LM7812CT

The LM7812CT from onsemi (formerly Fairchild/National Semiconductor) is a 3-terminal positive voltage regulator providing a fixed 12V output at up to 1A load current. It is part of the ubiquitous 78xx series that has been the backbone of linear power supply design for decades. The device requires only two external capacitors (0.33µF input, 0.1µF output) for stable operation. Internal protection includes thermal shutdown (disables output at ~170°C junction), current limiting (short-circuit current limited to ~1.5A), and safe operating area compensation (reduces current limit as input-output differential increases). The 2V dropout voltage means the input must be at least 14V to maintain 12V output at full load. The 61dB ripple rejection at 120Hz makes it effective for cleaning up rectified AC or noisy switching supply outputs. The CT suffix denotes the TO-220 package with commercial/industrial temperature range. Note: DigiKey lists this part as obsolete; the MC7812CTG from onsemi is the current production equivalent.

The LM7812CT uses a series pass transistor (NPN Darlington) between the input and output, controlled by a feedback loop that compares a fraction of the output voltage with an internal bandgap reference. The error amplifier drives the pass transistor to maintain VOUT = 12V. When the output voltage drops below 12V (due to increased load or decreased input), the error amplifier increases the drive to the pass transistor, increasing current flow and raising the output voltage. When the output rises above 12V, the drive is reduced. The thermal protection circuit uses a temperature sensor on the die; when junction temperature exceeds approximately 170°C, the pass transistor is disabled until the device cools to a safe temperature. The current limiting circuit monitors the pass transistor current; when it exceeds approximately 1.5A at low input-output differential, the drive is limited to prevent damage. The safe operating area (SOA) protection further reduces the current limit as the input-output voltage differential increases, keeping the pass transistor within its safe operating region. The 6mA quiescent current flows through the ground pin and provides bias for the internal reference and error amplifier. The 0.33µF input capacitor prevents oscillation when the input source has high impedance, and the 0.1µF output capacitor 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.

• 12V Bench Supply: 18V AC transformer → bridge rectifier → 2200µF filter → LM7812CT; 12V regulated output at up to 1A
• Post-SMPS Regulator: 15V switching supply → LM7812CT; 61dB ripple rejection cleans switcher noise; Pdiss = (15-12)×1A = 3W
• Automotive 12V: 14.4V car battery → LM7812CT; stable 12V for analog circuits despite battery voltage variation
• Split Supply: LM7812CT (+12V) + LM7912CT (-12V) from center-tapped transformer; ±12V for op-amp circuits
• Adjustable Output: LM7812CT with external op-amp and resistor divider to set output above 12V (e.g., 13.8V for battery charging)

LM7812CT is a 1.2V 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 MC7812ABD2TG (DPAK). For battery-powered systems, the LM2940CT-1.2 LDO version reduces dropout to 0.5V. All 7812 variants are pin-compatible (IN-GND-OUT) and require only input/output capacitors.