2N3904 transistor: pinout, ratings, gain and substitutes

A TO-92 printed 2N3904 is the through-hole part, and every maker checked here uses the same E-B-C order. A three-lead SMD part marked 1A (Nexperia adds a one-character site code in front) is the PMBT3904, the SOT-23 version, with pins numbered base, emitter, collector.

If the print is worn or the bag says only "NPN", a 2N3904 and a BC547 look the same and have opposite pin orders. Meter it, as described further down, before you solder it.

The three datasheets agree on the voltages: 40 V collector to emitter, 60 V collector to base and 6 V emitter to base. The TO-92 parts are rated 625 mW at 25 °C; the SOT-23 is rated 250 mW, because its body and pads shed heat far less well (500 K/W against 200).

The 6 V emitter-base rating is lower than it looks. Drive the base from a signal that swings negative, such as a transformer winding or an AC-coupled square wave, and a reverse 6 V can break the junction down. A small diode with its anode on the emitter and its cathode on the base clamps it.

The gain peaks around 10 mA. Between 100 and 300 is guaranteed there, and only 60 at 50 mA and 30 at 100 mA. The familiar "hFE 100 to 300" holds only at that best current. Design a 100 mA switch around it and you are three times too optimistic.

The makers part company in two places. MCC allows 0.4 V of saturation voltage at 50 mA where Diotec and Nexperia allow 0.3 V, and MCC guarantees 250 MHz where the others guarantee 300 MHz. Neither matters for a relay driver, but if you are sizing a part for an RF stage, design with MCC's figures and any maker's part will do.

The switching times are short: 35 ns delay, 35 ns rise, 200 ns storage and 50 ns fall at 10 mA. Storage time dominates. PWM at a few tens of kilohertz is comfortable, and the 2N3904 is a common choice for the level shifter in front of a MOSFET gate.

• Switching a small relay, a buzzer, an LED or a short LED string from a microcontroller pin, up to 50 mA. The BJT as a switch lesson covers the theory.
• Small-signal amplifiers around 1 to 10 mA, where its gain is highest, such as a common-emitter stage.
• Logic level shifting, pulse stretching and oscillators up to tens of megahertz.

Above 50 mA, use a PN2222A (same pins, rated 1 A) or a logic-level MOSFET. The 2N2222 page has its pinouts, and the trap in them.

A 12 V load drawing 50 mA, say a relay whose coil measures 240 Ω, switched from a 5 V microcontroller pin. The emitter goes to ground, the coil sits between +12 V and the collector, and a resistor feeds the base from the pin.

From a 3.3 V pin the same working gives R ≤ 470 Ω, which is itself an E12 value, so 470 Ω and a base current of 5.0 to 5.6 mA. Most microcontroller pins can supply that, but check your chip's per-pin and total limits first.

A relay coil or motor needs the diode across it (cathode to +12 V), or the spike at switch-off can pass the 40 V rating; see the flyback diode and relay driving.

• Read "rated 200 mA" as "survives 200 mA". The datasheets specify saturation only to 50 mA, so above that nobody tells you the voltage across the transistor or how hot it runs.
• Measure the voltage from collector to emitter with the load on. Under 0.3 V means the switch is saturated; a volt or more means the base is starved.
• Keep the 10 kΩ from base to emitter. Without it a floating pin during reset can half turn the transistor on and cook it slowly.
• For an LED, size the series resistor with the 0.2 V saturation voltage subtracted from the supply. The LED resistor calculator does the rest.
• A loose TO-92 marked only "3904" is almost always E-B-C, but a quick diode test costs ten seconds and saves a board.

The PN2222A is the drop-in upgrade: same E-B-C order, five times the current. The P2N2222A and the BC547 both use the opposite order, C-B-E, so they go in the other way round.

1. Set the meter to diode test. With the red probe on one lead, find the lead that conducts to both others, each reading about 0.6 to 0.7 V. That lead is the base, and the part is NPN.
2. Reverse the probes: both junctions should read open. The two outer leads should read open both ways.
3. The base-emitter reading is usually a few millivolts higher than the base-collector one. If your meter has an hFE socket, try the part both ways round; the orientation with the higher reading is the right one.

The transistor testing lesson goes through it step by step, and continuity and diode test explains what the meter is measuring.

Keep a strip of 2N3904s for anything under 50 mA: relay drivers for small relays, LED switching, level shifters. For a load between 50 mA and 500 mA, fit a PN2222A in the same footprint. Above that, or from a 3.3 V pin that cannot spare 5 mA, go to a logic-level MOSFET (the MOSFET as a switch).

onsemi's product page for its own 2N3904 listed all nine orderable versions as obsolete when we checked on 9 October 2026. Diotec, which revised its datasheet in March 2026, MCC and Nexperia (PMBT3904) still publish theirs. Buy from a seller who names the maker, and meter a sample from every new bag. Our guide to buying electronic components in India compares ten online sellers and covers how to spot fakes.