Part of the Day · LM317 — TO-220 adjustable voltage regulator

Part of the Day · 2026-10-23

A 3-terminal adjustable regulator whose output is set anywhere from 1.25 ~ 37V with two resistors. It delivers up to 1.5A and has long been used in adjustable bench power supplies.

3D view

3D drawing based on the dimensions of the TI datasheet TO-220 (KCS0003B) drawing. The grey plate at the back is the heatsink (bolt a heat sink through the hole). Not to scale.

Actual size · Front view

↓ Actual size, left edge aligned to 0 on the ruler

One ruler tick = 1mm. On PC this is actual size on a 24-inch FHD monitor at 100% zoom; on iPhone it adjusts automatically. For other screens, use “Calibrate ruler” below.

Enlarged (TO-220 · Front view · approx. 5×)

Pins 1 ADJ · 2 Output · 3 Input (heatsink tab = output)

leads partly shown · Package TO-220

Actual size · Right side view

Thickness 4.4~4.7mm · heatsink 1.22~1.32mm · lead length 12.7~13.1mm (TI KCS0003B drawing)

Enlarged · Right side view · approx. 4×

Right side view — the three leads overlap and look like one. The grey part on the left is the heatsink (back); the black part on the right is the front of the body.

Key specifications

Values are from the Texas Instruments datasheet.

Marking and pins

Typical uses

• Adjustable bench power supply — 1.25 ~ 37V constant voltage
• Constant-current source / charging circuit with a fixed resistor between ADJ and output

Companion & similar parts

• LM317M — 0.5A version of the same family (SOT-223, TO-252)
• LM317L — 0.1A small version of the same family (SOT-23, TO-92, etc.)
• LM317HV — High-voltage version with 4.25 ~ 60V input range

The heatsink tab is connected to the output (pin 2). Power loss is (input − output) × current, so a heatsink is needed when the voltage difference or current is large. With little or no load, draw the minimum load current (max 10mA) through a resistor.