حاسبة كود المقاومات السطحية (SMD)

Recognizing the right system before decoding

Decoding does not start with calculation but with identifying the marking. The rule is mechanical: three characters with no final letter → 3-digit code (tolerance ±5%, E24 series); four characters → 4-digit code (±1%, E96 series); two digits followed by a letter → EIA-96. The letter R, wherever it appears, takes the place of the decimal point: R22 = 0.22 Ω, 4R7 = 4.7 Ω, 1R00 = 1.00 Ω.

Three particular markings keep showing up on boards. A simple 0, or 00, 000, 0000, denotes a 0 Ω strap (a jumper, not a resistor). On 0402 packages and smaller, there is no printing at all: the value must be read from the board layout or measured with an instrument. Finally, a code underlined with a line indicates, for several manufacturers, a value in milliohms — typically a current-sense shunt resistor. For leaded resistors, it is a completely different system: see the resistor color code calculator.

Full example: a 472 on an 0805 package

On a power supply board, an 0805 package carries the marking 472. Three characters, no letter: 3-digit code. The mantissa is 47, the exponent is 2, so R = 47 × 10² = 4,700 Ω, i.e. 4.7 kΩ at ±5%. The allowed range runs from 4,465 Ω to 4,935 Ω: a multimeter reading 4.63 kΩ on the desoldered component indicates a healthy resistor.

To replace it, you need three pieces of information, not just the value: 4.7 kΩ, the 0805 package (about 2.0 × 1.25 mm), and the power this package can dissipate. Typical orders of magnitude: 0603 ≈ 0.1 W, 0805 ≈ 0.125 W, 1206 ≈ 0.25 W, 1210 ≈ 0.5 W, 2512 ≈ 1 W. A 4.7 kΩ in 0805 can be found among the 1/6 W and SMD resistors.

Going the other way, suppose you need 1.5 kΩ. In the 3-digit code this is written 152; in the precision 4-digit code, 1501; in EIA-96, 18B — rank 18 of the E96 series equals 150 and the letter B multiplies by 10. All three markings denote the same value; only the component's tolerance differs.

The pitfalls that can cost you a board

• Reading 103 as 103 Ω. The last digit is an exponent, not a significant digit: 103 = 10 kΩ. An actual 103 Ω would be marked 1030 in the 4-digit code.
• Confusing 3-digit codes with EIA-96. 22B does not equal 220 Ω: it is rank 22 of the E96 series (165) multiplied by 10, i.e. 1.65 kΩ. As soon as a letter other than R ends the code, you are dealing with EIA-96.
• Measuring in-circuit. Parallel branches always pull the reading downward. Desolder at least one leg before drawing conclusions, and use a suitable range on one of the multimeters.
• Looking for a value that does not exist. A 3-digit code can only produce an E24 value (10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91 × 10ⁿ). If your calculation gives 4.2 kΩ, use 3.9 kΩ (392) or 4.3 kΩ (432) instead.
• Ignoring power rating. Replacing an 1206 with a 0603 of the same value in a branch dissipating 0.2 W will cause overheating and drift in the resistance. Work out P = R × I² or P = U² / R with the Ohm's law calculator before choosing the package.

One last useful habit: tolerance can be inferred from the marking system. A 3-digit code implies ±5%, a 4-digit or EIA-96 code implies ±1% or better. Replacing an EIA-96 resistor with a ±5% one in a reference divider or a regulation loop will noticeably shift the setpoint.