to use the same basic regulator architecture for different output voltages.

For example, a designer can select appropriate resistor values to configure AS431 for a regulated output higher than the internal 2.5 V reference. This flexibility makes the device particularly useful in power-supply feedback circuits.

How AS431 Works

AS431 monitors the voltage applied to its reference terminal.

When the voltage at the reference terminal reaches the internal reference level, the device adjusts the current flowing through its cathode and anode terminals.

An external resistor network determines the voltage that the device regulates.

In a typical circuit, two resistors form a voltage divider between the regulated output and ground. The midpoint of this divider connects to the reference terminal of AS431.

As the output voltage changes, the AS431 responds by changing its cathode current. This feedback action can then be used to control another device, such as an optocoupler in an isolated switching power supply.

AS431 in Switching Power Supplies

AS431 is commonly found on the secondary side of isolated switching power supplies.

In this type of circuit, the device monitors the output voltage and controls an optocoupler. The optocoupler transfers the feedback signal across the isolation barrier to the primary-side controller.

If the output voltage rises above the desired level, the feedback circuit changes the optocoupler current. The primary controller can then reduce the energy delivered to the output.

If the output voltage falls, the feedback loop produces the opposite response.

This closed-loop control helps maintain a stable output voltage under changing load and input conditions.

AS431 Reference Voltage

The internal reference voltage of AS431 is approximately 2.5 V.

Reference accuracy is an important consideration when the device is used in precision voltage regulation.

Different versions can provide different reference-voltage tolerances. A tighter tolerance version can be selected when the final output voltage needs to remain closer to its nominal value.

However, overall output accuracy also depends on the external resistor tolerance, temperature characteristics and the complete feedback circuit.

AS431 Sink Current

AS431 can sink up to approximately 100 mA of current.

This capability allows the device to control the current in a feedback path or act as a shunt regulator.

The actual operating current should be selected according to the circuit requirements. The device must remain within its specified regulation region for the feedback loop to operate correctly.

When AS431 is used with an optocoupler, the LED current and AS431 cathode current should be considered together.

AS431 Package Options

AS431 is available in multiple package configurations depending on the manufacturer and specific ordering code.

Common package types associated with AS431 devices include small surface-mount packages and through-hole packages.

The package directly affects PCB layout, assembly method and thermal performance.

Engineers replacing an existing AS431 should therefore check the complete part number and pin configuration rather than assuming that every AS431 device has the same physical arrangement.

AS431 Applications

AS431 can be used in many electronic power and control circuits.

Typical applications include switching power supplies, AC-DC adapters, DC-DC converters, battery chargers, voltage references, voltage monitoring circuits and current-control circuits.

It is especially useful when the circuit requires an adjustable reference rather than a fixed-voltage reference.

The simple external resistor configuration also makes AS431 relatively easy to integrate into existing power-supply designs.

AS431 and Optocoupler Feedback

A common application combines AS431 with an optocoupler.

The AS431 monitors the secondary-side output voltage while the optocoupler provides electrical isolation between the secondary and primary sides of the power supply.

This architecture is widely used in isolated AC-DC power adapters and switching power supplies.

The resistor divider connected to the AS431 determines the voltage at which the feedback circuit responds. Proper resistor selection is therefore important for achieving the required output voltage and regulation performance.

AS431 Compared With a Zener Diode

AS431 can perform some functions that are traditionally handled by Zener diodes, but it provides an adjustable feedback-based architecture.

A Zener diode is generally selected according to its fixed breakdown voltage, while AS431 allows the desired regulation voltage to be established with external resistors.

This can reduce the number of different reference components required across a product family.

AS431 can also provide better control characteristics in suitable feedback circuits, although the actual performance depends on the complete circuit design.

AS431 Replacement Considerations

When selecting an AS431 replacement, designers should check several parameters rather than comparing the reference voltage alone.

Important specifications include reference-voltage accuracy, operating voltage range, cathode current, temperature range, package type and pin configuration.

The maximum cathode voltage is also important when the device is used in higher-voltage feedback circuits.

For a production replacement, the substitute should be evaluated under the actual operating conditions of the original circuit.

AS431 for Power Supply Design

AS431 remains a practical solution for adjustable shunt regulation and voltage-reference applications.

Its 2.5 V reference, programmable output voltage and relatively high sink-current capability make it useful in both conventional analog circuits and switching power-supply feedback loops.

For new designs, the exact AS431 variant should be selected according to the required accuracy, package and operating conditions. For replacement applications, matching the complete electrical and mechanical specifications is important for reliable operation.