part · Level 5 · Control · passive
What a 100k ohm resistor is used for
100 kΩ resistor — Ten 10 kΩs stacked. A pull-down for inputs that draw no current: MOSFET gates and logic pins.
What it is
A resistor limits current, sized in ohms (Ω). 100 kΩ, one hundred thousand ohms, bands brown, black, yellow, is ten 10 kΩ resistors in a row. At 4.5 V it lets through 4.5 ÷ 100 000 = 0.045 mA, forty-five microamps. Nothing you would notice; an LED fed through it stays dark.
Its job is to set a voltage without spending current. Some inputs, a MOSFET gate or a CMOS logic pin, draw no current at all, so even a resistor this large holds them firmly at 0 V (a pull-down) or at + (a pull-up) until a button overrides it. A transistor base would not be satisfied by 45 µA; a gate is, because it wants nothing.
It is also the slow resistor in timers: 100 kΩ charging a 10 µF capacitor takes about a second, which is why the 555 blinker uses one.
How it works
Pull-down for a gate. 100 kΩ from the gate to −. Button up: no current flows anywhere, so the gate reads 0 V and the MOSFET is off. Button down through a 10 kΩ: the two resistors form a divider, 4.5 × 100 ÷ (10 + 100) = 4.09 V at the gate, and 4.5 ÷ 110 000 = 41 µA flows down the chain. That is the Switch That Uses No Current.
Pull-down for a logic input. Same wiring without the 10 kΩ: button up, 0 V; button down, the input sits at the full 4.5 V and the resistor passes 45 µA. The Two-Key Lock has two of these.
Why not 10 kΩ? It would work, at ten times the current. 100 kΩ is the habit for inputs that take nothing, because the only current it ever wastes is while the button is held.
Why not 1 MΩ or more? A gate or a logic pin picks up stray charge from your hand and the air. The pull-down has to drain that faster than it arrives; 100 kΩ does, comfortably. Into the megohms, inputs start to flicker.
RC timing. A resistor filling a capacitor takes τ = R × C to reach 63 %: 100 000 × 0.00001 = 1 s with 10 µF, 10 ms with 100 nF. The 555 blinker's 0.7 s half-period comes from this resistor.
The numbers
| Resistance | 100 kΩ = 100 000 Ω — bands: brown, black, yellow |
|---|---|
| Current at 4.5 V | 45 µA |
| Divider with 10 kΩ on top | ≈ 4.09 V — 4.5 × 100 ÷ 110 |
| Red LED current | ≈ 25 µA — dark |
| τ with 10 µF | 1 s — 10 ms with 100 nF |
See it working
The The Switch That Uses No Current circuit, simulated live. Open it on the Workbench to change anything.
simulating…
Mistakes everyone makes
- Used as a transistor base resistor. 45 µA × 200 = 9 mA at the most, and only with a perfect part: the LED is dim or dark. Fix: 10 kΩ for a 2N2222 base; 100 kΩ is for gates and logic pins.
- Mixed up with 10 kΩ. Brown-black-yellow (100 kΩ) vs brown-black-orange (10 kΩ): one colour. Fix: a yellow third band means five zeros.
- Left out of a gate circuit. The gate floats and the MOSFET turns on and off at random. Fix: every gate and every logic input has a path to + or − through some resistor.
- Expecting it to light anything. 25 µA through an LED is invisible. Fix: it sets voltages; 220 Ω carries currents.
Questions people ask
- What are the colour bands for a 100k resistor?
- Brown, black, yellow, then gold: 1, 0, four zeros = 100 000 Ω = 100 kΩ at ±5 %.
- What value pull-down resistor should I use for a MOSFET gate?
- Anything from 10 kΩ to a few hundred kΩ works because the gate draws no current. 100 kΩ is the usual choice: it holds the gate at 0 V firmly and wastes only 45 µA while the button is pressed.
- Why does the 555 blinker use 100k?
- Because the timing is R × C. With a 10 µF capacitor, 100 kΩ gives about a second per step; a 10 kΩ would blink ten times faster.
Projects that use this part
- 🪶 The Switch That Uses No Current — A MOSFET turns the LED fully on, yet its gate draws nothing. Hold the button, then tap the 10 kΩ to make it 100 kΩ and see the gate still win. L5 · 30 min
- 🔐 The Two-Key Lock — Two NAND gates make an AND: the LED lights only while BOTH buttons are down. The first logic chip, and the reason NAND is called universal. L6 · 45 min
- 🚨 The Alarm That Remembers — Two NAND gates wired into each other make a memory. Tap SET and the red LED stays on after you let go, until someone presses RESET. L6 · 45 min
- 🏃 The Four-Step Chaser — A 555 ticks five times a second and a CD4017 counter walks the light from LED to LED, round and round. Hold a button and the chase freezes. L6 · 60 min
- 💡 The Two-Switch Hall Light — A light with a switch at each end of the hall: flip either and the light changes. Four NAND gates in one CD4011 make an XOR. L6 · 60 min
verified against: SparkFun — Pull-up Resistors · SparkFun — Voltage Dividers · SparkFun — Resistors