39 kΩ resistor color code

Where you'll meet it

A low-battery warning for a 3S LiPo pack

A 3S LiPo pack (three cells in series) is 11.1 V, and 12.6 V when full, while a Raspberry Pi Pico's analog pins read 0 to 3.3 V. A 39 kΩ resistor from the pack's plus to GP26 and a 10 kΩ resistor from GP26 to ground scale it down.

V_out = 12.6 V × 10 kΩ / (39 kΩ + 10 kΩ) ≈ 2.6 V

At 11.1 V the pin sees 11.1 V × 10 kΩ / 49 kΩ ≈ 2.3 V. In code, multiply the pin voltage by 4.9.

The pin would reach 3.3 V only at about 16.2 V, so a 4S pack (16.8 V when full) needs a bigger top resistor.

The pair draws 12.6 V / 49 kΩ ≈ 0.26 mA from the pack.

The 39 kΩ resistor turns (0.26 mA)² × 39 kΩ ≈ 0.003 W into heat, so any size will do.

The pack's minus must connect to the Pico's GND. The divider draws from the pack all the time, so disconnect it when the pack is stored. It sees the whole pack, not each cell. Resistor tolerances shift the reading by a few percent, so check it once against a meter.

On SMD parts

Look for 393 or 3902

• 393393 = 39 × 10³ = 39,000 Ω (3-digit code).
• 39023902 = 390 × 10² = 39,000 Ω (4-digit code, usually ±1% parts).

There is no EIA-96 code, because 39 kΩ is not an E96 value (the nearest E96 values are 38.3 kΩ and 39.2 kΩ).

Check it with a meter

Read 37.05–40.95 kΩ

1. Take the resistor out of the circuit.
2. Set the meter to Ω, range 200 kΩ (or auto).
3. Touch one probe to each lead. Keep your fingers off the metal: your body is a resistor too.
4. Anything from 37.05 to 40.95 kΩ is a good ±5% part. A 5-band ±1% part should read 38.61 to 39.39 kΩ.

A meter has a small error of its own, so a reading a hair outside the range does not prove the part is bad. Measure again with firm contact before you swap it.