Voltage Regulator Symbol
Definition: The Voltage Regulator symbol represents a three-terminal linear or switching power-regulation block, standardised in schematic practice under ANSI Y32.2 / IEEE 315 conventions, that maintains a fixed or adjustable output voltage (V_out) regardless of input voltage or load current variations, with terminals labelled V_in, V_out, and GND.
Also known as: linear regulator, LDO regulator, 78xx regulator, fixed voltage regulator, three-terminal regulator.
What the Voltage Regulator symbol means
The Voltage Regulator symbol denotes a self-contained circuit block that accepts an unregulated input voltage on its V_in terminal and delivers a stable, regulated output voltage on its V_out terminal, referenced to the GND terminal. In schematic diagrams, the symbol is typically drawn as a rectangular block (or an IC package outline) with the three terminal labels explicit, indicating that the designer has selected a prepackaged regulator IC rather than building a discrete regulator circuit.
The Voltage Regulator symbol communicates an important circuit design decision: the rail downstream of the regulator is protected from input supply variations, transients, and ripple. Common fixed-output variants include the 78xx series (positive output) and 79xx series (negative output), where 'xx' denotes the output voltage in volts — for example, the 7805 regulates to 5 V. Adjustable regulators such as the LM317 and LM337 set their output voltage through an external resistor divider connected between the output and adjust pins.
How to identify the Voltage Regulator symbol
The Voltage Regulator symbol in a schematic is most commonly drawn as a rectangular box or a three-pin IC outline with three labelled connections: V_in (or IN) on the left, V_out (or OUT) on the right, and GND (or ADJ for adjustable types) at the bottom. The device reference designator VR or U precedes a number (e.g. VR1, U1). Some schematics use a simplified TO-220-style three-pin package outline showing the physical tab, though a plain box with three stubs is equally common and preferred for clarity.
Function in a circuit
In a circuit, the Voltage Regulator maintains a constant output voltage by dissipating the excess input-to-output voltage difference as heat (in linear regulators) or by switching energy storage elements (in switch-mode regulators). A linear regulator such as the LM7805 holds V_out at 5 V for any V_in between approximately 7 V and 35 V and any load current up to 1 A, providing a clean, low-noise supply rail. The GND pin provides the reference potential; in adjustable types, the ADJ pin sets the output voltage via an external resistor divider, allowing V_out to be programmed anywhere within the device's specified range.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 does not define a dedicated voltage regulator block symbol; it is represented as a general functional block (a rectangle with labelled terminals) under IEC 60617-02 (binary logic and analogue elements) conventions, with terminal functions labelled explicitly. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315-1975 similarly treats the voltage regulator as a labelled functional block; the reference designator VR (voltage regulator) or U (integrated circuit) is specified in IEEE 315 Table 22. |
| Key difference | No standardised glyph exists that differs between IEC and ANSI for a voltage regulator; both standards use a labelled rectangular block. The difference lies in the reference designator convention: IEC-influenced European schematics often use 'IC' or 'U', while IEEE 315 specifies 'VR' for voltage regulators specifically. |
Terminals / pins
| Pin | Name |
|---|---|
| in | Vin |
| out | Vout |
| gnd | GND |
Typical values
Input voltage V_in: typically 3 V to 40 V depending on device. Output voltage V_out: fixed (1.2 V, 1.8 V, 2.5 V, 3.3 V, 5 V, 12 V common) or adjustable (1.25 V to 37 V for LM317). Maximum output current: 100 mA to 5 A for common discrete packages. Dropout voltage (LDO): as low as 100 mV. Junction temperature: −40°C to +125°C.
Where the Voltage Regulator symbol is used
- Generating a stable 3.3 V or 5 V supply rail from a 9 V or 12 V unregulated input in microcontroller and Arduino-based projects
- Post-rectification smoothing in linear power supply circuits, after a bridge rectifier and filter capacitor
- Providing a low-noise analogue reference voltage for ADC circuits and precision measurement systems
- USB charger designs requiring a fixed 5 V output from a higher-voltage DC bus
- Automotive electronics converting a nominally 12 V vehicle battery supply to regulated 5 V or 3.3 V for logic circuits
- Laboratory bench power supplies with user-adjustable output using LM317-type adjustable regulators
- Powering RF circuits and audio preamps where switch-mode noise would degrade performance
Example
In a classic Arduino Uno power supply section, an LM7805 voltage regulator symbol appears between the DC barrel jack (7–12 V input) and the 5 V supply rail that powers the ATmega328P microcontroller. The V_in terminal connects to the smoothing capacitor on the raw input, the GND terminal connects to the circuit common, and the V_out terminal feeds the 5 V bus. A 100 nF decoupling capacitor is placed on each terminal to suppress oscillation and high-frequency noise, a detail often shown in the schematic alongside the regulator symbol.
Key facts
- The Voltage Regulator symbol is a three-terminal functional block with terminals V_in, V_out, and GND (or ADJ for adjustable types); the reference designator is VR or U per IEEE 315.
- Linear voltage regulators (e.g. LM78xx series) dissipate excess power as heat; the power dissipated equals (V_in − V_out) × I_load, requiring heat-sinking at high currents.
- Low-dropout (LDO) regulators require a minimum input-to-output voltage differential (dropout voltage) as low as 100 mV, versus 2–3 V for standard linear regulators, making them essential in battery-powered designs.
- The LM7805 (5 V output), LM7812 (12 V output), and LM7833 (3.3 V output) are among the most widely used fixed positive-voltage regulators, manufactured to the 78xx pinout standard across multiple vendors.
- The LM317 adjustable regulator sets its output voltage via the formula V_out = 1.25 × (1 + R2/R1), where R1 and R2 are the external programming resistors connected to the ADJ pin.
- Switch-mode voltage regulators (buck, boost, buck-boost) are also represented as block symbols in schematics but are distinguished by the presence of an inductor in their associated circuit, unlike the simple three-terminal linear type.
- IEC 60617 and ANSI Y32.2 / IEEE 315 both represent the voltage regulator as a labelled rectangular block; no unique glyph is standardised for it in either specification.
Diagrams that use this symbol
- electrical line diagram
- circuit diagram online
- electrical diagram online
- battery charger circuit diagram
- bms connection diagram
- bms wiring diagram
- controller diagram
- brake controller wiring diagram
Frequently asked questions
What does the voltage regulator symbol look like in a schematic?
The voltage regulator symbol is a rectangular block with three terminals: V_in on the left, V_out on the right, and GND at the bottom. The block is labelled with the device reference designator (VR1 or U1) and sometimes the part number (e.g. LM7805). It may also appear as a stylised three-pin IC package outline.
What does the voltage regulator symbol mean in a circuit diagram?
The voltage regulator symbol means a device is present that converts an unregulated input voltage to a stable, fixed or adjustable output voltage. It tells the reader that the downstream rail is regulated and protected from input supply variations and load transients.
What is the designator letter for a voltage regulator?
The standard IEEE 315 reference designator for a voltage regulator is VR. However, many modern schematics use U (integrated circuit) instead, especially when the regulator is an IC such as the LM7805 or LM1117.
What is the difference between a linear and a switching voltage regulator symbol?
Both are drawn as labelled block symbols in schematics. A linear regulator block typically shows only V_in, V_out, and GND terminals. A switching regulator block usually shows additional terminals such as SW (switch node), FB (feedback), EN (enable), and a separate inductor symbol connected externally, indicating its topology.
How do I read the voltage regulator symbol in a schematic?
Identify the three terminals: V_in receives the higher unregulated supply, V_out delivers the regulated voltage, and GND is the common reference. The reference designator (VR1) identifies the device; the part number or net label on V_out (e.g. '+5V') tells you the regulated output voltage.
What standard defines the voltage regulator schematic symbol?
No single dedicated glyph is standardised for the voltage regulator in IEC 60617 or ANSI Y32.2 / IEEE 315; both standards treat it as a labelled functional block. The reference designator VR is specified in IEEE 315-1975 Table 22. In practice, the symbol is drawn as a three-terminal box per the component manufacturer's datasheet.
What is the dropout voltage shown in a voltage regulator circuit?
Dropout voltage is the minimum difference between V_in and V_out required for the regulator to maintain regulation. For standard 78xx linear regulators it is approximately 2–3 V; for LDO regulators it can be as low as 100–300 mV. Circuits that must operate near battery end-of-life require LDO types to maintain regulation.
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