NPN Transistor for Relay Coil Driver

Question

Problem: NPN Transistor for Relay Coil Driver. Solve this hands-on electronics task on EWskills.

Accepted Answer

1. Component Purpose

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

2. How to Find It on DigiKey

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.

3. Key Specifications & What They Mean

Specification	Required	Why
Transistor Type	NPN	Low-side switch; collector to load, emitter to GND.
Ic (Max)	≥200mA	Coil is 70mA; ~3× margin for safe operation.
Vceo (Max)	≥30V	Must withstand 12V + flyback spike margin.
hFE (Min)	≥80	Ensures saturation with ~1mA base drive.
Vce(sat)	≤400mV	Low voltage drop when ON; less heat.
Power	≥200mW	P = 12V × 70mA ≈ 0.84W is through coil, not transistor. Transistor dissipates Vce(sat) × Ic ≈ 28mW.
Package	SOT-23-3	Compact SMD; standard for small switches.
Temp Range	−40°C to +125°C	Industrial conditions.
RoHS	Yes	Compliance required.

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.

4. Selecting a Safe, Production-Ready Part

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

5. Filtered DigiKey Link

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

72. NPN Transistor for Relay Coil Driver