PAGE 068 OF 150 · DRIVING WITH SWITCHES & TRANSISTORS
Driving Many LEDs with One Transistor
The transistor must handle the total current.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
Adding up the current
The collector current is the sum of all LED currents. Ten LEDs at 15 mA each need 150 mA, which approaches the limit of a small signal transistor.
Better ways
- Wire LEDs in series strings with one resistor each, so the current is the same as one LED per string.
- Use several strings in parallel and add their currents.
- For larger loads use a MOSFET or a Darlington array.
Keep a resistor in every string, and check the transistor current and heat rating.
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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