How does a resistor work?

An LED needs electricity to light up, but too much burns it out. A resistor makes it harder for electricity to flow, so only the right amount gets through. Turn the knobs and watch the dots: they are the electricity.

mA
Moving dots: the electric currentLED light: brighter with more currentBlue parts: you can turn or press themThe supply pushes electricity around one loop: out of the red post, through the resistor, through the LED, through the meter and back into the black post. The same current flows everywhere in the loop, so the dots all move at the same speed. A bigger resistance lets less current through, and the LED gets dimmer. The two LEDs lying in the tray are spares, waiting in case the one in the socket burns out.

The sum the circuit does

push from the supply
−
the LED keeps for itself
÷
the resistor
=
current through the loop

Try this

1 The push: volts

The power supply is like a water pump. It pushes electricity out of the red post, around the loop, and back into the black post. More volts means a harder push.

2 The squeeze: the resistor

A resistor is like a narrow, rough bit of pipe. Electricity can get through, but not easily. The bigger its resistance (in ohms, Ω), the less electricity gets through each second.

3 The flow: current

Current is how much electricity goes past each second. We measure it in milliamps (mA). There is only one path, so the same current goes through everything: the dots move at the same speed everywhere.

4 Why the LED needs a resistor

An LED does not slow the flow down much once it lights. Without a resistor, the current rushes through, the LED gets hot and burns out. The resistor keeps the current small and safe, about 10 to 20 mA. The rest of the push is turned into a little warmth in the resistor.

5 Reading the stripes

Resistors are too small to print numbers on, so they wear coloured stripes. Stripe 1 and stripe 2 are digits. Stripe 3 says how many zeros to add. Gold means it is within 5%.

  • 0black
  • 1brown
  • 2red
  • 3orange
  • 4yellow
  • 5green
  • 6blue
  • 7violet
  • 8grey
  • 9white

For grown-ups

The model is a red LED with a diode curve (about 1.75 V at 1 mA, 2.0 V at 20 mA) in series with the resistor; the page solves V = I·R + VLED(I) for I. The dots show conventional current, from + to −; the electrons in the copper drift the other way. The dots' speed is in step with the current, and their spacing never changes, because the wire is always full of charge. Burnout is simplified: a real LED over its 30 mA rating may last a little longer, but it is overheating.