3.9 kΩ resistor color code
A 3.9 kΩ resistor is Orange, White, Red, Gold: 39 × 100 = 3,900 Ω, ±5%, so anything from 3.705 to 4.095 kΩ is in spec.
39 × 100 = 3,900 Ω = 3.9 kΩ
5-band version
Orange · White · Black · Brown · Brown = 3.9 kΩ ±1%
390 × 10 = 3,900 Ω = 3.9 kΩ. Range 3.861–3.939 kΩ (±1%).
Open the 5-band version
Where you'll meet it
A transistor switch for a small load
When the load is small, such as an LED with its own resistor, an NPN transistor (such as a BC547 or 2N2222) needs only a little base current to switch it. The 3.9 kΩ resistor sits between pin D6 and the base and sets that current. The load sits between +5 V and the collector, and the emitter goes to ground.
I_B = (5 V − 0.7 V) / 3.9 kΩ ≈ 1.1 mA
1.1 mA is a light touch on the pin, far below the 20 mA it should give.
For a switch that turns fully on, give the base about a tenth of the load current: 1.1 mA × 10 ≈ 11 mA, so loads up to about 10 mA. A BC547 is rated 100 mA, so it has plenty in reserve.
From a 3.3 V board: (3.3 V − 0.7 V) / 3.9 kΩ ≈ 0.7 mA, good for loads up to about 7 mA.
The resistor turns (1.1 mA)² × 3.9 kΩ ≈ 0.005 W into heat, so a ¼ W part is plenty.
0.7 V is the usual base-emitter estimate; with a load this small a BC547 switched fully on drops about 0.7 to 0.8 V, so the real base current is a hair lower, still about 1.1 mA. Check your transistor's datasheet. A bigger load needs a smaller base resistor, sized the same way. A typical 5 V relay coil draws far more than 10 mA, so check its current first, and put a flyback diode across the coil.
On SMD parts
Look for 392 or 3901
- 392392 = 39 × 10² = 3,900 Ω (3-digit code).
- 39013901 = 390 × 10¹ = 3,900 Ω (4-digit code, usually ±1% parts).
There is no EIA-96 code, because 3.9 kΩ is not an E96 value (the nearest E96 values are 3.83 kΩ and 3.92 kΩ).
Don't mix it up
Only band 3 changes
These four share Orange, White and Gold. Red (×100) is 3.9 kΩ; brown is ten times less and orange ten times more.
390 Ωband 3 brown- 3.9 kΩband 3 redThis page
39 kΩband 3 orange
390 kΩband 3 yellow
Check it with a meter
Read 3.705–4.095 kΩ
- Take the resistor out of the circuit.
- Set the meter to Ω, range 20 kΩ (or auto).
- Touch one probe to each lead. Keep your fingers off the metal: your body is a resistor too.
- Anything from 3.705 to 4.095 kΩ is a good ±5% part. A 5-band ±1% part should read 3.861 to 3.939 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.
Spot it
Can you find 3.9 kΩ in a pile?
Practise reading it, and its look-alikes, in the resistor challenge.
Try the resistor challenge3.9 kΩ resistor FAQ
Why use 3.9 kΩ instead of 1 kΩ on a transistor base?
A small load needs only a small base current. From a 5 V pin, 3.9 kΩ gives (5 V − 0.7 V) / 3.9 kΩ ≈ 1.1 mA, enough to switch a load of up to about 10 mA fully on, and it takes less from the pin than 1 kΩ would. A heavier load needs a smaller resistor: give the base about a tenth of the load current.
What is the SMD code for a 3.9 kΩ resistor?
392 on 3-digit parts (39 × 10²) and 3901 on 4-digit parts (390 × 10¹). There is no EIA-96 code, because 3.9 kΩ is not an E96 value.
What are the 5-band colors for 3.9 kΩ?
Orange, White, Black, Brown, Brown: 390 × 10 = 3.9 kΩ with ±1% tolerance, so 3.861 to 3.939 kΩ.
What does 3k9 mean?
3k9 is another way to write 3.9 kΩ. The k sits where the decimal point would be, so the point cannot get lost in small print or on a schematic. You will also see it written as 3.9k or 3K9.
Bands, ranges and codes come from the same engine as the calculator. Last reviewed .