71. NPN Transistor for Relay Coil Driver

A 3.3V MCU GPIO pin can typically source only 4–20mA. But the relay coil needs 70mA at 12V. The GPIO cannot drive the relay directly — it doesn’t have enough current capacity, and the voltage levels don’t match.

An NPN transistor as a low-side switch solves both problems. The GPIO drives the transistor’s base through a resistor. When the base current flows (~1–2mA), the transistor saturates and connects the relay coil to ground, allowing the full 70mA to flow from the 12V rail through the coil.

The transistor acts as a current amplifier: a small base current controls a much larger collector current. The ratio is the DC current gain (hFE).

Required: NPN · Ic ≥200mA · Vceo ≥30V · hFE ≥80 · Vce(sat) ≤400mV · ≥200mW · SOT-23 · SMD

Go to DigiKey.com → Discrete Semiconductor Products → Transistors → Bipolar (BJT) - Single

Apply these filters:

• Transistor Type → NPN
• Current - Collector (Ic) (Max) → 200mA or higher
• Voltage - Collector Emitter Breakdown (Max) → 30V or higher
• DC Current Gain (hFE) (Min) → 80 or higher
• Vce Saturation (Max) → 400mV or lower
• Package / Case → SOT-23-3 (TO-236)
• Part Status → Active

Tip: On DigiKey, the SOT-23 package may appear as "TO-236-3, SC-59, SOT-23-3" or similar. These are all the same physical package with different naming conventions.

Important: Always add a flyback diode across the relay coil (cathode to +12V, anode to transistor collector). Without it, the voltage spike when the relay turns OFF can destroy the transistor.

The filtered list will show matching parts. Before you pick one, check: Is it Active? Is stock available? Is the manufacturer reputable? Is a datasheet available?

Full checklist: How to Select a Safe, Production-Ready Component — EWskills Guide

➡ View Matching Parts — NPN, SOT-23, Ic≥200mA, Vceo≥30V, hFE≥80