LM317T

The LM317T from Texas Instruments is a classic adjustable 3-terminal positive voltage regulator capable of supplying more than 1.5A over an output range of 1.25V to 37V. It requires only two external resistors to set the output voltage: VOUT = 1.25V × (1 + R2/R1) + IADJ × R2, where IADJ is typically 50µA. The device offers superior line and load regulation compared to fixed regulators (0.01%/V and 0.1% typical, respectively) and 80dB ripple rejection. Full overload protection includes current limiting, thermal shutdown, and safe operating area protection—all of which remain functional even if the ADJ pin is accidentally disconnected. The floating architecture means the regulator only sees the input-to-output differential voltage, allowing it to regulate supplies of several hundred volts as long as the differential does not exceed 40V. The T suffix denotes the TO-220-3 through-hole package, ideal for heatsink mounting.

The LM317T maintains a constant 1.25V reference between the OUT and ADJ pins. Current flowing through R1 (connected between OUT and ADJ) is I_R1 = 1.25V / R1. This same current (plus the small IADJ of ~50µA) flows through R2 (connected between ADJ and GND), setting the output voltage: VOUT = 1.25V × (1 + R2/R1) + IADJ × R2. For practical purposes, IADJ × R2 is negligible, so VOUT ≈ 1.25V × (1 + R2/R1). A typical design uses R1 = 240Ω, giving I_R1 ≈ 5.2mA. The output voltage is then set by R2: for 5V output, R2 ≈ 720Ω; for 12V, R2 ≈ 2.06kΩ; for 3.3V, R2 ≈ 394Ω. The minimum load current is approximately 3.5mA (the internal reference current), so R1 should not exceed approximately 375Ω to guarantee regulation under no-load conditions. The input-to-output differential voltage must be at least 3V (dropout voltage) for proper regulation, meaning VIN must be at least VOUT + 3V. The internal current limit is typically 2.2A at 25°C, and it remains relatively constant over temperature. The thermal shutdown activates at approximately 170°C junction temperature. The ADJ pin can be bypassed with a 10µF capacitor to improve ripple rejection to over 80dB.

LM317T uses the TO-220-3 package with adjustable regulator pinout: Pin 1 = ADJ (Adjustment), Pin 2 = VOUT (tab), Pin 3 = VIN. This pinout differs from fixed 78xx regulators where Pin 1 = INPUT. The tab is electrically connected to VOUT (Pin 2), not GND — this is critical for heatsink mounting. When VOUT is not at ground potential, an insulating washer is required between tab and heatsink. The ADJ pin maintains 1.25V below VOUT with 50µA bias current. Output voltage is set by R1 (VOUT to ADJ, typically 240Ω) and R2 (ADJ to GND). Protection diodes are recommended: D1 (1N4002, ADJ to VOUT) for output capacitors >10µF, and D2 (1N5400, VIN to VOUT) for reverse input conditions.

• Adjustable Bench PSU: Replace R2 with a 5kΩ potentiometer for 1.25V–26V adjustable output; add 10µF cap on ADJ for clean output
• Battery Charger: Set VOUT to float voltage (e.g., 13.8V for 12V lead-acid); add series diode and current-limiting resistor
• LED Current Regulator: Connect ADJ to load (LED cathode) with R1 setting ILED = 1.25V / R1; e.g., R1=12.5Ω gives 100mA
• High-Voltage Regulation: Floating topology allows regulating 100V+ rails; only VI-VO differential must stay under 40V
• Electronic Shutdown: Drive ADJ to GND with a transistor; VOUT drops to 1.25V where most loads draw negligible current

LM317T is a 1.25-37V adjustable positive voltage regulator in TO-220-3 with 1.5A output and ±4% reference tolerance. The LM317AT offers ±1% precision. For surface-mount, use LM317EMP (SOT-223). For higher current, LM1085 provides 3A output. The LM337T is the negative complement for split-rail supplies. All require only two external resistors to set output voltage.