part · Level 5 · Control · passive
What a 100 ohm resistor is used for
100 Ω resistor — Half a 220. The LED resistor for a regulated 3.3 V rail, where there is less voltage to burn off.
What it is
A resistor limits current, sized in ohms (Ω). 100 Ω, bands brown, black, brown, is less than half of the 220 Ω you have used on every LED so far. That makes it the wrong resistor for a 4.5 V battery and exactly the right one for a 3.3 V rail.
The reason is that the resistor only has to burn off the voltage the LED does not take. A red LED keeps about 2.0 V for itself. On 4.5 V that leaves 2.5 V for the resistor; on 3.3 V it leaves only 1.3 V. Less voltage to burn off means a smaller resistor for the same current.
You meet 100 Ω wherever the supply is low: regulated 3.3 V rails, logic outputs that cannot swing all the way up, single lithium cells.
How it works
On the 3.3 V rail of the zener project. The node sits at about 3.36 V and the red LED takes 2.02 V, so the resistor sees 1.34 V: 1.34 ÷ 100 = 13.4 mA. Bright and under the 20 mA rating. With 220 Ω the same lane would give only 6 mA, a dim LED, which is why the resistor shrank.
On 4.5 V it is too small. (4.5 − 2.0) ÷ 100 = 25 mA: over the LED's 20 mA rating, though under the 30 mA that pops it on the Workbench. It will light, brightly, and shorten its life. Stay with 220 Ω on the AA pack.
On 9 V it is dangerous. (9 − 2.0) ÷ 100 = 70 mA. Pop.
Power. At 13 mA the resistor dissipates 0.013² × 100 ≈ 0.017 W. Cold. Even at 25 mA it is 0.06 W, a quarter of a ¼ W part's rating.
Reading the bands. Brown (1), black (0), brown (×10): 10 × 10 = 100 Ω. The fourth band, gold, is the ±5 % tolerance, so a real part is between 95 and 105 Ω.
The numbers
| Resistance | 100 Ω — bands: brown, black, brown |
|---|---|
| Red LED on a 3.3 V rail | ≈ 13 mA — (3.36 − 2.02) ÷ 100 |
| Red LED on 4.5 V | ≈ 25 mA — over the 20 mA rating — use 220 Ω |
| Red LED on 9 V | ≈ 70 mA — pops |
| Power at 13 mA | ≈ 0.02 W |
See it working
The The 3.3-Volt Rule circuit, simulated live. Open it on the Workbench to change anything.
simulating…
Mistakes everyone makes
- Grabbed instead of 1 kΩ. Brown-black-brown (100 Ω) and brown-black-red (1 kΩ) differ only in the third band. Fix: compare the third band under good light.
- Used as the LED resistor on the AA pack. 25 mA: a bright LED with a short life. Fix: 220 Ω on 4.5 V, 100 Ω only on a 3.3 V rail.
- Used as a pull-down or base resistor. It passes 45 mA at 4.5 V, a waste that may overload the part driving it. Fix: 10 kΩ or 100 kΩ for anything that is only setting a voltage.
Questions people ask
- What are the colour bands for a 100 ohm resistor?
- Brown, black, brown, then gold: 1, 0, one zero = 100 Ω at ±5 %.
- Can I use a 100 ohm resistor with an LED?
- Yes on a 3.3 V supply (about 13 mA for a red LED). On 4.5 V it gives 25 mA, over the LED rating, so use 220 Ω there; on 9 V it destroys the LED.
- What happens if the resistor is too small?
- More current than the LED is rated for. Up to about 30 mA it just runs hot and ages fast; beyond that it fails outright, which the Workbench shows as a popped LED.
Projects that use this part
- ⚖️ The 3.3-Volt Rule — A zener diode holds 3.3 V for the LED no matter what the 9 V battery wants. Flick the switch to add a second load and watch the zener share. L5 · 30 min
- 🔋 The Zener Battery Tester — A zener in series with an LED makes it ignore the first 3.3 V. A fresh 9 V lights it; a run-down battery barely can. A sharper gauge than a bare LED. L5 · 25 min
verified against: SparkFun — Resistors · SparkFun — Light-Emitting Diodes (LEDs)