Linear Regulator (78xx / LDO)
IC (Power)
Turns a higher, messy DC voltage into a clean fixed lower voltage by dropping the difference as heat. Simple and quiet, but inefficient for big drops.
Schematic symbol
Integrated circuit (DIP)
How linear regulator (78xx / ldo) appears in a circuit diagram.
Types & variants
Key specs
Output voltage
VFixed (e.g. 5V, 3.3V) or set by resistors (LM317).
Max output current
Ae.g. 1A for 7805/LM317, 1A for AMS1117.
Dropout voltage
VMin Vin-Vout needed: ~2V for 78xx, ~1.1V AMS1117, <0.3V good LDO.
Quiescent current
mACurrent the regulator itself uses; matters on battery.
Markings
Part number on the tab/body (e.g. 7805, LM317, AMS1117-3.3). Pinout printed in datasheet.
Standard values
7805/7812/7833, 7905, LM317 (adjustable), AMS1117-3.3/-5.0, MCP1700 (low-Iq).
How to choose
Pick Vout and current with margin. Ensure Vin >= Vout + dropout. Estimate heat P = (Vin-Vout) x I and add a heatsink (or use a buck converter for big drops). Add input/output caps per datasheet.
Pinout & package
TO-220 — front view, tab at back
Check first: Fixed 78xx regulators use IN–GND–OUT. Adjustable types (LM317) are ADJ–OUT–IN instead, so confirm before wiring.
78xx (TO-220): IN, GND, OUT. LM317: ADJ, OUT, IN. AMS1117: GND, OUT, IN. Always add 0.1µF + bulk caps.
Example circuits
- 5V supply from a 9-12V input
- 3.3V rail for an MCU (AMS1117)
- Adjustable bench reference (LM317 + 2 resistors)
Common failures
Overheats and shuts down/dies with too much drop x current; outputs wrong voltage if caps missing (oscillation); damaged by reverse input.
How to test
Apply Vin above dropout, measure OUT vs GND - should be the rated voltage. Hot with no load or wrong Vout = faulty or missing caps.
Substitutes
Same Vout, equal/higher current, dropout that fits your headroom. For efficiency swap a 78xx for a switching buck module.