BC547 vs 2N2222 vs 2N3904: the same job, mirrored pins

The Diotec datasheets number the leads 1, 2, 3 from the left with the flat face toward you. The BC547 sheet assigns C (1), B (2), E (3). The 2N3904 and 2N2222A sheets assign E (1), B (2), C (3). The drawings above show each one.

Because the base sits in the middle either way, rotating the transistor 180 degrees (flat face away from you) puts collector and emitter back where the board expects them. That is the quickest way to fit a 2N3904 into holes drawn for a BC547, or the other way round. Check the silkscreen first. If it prints the transistor's flat side, the turned part will face the other way to that outline, which is what you want.

Not every part sold as a 2N2222 follows the Diotec order. onsemi's P2N2222A in TO-92 is collector, base, emitter, the same as a BC547, while Central's 2N2222A in the TO-18 metal can is lead 1 emitter, 2 base, 3 collector. Read the part number printed on the body, then the datasheet for that exact part. Our 2N2222 page draws all the variants.

In the saturation row the base current is a tenth or a twentieth of the collector current, and the gain figures come from different test currents and voltages, so don't compare them directly. The BC547 letter suffix (A, B or C) is its gain group; BC547B is the one most kits carry.

BC547 suits low-current work: LED indicators, a buzzer, a small-signal amplifier stage, a sensor buffer. Its gain is high (group B is 200 to 450 at 2 mA), so a small base current switches it, and its saturation voltage at 10 mA is 250 mV at most. It runs out of room at 100 mA.

2N3904 sits in the middle. It takes 200 mA continuous, has the lowest collector-base capacitance of the three (4 pF max), and its sheet gives switching times (35 ns delay and rise, 200 ns storage, 50 ns fall, all maximums). Its saturation figures stop at 50 mA, and that is where we stop counting on it as a hard switch.

2N2222A is the one for relays, small motors and LED strings between 100 and 500 mA. The sheets guarantee 0.3 V at 150 mA, but only with 15 mA into the base, and 1 V at 500 mA with 50 mA into the base. That base current has to come from whatever drives the pin, and Figure 3 shows how much each test point asks for.

All three run from a 5 V microcontroller pin and use the datasheet's own test conditions. Heat is estimated with the 200 K/W junction-to-air figure all three sheets give.

1. An 80 mA load with a BC547

The load is under the 100 mA rating. Use the 100 mA test point: base current 5 mA, VCE(sat) at most 600 mV, VBE(sat) typically 900 mV. The base resistor is (5 − 0.9) V ÷ 5 mA = 820 Ω. The worst-case heat is 0.6 V × 80 mA = 48 mW, a rise of under 9.6 °C. It works, with little margin on current. If the load could ever reach 100 mA, move up to a 2N2222A.

2. A 150 mA load with a 2N2222A

This is past the BC547's rating and past the 2N3904's tested saturation range. The 2N2222A sheets give 0.3 V max at 150 mA with 15 mA into the base, and Central gives VBE(sat) between 0.6 and 1.2 V at that point. For at least 15 mA with the worst VBE: (5 − 1.2) V ÷ 15 mA = 253 Ω, so take the next standard value down, 220 Ω. The pin then supplies 17.3 mA at the high VBE and 20 mA at the low one. Heat is 0.3 V × 150 mA = 45 mW, about 9 °C above the room.

The transistor is comfortable. The 17 to 20 mA from the pin is what to check against your microcontroller's datasheet before you build it.

3. Why not 500 mA from a pin?

The 2N2222A is rated for 600 mA, so 500 mA looks allowed. But the 1 V saturation figure at 500 mA assumes 50 mA into the base, far more than a typical GPIO pin is rated for. With less base current the transistor comes out of saturation and VCE rises. Even at the guaranteed 1 V, the heat is 1 V × 0.5 A = 0.5 W, a rise of 100 °C, which puts the junction at 125 °C in a 25 °C room against a 150 °C limit.

For loads of several hundred milliamps from a 5 V or 3.3 V pin, use a logic-level MOSFET. Our IRFZ44N vs IRLZ44N page works through that case.

• Check the flat face and the silkscreen before soldering. A transistor fitted the wrong way round can still pass some current, so the circuit may half-work and mislead you.
• Read the gain letter on a BC547. A BC547A has about a quarter of the minimum gain of a BC547C (110 against 420).
• Keep the base resistor. A base driven straight from a pin overloads both the pin and the transistor.
• Fit a diode across any relay coil or motor. The inductive kick from a 12 V coil can easily pass the 40 to 45 V VCEO ratings.
• When you need the PNP twin for the high side, use the BC557, 2N3906 or 2N2907A. Each maker lists them as the complementary parts.

1. Set the meter to diode test. Put the red probe on each lead in turn until you find the one that conducts to both others, about 0.6 to 0.7 V each. That is the base, and on all three of these parts it is the middle lead.
2. Reverse the probes: both junctions should read open, and the two outer leads should read open both ways.
3. With an hFE socket, try both orientations. The right one reads in the hundreds; the wrong one reads far lower. That tells you which outer lead is the collector.

The transistor testing lesson goes through it step by step.

Keep BC547B for signal work and loads under about 50 mA, where its high gain helps. Use the 2N2222A for anything from 100 mA to about 300 mA driven by a pin, and size the base resistor from the 15 mA test point. Reach for the 2N3904 when you want the faster, lower-capacitance part at small currents, or when it's simply what you have. When you swap between the C-B-E part and the E-B-C pair, turn the transistor around.

Each part has its own page with every package variant: BC547, 2N2222 and 2N3904. For heavier loads, read choosing a MOSFET for Arduino and ESP32. To buy them in India, see where to buy electronic components in India, and check the printed part number and gain letter when the bag arrives.