Circuits / Getting started
Potentiometer dimming an LED
The wiper is an adjustable divider: its voltage goes from 0 to the supply
Tap a part to see its voltages and currents. It is a model, not a measurement.
Predict, measure, explain
- Predict Where does the LED shine brightest: with the knob at one end, in the middle or at the other end?
- Measure Move RV1 from one end to the other and tap the LED at each end.
- Explain
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At one end the potentiometer adds nothing and the LED gets 9.8 mA; at the other it adds all its 10 kΩ and the LED goes out.
What do you need?
- Potentiometer 10 kΩSearch on Amazon
- Resistor 330 ΩSearch on Amazon
- Red LEDSearch on Amazon
- Jumper wires × 3Search on Amazon
- BreadboardSearch on Amazon
How does a potentiometer dim an LED?
A potentiometer is a resistor with a sliding contact, the wiper, so it works as an adjustable voltage divider: the wiper's voltage goes from 0 to the supply as you turn it. Here the wiper feeds the LED through R1 (330 Ω) on 5 V. With the knob at the end that touches the supply, about 9.8 mA flows and the LED is bright. A quarter of the way along, the current is already down to 1.0 mA, and at the middle it is only 0.36 mA. The dimming is not even, for two reasons. The LED needs about 1.5 V before it lights at all, and the LED and R1 load the divider: they draw current through the upper half of the 10 kΩ track, so the wiper sags below the voltage it would show unloaded. At the far end the LED is off.
Values from the Protoboard ISO simulation: a model, not a bench measurement.
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