LM7809CT

The LM7809CT from onsemi (formerly Fairchild) is a 3-terminal fixed positive voltage regulator providing a regulated 9V output at up to 1A. It is part of the industry-standard LM78XX family, sharing the same TO-220 package, pinout, and internal protection circuitry as the LM7805 and LM7812. The 9V output is particularly useful for powering 9V battery-operated equipment from a 12V supply (common in automotive and battery-backed systems), guitar effects pedals, and audio preamplifiers. Like all 78XX regulators, it requires only two external capacitors (0.33µF input, 0.1µF output) for stable operation. The 2V dropout voltage means the input must be at least 11V to maintain regulation at full load. The internal thermal shutdown protects against overheating (activates at ~150°C junction), and the short-circuit current limiter restricts output current to ~230mA during a fault. The TO-220 package with its metal tab can be heatsink-mounted for continuous 1A operation; without a heatsink, the package dissipates about 2W in free air. The CT suffix denotes the TO-220 package from onsemi.

The LM7809CT uses an internal bandgap reference, error amplifier, and series pass transistor to maintain a constant 9V output. The error amplifier compares a fraction of the output voltage (via an internal resistor divider) with the bandgap reference (~1.25V). If the output voltage drops below 9V, the error amplifier increases drive to the pass transistor, raising the output. If the output rises above 9V, the drive is reduced. The thermal protection circuit monitors the junction temperature: when it exceeds approximately 150°C, the pass transistor is turned off, preventing damage. The current limiter senses the voltage across an internal sense resistor in series with the pass transistor: when the voltage exceeds ~0.6V (corresponding to ~230mA in short-circuit condition), the drive is reduced to limit current. The safe-area protection reduces current limit as the input-output differential increases, keeping the pass transistor within its safe operating area. Input capacitor (0.33µF) prevents oscillation when the input source has high impedance; output capacitor (0.1µF) improves transient response.

Pin assignment follows standard TO-220-3 layout: Pin 1 (left) = Input, Pin 2 (center) = Ground/Tab, Pin 3 (right) = Output, when viewed from the front with pins pointing down. The metal tab on the back is electrically connected to Pin 2 (GND) and must be insulated from the heatsink when the heatsink is not at ground potential. Output capacitance of 0.1µF is required for stability; input capacitance of 0.33µF is recommended when the input supply is remote.

• Basic 9V Supply: 12V battery → INPUT; GND to ground; 0.33µF input cap, 0.1µF output cap; OUTPUT = 9V regulated for guitar pedals
• 12V to 9V Converter: 12V automotive → INPUT; OUTPUT = 9V for 9V-rated equipment; dropout 2V allows regulation from 12V (9V + 2V = 11V minimum)
• Split Supply (±9V): LM7809CT for +9V; LM7909CT for -9V; center-tapped transformer; dual supply for op-amp circuits
• Adjustable Output: Add resistor between GND and ground: VOUT = 9V + (IQ × R); or use LM317T for full adjustability

LM7809CT is a 0.9V fixed positive voltage regulator in TO-220-3 with 1A output and ±4% tolerance. The ACT suffix offers ±2% precision. For surface-mount designs, use MC7809ABD2TG (DPAK). For battery-powered systems, the LM2940CT-0.9 LDO version reduces dropout to 0.5V. All 7809 variants are pin-compatible (IN-GND-OUT) and require only input/output capacitors.