EMC Question of the Week: October 10, 2022

If a resistor network is labeled 101, the value of each resistor in the network is likely to be 

1. 1 Ω
2. 10 Ω
3. 100 Ω
4. 101 Ω

Answer

The correct answer is “c.” When the code used to label a resistor consists of 3 numbers, the first two digits are the first two significant digits of its nominal value. The last digit indicates the power of ten by which to multiply the given resistor value. In this case, 101 indicates 10 x 10¹ = 100 Ω.

Four-digit numerical codes can be used to express a nominal value to three significant figures. For example, 1010 indicates 101 x 10⁰ = 101 Ω. The letter "R" is used to indicate the position of a decimal point for resistance values lower than 10 Ω. Thus, 0R5 would be 0.5 Ω and 0R01 would be 0.01 Ω.

Resistor labels with a last digit that is a letter rather than a number are likely to be using the EIA-96 code. This code is often used for high-precision resistors.

Note that resistor networks are often an excellent way to keep high-frequency noise currents in an integrated circuit from flowing out onto traces that nominally carry low-frequency signals. They also help protect IC inputs from ESD and other fast transient noise. 

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