The standard symbol of voltage regulator on a schematic is a rectangular box with an input line on the left, an output line on the right, and a ground or reference line pointing down. However, the exact geometry changes depending on whether your schematic follows US (ANSI/IEEE 315) or international (IEC 60617) standards, and whether the regulator is fixed, adjustable, or a switching type. Below is the immediate reference data you need to read schematics and wire physical packages correctly.
Schematic Symbols: ANSI/IEEE 315 vs. IEC 60617
Schematic symbols are not universal. If you are reading a board designed in the US, you will likely see ANSI/IEEE 315 symbols. If you are reading European or modern global datasheets (like those from STMicroelectronics or Infineon), you will see IEC 60617 symbols. Here is how the symbol of voltage regulator types translates across both standards.
| Regulator Type | ANSI/IEEE 315 (US Standard) | IEC 60617 (Global Standard) | Key Identifier on Schematic |
|---|---|---|---|
| Fixed Linear (e.g., 7805) | Rectangle with 'VREG' inside, DC input/output arrows | Rectangle with '*U' or specific part number, straight lines | 3 terminals: IN, OUT, GND |
| Adjustable Linear (e.g., LM317) | Rectangle with an internal diagonal arrow indicating variability | Rectangle with a diagonal arrow crossing the box boundary | 3 terminals: IN, OUT, ADJ |
| Low Dropout (LDO) | Same as fixed linear, often annotated with 'LDO' or 'Vdrop' | Same as fixed linear, annotated with part number | Often includes an EN (Enable) pin |
| Switching Buck (e.g., LM2596) | Box with internal switch symbol (transistor/diode) or 'SW' label | Complex block showing PWM controller, inductor, and diode nodes | Includes SW (Switch), FB (Feedback), and BOOT pins |
Source: IEEE 315-1975 (Reaffirmed 2008) and IEC 60617 database standards for graphical symbols for diagrams.
Physical Pinouts & Package Variants
Knowing the symbol of voltage regulator components on paper is only half the battle. When you have the physical component on your bench, the pinout depends entirely on the silicon family and the physical package. A classic bench mistake is assuming the pinout of a TO-220 package applies to a TO-92 package for the same silicon.
The metal tab on a TO-220 package is electrically connected to a specific pin. On a 78xx series, the tab is GROUND. On an LM317, the tab is OUTPUT. If you bolt an LM317 to a grounded chassis or shared heatsink without a mica/silicone insulator pad, you will instantly short your output to ground and destroy the regulator or your power supply.
| Part Number | Package | Pin 1 (Left) | Pin 2 (Middle) | Pin 3 (Right) | Tab Connection | Typical Dropout |
|---|---|---|---|---|---|---|
| LM7805 | TO-220 | INPUT | GROUND | OUTPUT | Ground (Pin 2) | 2.0V |
| 78L05 | TO-92 | OUTPUT | GROUND | INPUT | N/A (Plastic) | 2.0V |
| LM317 | TO-220 | ADJUST | OUTPUT | INPUT | Output (Pin 2) | 2.0V |
| AMS1117-3.3 | SOT-223 | GROUND | OUTPUT | INPUT | Output (Pin 2) | 1.1V |
| MCP1700-33 | SOT-23 | GROUND | INPUT | OUTPUT | N/A | 0.178V |
Note: Pinouts are read with the component facing you, pins pointing down, and the text readable left-to-right. Always verify against the specific manufacturer's datasheet (e.g., Texas Instruments LM317 Datasheet) as clone parts from obscure foundries occasionally swap pins.
Rows People Get Wrong: Faded Markings & Pinout Traps
When working with salvaged boards, counterfeit parts, or components that have overheated, the laser etching on the regulator often fades or burns off entirely. Here is how to safely interpret the hardware when the markings are missing, and the specific rows in the tables above that cause the most blown prototypes.
The TO-92 vs. TO-220 Pin Swap Trap
Look at Table 2 for the LM7805 (TO-220) and the 78L05 (TO-92). Both are 5V fixed linear regulators. However, the TO-92 package pinout is reversed (Out-GND-In) compared to the TO-220 (In-GND-Out). If you design a PCB footprint for a TO-92 based on the TO-220 symbol and pinout, you will feed raw input voltage directly into your 5V logic rail the moment you apply power, instantly bricking your microcontroller.
Identifying Pins on Unmarked TO-220 Regulators
If you have a 3-pin TO-220 component with completely faded markings, you can identify its family using a digital multimeter (DMM) in continuity or diode-test mode:
- Find the Tab Connection: Place your black probe on the metal tab. Touch the red probe to Pins 1, 2, and 3.
- If Pin 2 (Middle) shows continuity to the tab: You likely have a 78xx series fixed regulator (Tab is Ground) OR an LM317/LM1117 adjustable regulator (Tab is Output).
- Differentiating Fixed vs. Adjustable: Measure the resistance between Pin 1 and Pin 2. On a 7805, the internal circuitry will show a high resistance or a specific diode drop. On an LM317, the Adjust pin (Pin 1) has a very specific internal resistance network relative to the Output pin (Pin 2).
- The Safest Bench Test: Do not guess. Wire the component on a breadboard with a current-limited bench power supply set to 9V and a 100Ω dummy load. Measure the output. If it outputs exactly 5.00V, it's a 7805. If it outputs roughly 1.25V (the internal reference voltage of an LM317 with no resistor divider), it's an adjustable type.
The SOT-223 Thermal Pad Misinterpretation
On surface-mount packages like the SOT-223 (commonly used for the AMS1117 LDO), the large thermal pad on the bottom is not just for heat dissipation—it is electrically active. As noted in Table 2, the tab on the AMS1117 is tied to the OUTPUT pin. If your PCB footprint routes the thermal pad to a ground pour for cooling, you will short the 3.3V output directly to ground. The pad must be tied to the output trace or left electrically isolated on an internal layer.
Regional Standards and Safe Verification Practices
While the symbol of voltage regulator components is dictated by drafting standards (ANSI vs IEC), the application is governed by electrical safety standards, particularly when these regulators are used in offline power supplies or mains-connected equipment.
If you are designing or repairing a board where the voltage regulator sits on the primary (mains) side of an isolation transformer, or is part of a non-isolated buck converter tied directly to 120V/230V AC, you must adhere to creepage and clearance distances defined by IEC 62368-1 (Audio/video, information and communication technology equipment safety). A standard 7805 symbol on a schematic does not convey the lethal hazard present on the copper traces feeding it if the input is derived from a rectified mains line without proper galvanic isolation.
When the Schematic Symbol Doesn't Match the Datasheet
It is incredibly common in hobbyist and open-source hardware to find a custom symbol of voltage regulator drawn in KiCad or Altium that omits critical pins, such as the Enable (EN) or Power Good (PG) pins found on modern LDOs and switching regulators.
- Enable (EN) Pins: If a schematic symbol shows only IN, OUT, and GND, but the physical BOM calls for an LDO with an EN pin (like the AP2112), the EN pin must be pulled HIGH (tied to IN) for the regulator to output voltage. If left floating, the regulator's internal logic may keep it in shutdown mode.
- Feedback (FB) Pins: On switching regulators (like the LM2596), the symbol will include an FB pin. Never leave this floating. It must be tied to a precision resistor divider network connected to the output. A floating FB pin will cause the regulator to drive the output to maximum duty cycle, resulting in catastrophic overvoltage and exploded output capacitors.
Always cross-reference the schematic symbol against the physical component's datasheet pinout table before applying power. The symbol tells you the logical function; the datasheet tells you how to keep it from catching fire.
