Know the BJT: reading the datasheet
16 minutes · Intermediate
Simulations and quizzes aren't read aloud; pause when you reach them and try them out.
In the Basic Components course you learned that a transistor controls a large collector current with a small base current. In this course we'll apply the same idea to real loads: relays, motors, lamps, LED strips. The first step in choosing the right transistor and not burning it out is reading the datasheet.
Quick recap: currents
In an NPN transistor, the current into the base and the current into the collector leave together through the emitter:
IE = IB + IC
Recall the transistor's three operating regions in the simulator. Make the base resistor larger and smaller to move between regions:
Saturation: the transistor acts like a closed switch. Only the LED and the 330 Ω now set the current.
What is a datasheet?
The manufacturer of every electronic part publishes a document describing all the part's limits and characteristics. Search for the part's name and the word "datasheet" and you'll find it as a PDF. It looks intimidating at first, but only a few lines matter to start with.
The most important values (BC547 example)
| Symbol | Meaning | BC547 | In practice |
|---|---|---|---|
| VCEO | Highest voltage that can be applied between collector and emitter | 45 V | The load's supply voltage must stay below this (leave a margin). |
| IC (max) | Highest continuous current through the collector | 100 mA | The load current must be comfortably below this. |
| Ptot | Highest power that can be dissipated at 25 °C ambient | 500 mW | The transistor's heat: P ≈ VCE × IC. |
| hFE | Current gain (β). Given as min/typ/max | 110–800 | Always use the lowest (min) value in calculations. |
| VCE(sat) | Collector-emitter voltage in saturation | 0.09–0.6 V | This much is lost from the voltage reaching the load. |
| VBE(on) | Base-emitter voltage when conducting | ≈0.7 V | Used in the base resistor calculation. |
Why does the gain (β) vary so much?
The BC547's gain is given in the datasheet as between 110 and 800. Two transistors from the same bag can have very different gains. The gain also changes with temperature and collector current. That's why there are gain groups such as BC547A (110–220), BC547B (200–450) and BC547C (420–800).
The golden rule
When designing a switching circuit, always use the lowest gain (hFE min). A circuit designed for the typical value won't saturate and will heat up if the transistor you have turns out to have low gain.Commonly used transistors
| Part | Type | IC max | VCEO | Package | Typical use |
|---|---|---|---|---|---|
| BC547 / BC557 | NPN / PNP | 100 mA | 45 V | TO-92 | LEDs, small relays, signals |
| 2N2222A / 2N2907 | NPN / PNP | 600 mA | 40 V | TO-92 | Relays, small motors |
| BD139 / BD140 | NPN / PNP | 1.5 A | 80 V | TO-126 | Medium-power loads |
| TIP120 | NPN Darlington | 5 A | 60 V | TO-220 | Motors, solenoids |
The pin order varies by model and manufacturer. On the BC547 it's C-B-E with the flat face towards you; on the plastic version of the 2N2222A (PN2222A) it's E-B-C. Always check the drawing in the datasheet before fitting it.
Test yourself
1. Which gain value is used when designing a switching circuit?
2. If IB = 1 mA and IC = 99 mA, what is IE?
3. Can a 12 V load be driven with a BC547?
4. Which datasheet value is the highest continuous current the transistor can carry?
This was the free first lesson of the course
The remaining 8 lessons of Switching with Transistors, with their simulations and quizzes, unlock when you buy the course.
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