Reference chart

Common transistors chart

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A transistor lets a small signal switch a bigger load. Most hobby circuits use the same few parts, and this chart gives each one's limits from its datasheet, its gain or threshold, and which pin is which, so you can pick the right one for the job.

Bipolar transistors (BJTs)

Part	Type	Max V_CE	Max I_C	Gain (h_FE)
2N3904	NPN	40 V	200 mA	100–300 at 10 mA
2N3906	PNP	40 V	200 mA	100–300 at 10 mA
2N2222A	NPN	40 V	600 mA	100–300 at 150 mA
BC547	NPN	45 V	100 mA	110–800, by gain group
BC557	PNP	45 V	100 mA	125–800, by gain group
TIP120	NPN Darlington	60 V	5 A	1000 or more

Gain, h_FE, is the datasheet's name for β. It changes with current and from part to part, so design with the smallest value. A BC547B or BC547C is a BC547 sorted into a higher gain group. PNP datasheets give their ratings as negative numbers, like −40 V.

A TIP120 is two transistors in one (a Darlington pair), so its gain is huge, but it drops up to 2 V when fully on and needs a heatsink above about an amp.

Logic-level MOSFETs

Part	Max V_DS	Max I_D	V_GS(th)	R_DS(on) (max)
2N7000	60 V	200 mA	0.8–3 V	5.3 Ω at 4.5 V
IRLZ44N	55 V	47 A on a big heatsink	1–2 V	25 mΩ at 5 V
IRFZ44N (not logic-level)	55 V	49 A on a big heatsink	2–4 V	17.5 mΩ at 10 V

The threshold, V_GS(th), is where the MOSFET only just starts to conduct. To switch a load fully on, the gate needs the voltage its R_DS(on) is quoted at. That's why the IRFZ44N, which needs 10 V, is a poor choice for a 5 V or 3.3 V board, even though it looks almost the same.

The huge I_D figures assume the case is held at 25 °C. In free air, a TO-220 MOSFET can only get rid of a watt or two, so work out the heat with P = I² × R_DS(on).

Which pin is which

Hold a TO-92 part (the small black half-cylinder) with the flat face toward you and the legs down. Hold a TO-220 part (with the metal tab) with the writing toward you. Then, left to right:

Part	Package	Pins, left to right
2N3904, 2N3906	TO-92	E, B, C
2N2222A	TO-92	E, B, C (PN2222A) or C, B, E (P2N2222A)
BC547, BC557	TO-92	C, B, E
TIP120	TO-220	B, C, E (the tab is C)
2N7000	TO-92	S, G, D
IRLZ44N, IRFZ44N	TO-220	G, D, S (the tab is D)

These are the usual pinouts, but makers differ, especially for the 2N2222A and the BC547 family. Check the datasheet for your exact part before you solder.

What each is for

Part	Use it for
2N3904, BC547	Small loads like LEDs, buzzers and small relays, and signal amplifiers
2N3906, BC557	The same, as a PNP high-side switch
2N2222A	Relays, small motors and lamps up to about 300 mA
TIP120	Motors, solenoids and LED strips of an amp or more, from a weak control signal
2N7000	Small loads from a 5 V pin, and shifting logic levels
IRLZ44N	Motors, solenoids and LED strips of several amps from a 5 V pin, running cool

Picking a switch

Load current: pick a part rated for at least twice it.
Load voltage: pick a V_CE or V_DS rating well above the supply, with margin for spikes from coils.
Control voltage: a 5 V pin drives every part here except the IRFZ44N. On 3.3 V, pick a MOSFET whose R_DS(on) is quoted at 2.5 V.
Base or gate: give a BJT a base resistor, and a MOSFET a gate pull-down.

Free lesson

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Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.

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Work it out

The ideas behind it

Questions people ask

What is the pinout of a 2N2222?

It depends on the maker. The PN2222A reads E, B, C with the flat face toward you and the legs down, but the P2N2222A reads C, B, E. Check the datasheet for your exact part, or find the base with a multimeter's diode test.

Is the IRLZ44N a logic-level MOSFET?

Yes. Its on-resistance is quoted at 4 V and 5 V gates, so a 5 V microcontroller pin turns it fully on. The IRFZ44N, without the L, is a standard part that needs about 10 V on its gate.

What's the difference between a 2N2222A and a 2N3904?

Both are 40 V NPN transistors with a gain of 100–300. The 2N3904 is rated for 200 mA and the 2N2222A for about 600 mA, so the 2N2222A suits bigger loads like relays and small motors.

Should I use a TIP120 or a MOSFET?

For a new design, a logic-level MOSFET. At 3 A a TIP120 can drop up to 2 V and burn about 6 W. An IRLZ44N with 25 mΩ on-resistance burns about 0.23 W at the same current, so it stays cool without a big heatsink.