The 2N3904 is a small signal general-purpose NPN transistor. It is one of the lowest-cost general purpose transistors currently available on the market, and compares well with the BC range of NPN transistors.
This transistor is also available in a through-the-hole TO-92 Bulk, and Ammopack versions. The SMD version is MMBT3904 in a SOT-23 package. It is also called PZT3904 and available in a SOT-223 SMD package.
It has many modern uses and applications. It is often used in small signal preamplifiers, oscillators, and even radio circuits.
It can be used as a switch to drive small loads up to 200 mA. It can drive LEDs, NTC temperature sensor circuits, low power motors, and miniature relays.
The 2N3904 has a maximum collector current rating of 200 mA and power dissipation of 625 mW. This compares well with the BC547, BC548, and BC549 general purpose transistors which have a peak collector rating of 200 mA and power dissipation of 500 mW.
2N3904 Vs BC547
Its electrical properties compare much more favourably with the BC547, where Vcbo = 50 V, Vces = 50 V, Vceo = 45 V, Vebo = 6 V. This is similar to the characteristics of the 2N3904 where Vcbo = 60 V, Vceo = 40 V, Vebo = 6 V.
However, the BC550 is an even closer match with Vcbo = 50 V, Vceo = 45 V, Vebo = 5 V.
For non-critical low voltage applications, it is often possible to make a direct replacement on some circuits; if you have a hobby project to light a couple of LEDs, or drive a small relay, then it is generally fine.
However, please note, that the pinout orientation is different between the two transistors. The 2N3904 pinout is emitter, base, collector, whilst for the BC range, the pinout is collector, base, emitter. The BC replacement transistor would therefore have to face in the opposite direction.
The pin configuration of this transistor is very simple. As you can see from the image above, when the flat side is facing you, the leads are emitter, base, collector. It is a very simple orientation.
Here is the symbol for an NPN transistor.
The DC current gain of this transistor is also in the same range as the BC range of general purpose NPN transistors. A maximum value of 300 can be achieved with Ic = 10 mA, and Vce = 1V.
The base-emitter saturation voltage range is 0.65 V minimum, and 0.95 V maximum. Assuming base current Ib is 5 mA, then the collector current Ic will be 50 mA.
The 2n3904 is one of the lowest-cost general purpose transistors. It can be found in many electronics shops. Your local Maplin shop will sell one for around 49 pence. Order code: QR40T.
The current market rate on eBay is around two pounds for 100 pcs from sellers in China. Or two pounds for 20 pcs from UK sellers.
It is one of many general purpose transistors currently available and the prices should remain low for the foreseeable future.