PAGE 071 OF 150 · DRIVING WITH SWITCHES & TRANSISTORS
Darlington Pairs
Two transistors in one give high current gain.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
Two transistors in one
A Darlington pair connects two transistors so that the first drives the second. Their gains multiply, giving a total gain of hundreds to thousands.
Trade-offs
- Very little base current is needed.
- Voltage drop across it is larger, about 1 V or more when on.
- Base needs about 1.4 V to turn on.
Examples
The TIP120 is a single power Darlington. The ULN2003 packs seven Darlingtons in one chip, each handling around 500 mA, which suits driving many LED channels.
REMEMBER
A transistor lets a tiny signal control a much larger LED current.Try it yourself
Sketch the circuit on paper first, calculate the values, then build it on a solderless breadboard using a low-voltage supply. Measure with a multimeter before you make anything permanent.
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