Understanding Resistors
Striped vs Labelled
Throughout history, the vast majority of resistors indicated their value with a series of painted stripes, also called color bands.
Striped resistors have the advantage that their value can be read after installation, because the color bands surround the entire circumference of the component body. In contrast, resistors whose value is printed in text normally cannot be read after installation, unless care was taken to bend the leads such that the value faces away from the PCB. That's easy for manual installation, but impossible for automated lead bending and insertion machines.
How to read resistor color bands
Two to four bands indicate significant digits of the value. One band indicates a multiplier. One band indicates the tolerance.
The first problem you will encounter is to figure out what the order of bands is. Said another way, given any resistor, do you read the colors from left to right or right to left?
There are no certain visual cues. Theoretically, the tolerance band is spaced away from the bands that indicate significant digits and multiplier. But that is not always the case.
The next potential visual cue: the tolerance band typically would not be black, orange, yellow, or white. Color fidelity is typically not a concern of resistor manufacturers, though, and the color bands' perceived color can change due to contrast (or lack thereof) with the resistor body coating. Blue and purple, red and brown can be difficult to distinguish. A yellow stripe on a blue resistor often looks green.
Here are a few examples:
The color bands on the resistor at right appear to be:
Neither of the bands on the ends seem to be separated from the rest, so we don't know whether the yellow band or the brown band indicates tolerance... except that yellow is never used to indicate tolerance, whereas brown would indicate 1%, so chances are this is a 1% tolerance resistor, and the actual order of the color bands for the resistor's value is:
Armed with that, we can jump to any of the online resistor color code calculators. Choose the 5-band calculator, to accommodate the 4 value bands and 1 tolerance band. Set the menus to the colors we identified, and see the resulting interpretation (pictured at right).
This resistor does show obvious separation to indicate the tolerance band. The tolerance band is always at the end, so the correct order of color bands for this part is:
Our nearby 4-band color code calculator shows this to be a 51MΩ component, with 5% tolerance.
This example illustrates the ambiguity mentioned above: the tolerance band is not obviously spaced away from the others, and both end bands are brown. Which of these is the correct order?
BROWN BLUE RED BROWN BROWN
We have to put both values into a 5-band decoder to try to figure out which is more likely. Both sequences yield valid results:
- BROWN BROWN RED BLUE BROWN: 112M Ω, 1%
- BROWN BLUE RED BROWN BROWN: 1.62kΩ, 1%
Within the context of one of the MicParts DIY kits, the documentation identifies the resistor value you should expect to have. In other contexts, you would have to measure the resistor to determine its actual value.
How to read resistor value labels
We have seen two approaches to resistor value labeling. The first is obvious and easy: the value is literally printed in readable text, e.g. "2.21K". This means 2.21 kΩ, aka 2210 Ohms. Most of the resistors we have used in our DIY kits since 2012 have taken this approach.
Beginning in 2023, our go-to resistor family for DIY kits began using an alternative labelling system, based on the IEC 60062 standard. In this notation, a value multiplier is substituted for the decimal. R indicates Ohms. K indicates Kiloohms, or "1000 Ohms." M indicates megohms (1 million ohms). Examples:
- 2.21kΩ → 2K21
- 720 Ω → 720R
- 1.0 MΩ → 1M0
- 4R7 → 4.7 Ω
Why would Vishay change their marking style? We have no inside information, but we can speculate that decimal marks are too small to print accurately, and too small to be read with certainty. The capital letters R, K, M are much less likely to become smeared or overlooked.
The photo at right shows, top to bottom: Vishay 2.21 kΩ (pre-2023), Vishay 2.21 kΩ (post-2023), PRP 2.2 kΩ. These are all essentially equivalent values, even if the markings are different.
Measuring resistor values
Needless to say, a less ambiguous way to determine the value of a resistor is to measure it. Switch your multimeter to "Ω" mode, then connect the DMM leads to the resistor. Then the resistor's value can be read from the meter's screen.
Caution: some auto-ranging meters will display values in confusing ways, e.g. 750Ω might show as ".750 kΩ". You must read the units on the screen, not just the numbers. See the image at right.
If the resistor is already installed in a circuit, its value cannot be reliably read until one end of the resistor is de-soldered and lifted away from the PCB.
Be aware that some high-value resistors cannot be measured by typical multimeters. Even pricey DMMs often max out at 50MΩ. Therefore if you are trying to read a 51M, 200M, or 1G resistor in one of our DIY kits, you'll see an overload or other error from the DMM. This does not mean the resistor has failed, but rather than you need to review your DMM's specifications page.
For such resistors, decode the color bands.
What is resistor "tolerance" ?
Resistors, like any electronic component, have a stated or nominal value. But manufacturing processes are not capable of producing exact values. Therefore a resistor whose nominal value is 1000 Ω might actually measure 980Ω or 1050Ω, depending on the tolerance.
A batch of resistors with 5% tolerance means the actual component value could be 5% lower, or 5% higher than nominal.
Resistor Power Ratings
Resistors are rated for the amount of power they can handle. Within a phantom-powered FET microphone, 1/4-watt or 1/8-watt resistors are typically fine; a typical phantom power supply would only allow the entire microphone to dissipate 0.48W (P=V×I; 48V × 0.01A = 0.48 W).
The resistors supplied with MicParts kits are typically 0.5W rated, which is overkill for the intended application.
The VPS power supply is an exception to this rule; the power transformer in a tube mic power supply delivers significantly higher voltage and current than a preamp's phantom power rail. Some of the resistors in the VPS are rated for 1W to 3W.