Arguably the most utilized electronic component, the resistor gets little to no attention when compared to other components in a design. Resistors are passive components which can reduce current and divide voltage, allowing a designer to achieve control over a circuit. If you have any background in electronics, Ohm’s law has undoubtedly been permanently etched in your head. However, to be fair let’s provide a little background:
Resistance is denoted by the Ohm (Ω) which is named after Georg Simon Ohm. The Ohm is the equivalent to one Volt (V) per Ampere (A). Ohm’s law dictates the behavior of a resistor in an ideal setting with the formula: V = I * R, where V is Voltage, I is Current and R is Resistance.
This article aims to give an overview on resistors at Digi-Key: how our resistor products are organized on our website, how the color coding operates on through-hole resistors, and some variations on resistors that do not fit the conventional color code layout. We will also touch on some circumstances that may make resistors difficult to read without use of a tool.
Resistors are available in a multitude of packages, materials, and with a variety of features. Delving into each type of resistor and their intended application is beyond the scope of this article, however, I want to touch on some of the common types Digi-Key carries. Chassis mount resistors are the monsters of the resistor category. Typically these can have power ratings starting around 5 watts, with the highest power rating we currently stock being upwards of 2.5 kilowatts. As the name implies, chassis mount resistors are typically mounted to a chassis or heatsink to assist in heat transfer. Through-hole resistors can also fall in to a high power category based on the composition and package type. A common high-power package for through-hole resistors is the TO-247 package which lends itself to better heat dissipation. I would suggest reading our article on heat transfer for power resistors for more information. Within the through-hole family are the more common resistor types: axial leaded carbon film and metal film. They range typically from 1/8 watt up to a few watts, granted there are always exceptions. The chip resistor family includes resistors in the typical 0805, 0603 and 0402 packages, as well as a plethora of other physical sizes. Although there are a few higher power options in the chip resistor family, it is rare. They are typically utilized in mid- to low-power applications. Designs that use an array of resistors, typically with the same ohm value, can take advantage of resistor networks. Resistor networks combine two to upwards of twenty-eight resistors in one package; with the most common being four and eight resistors within the same package. Packages are available in chip, SIP (Single In-line Package) and DIP (Dual In-line Package) layout. On a side note: Digi-Key has an additional family within the resistor category that is for specialized resistors. These are typically temperature sensitive resistors focused on temperature compensation circuits.
Resistor color codes
The electronic color code has been used since the early 1920s. Developed by the Radio Manufacturers Association (RMA), it evolved into the color bands we see on resistors due to the fact that it was a cheap and simple way to identify small components. The color code is taught in entry level electronics classes with most students memorizing the chart (Figure 1). Most people in the industry know of the mnemonics used to remember the color code order with one of the most popular being “Big Boys Race Our Young Girls But Violet Generally Wins”, which correspond to the colors; Black, Brown, Red, Orange, Yellow, Green, Blue, Violet, Gray, and White, respectively. Obviously, this does not account for the tolerance bands Gold, Silver, and None (no band). There are literally hundreds of different mnemonics, some of which include these missing bands. It is up to the user to pick whatever sticks in their head the best.
Figure 1: Example of resistor color code chart.
An issue arises, however, when precision resistors come into play. Adding a third digit and possibly a temperature coefficient while keeping the order straight drives engineers to still refer to the color code chart on a regular basis. They either have a small reference chart hanging in their lab/workstation or refer to one with a simple search on the internet. That being said, at Digi-Key, we want to assist you in any way we can to keep you focused on your project – that’s where our online tools come in. The Digi-Key Resistor Color Code Calculator is an easy-to-use tool to quickly identify the value of your typical through-hole resistors with 4, 5, or 6 color bands.
Color band exceptions
There are always exceptions to every rule, and resistors are no different. The most common exception is zero ohm resistors, which are easily identified by a single black band (Figure 2). These are a common way of connecting traces on a printed circuit board. Most likely these have been used by a designer to take advantage of pick and place machines in automated assembly. Additionally, in some cases, military qualified resistors have a fifth band to indicate failure rate. There have also been cases of resistors having odd colors that do not fit the color chart. This can be due to damage, wear and tear, or due to an issue in color perception by the user.
Figure 2: A common zero ohm resistor identified by a single black band.
Color blindness is a very common issue with identifying resistors that adhere to the color band standards. According to the National Eye Institute (NEI), 1 in 12 males and 1 in 200 females have some form of color blindness. For such a common problem there must be a solution, right? After doing some research the answer is, well, not really. The general consensus is that people who are color blind usually have to get a second opinion when identifying color codes. Since most designers have access to a multimeter, this is another solution. Multimeters can be relatively inexpensive to very expensive depending on the brand and features you are looking for. A very simple handheld version should satisfy any resistance measuring needs. It is important to keep in mind that resistance measurement is most accurate out of the circuit as it can be affected by other components on the board. It should go without saying, but just in case, make sure you disconnect power before measuring resistance to prevent blowing a fuse or damage to the meter. This measurement will be the actual resistance value plus or minus its tolerance. An example of this is a 1K Ω resistor with a 5% tolerance can measure as low as 950 Ω or as high as 1.05K Ω.
The fundamental operation of a resistor is something that is taught at the very early stages of electronics, hand-in-hand with decoding color bands. While the technology to physically print the actual specifications on the component is available, common through-hole resistors still utilize color codes to this day. The reasoning behind this, in my opinion, is to give future engineers an introduction to decoding values for other devices such as surface mount components, integrated circuits and the like. While there are other ways to identify the value of a resistor, it seems for the foreseeable future, color codes are here to stay.