By Jean-Jacques DeLisle, contributing writer
The maker movement, or just good old-fashioned tinkering, is inspiring more engineers, technical professionals, and hobbyists to pick up a soldering iron and dive into their latest home automation, IoT, robotic, or otherwise shock-tastic projects. One of the most common components that every professional or hobbyist circuit designer encounters is the trusty electrical resistor. As one could imagine, there is more to this technology than the rainbow-banded through-hole resistors that come in hobby kits or that were pulled from a defunct VCR.
A resistor’s job is to convert electrical energy, either DC or AC, into thermal energy by having higher resistance than the typical electrical conductor used in a circuit. By doing so, they can control the current flow through or voltage drop across them, offering a handy way to manipulate such things in an electronic circuit. In fact, it’s fairly easy to combine resistors to achieve the specific current value desired or to structure resistor networks to achieve a specific voltage, which is why these devices are so common.
Given this broad definition, though, there can be many types of resistors that optimize different features of this component, including cost, power-handling, size, weight, temperature range, high reliability, accuracy, noise, and frequency response. These many types of resistors offer a wide range of strengths and weaknesses. The chart below lays out many of the more common types.
Fig. 1: There are many members of the electrical resistor family.
Resistor type
Typically, though, when just the term “resistor” is used, it implies either a through-hole or surface-mount (SMT/SMD) linear, fixed resistor. But within this smaller category, there are still a multitude of types with very different physical and electrical properties.
Through-hole resistors of many sorts have a wire either coming from the bottom or sides of the resistor body. These are the type that come in many DIY kits and are responsible for many soldering iron burns. Yet even these resistors come in many shapes and sizes. It’s likely that anyone who has purchased a few resistors hasn’t accidentally overlooked the mechanical drawing in a datasheet to confirm the actual size and shape of a resistor. With through-hole resistors, the size and resistance or power rating are sometimes correlated, sometimes not. This makes checking the datasheet mechanical drawing necessary if footprint is a consideration. Sometimes it makes for an ugly and hard-to-troubleshoot circuit if the resistor has to take on a yoga position to fit via hole spacing that’s much smaller than its body.
SMD resistors are some of the small black squares on a highly dense or small-form-factor PCB. Hobbyist projects may contain professionally assembled parts with SMD resistors, but without the right hot air or specialized soldering tools, SMT resistors are difficult to work with. These resistors usually have much lower power handling and are much smaller than through-hole resistors.
Image source: Ohmite.
With SMD resistors, it’s relatively easy to determine the footprint as the resistor package is typically given in XX-XX, such as 04-02. This metric is a physical measurement given in standard or metric and applies to the length and width of the component. It may be advisable, even with SMD resistors, to check the height in the mechanical drawing if low profile is desirable.
There is another type of resistor less commonly encountered, which is the screw-mounted or bracket-mounted resistor. These are typically used in high-power, high-reliability, or testing scenarios in which precise resistances with low noise operate at DC or with low-frequency signals.
Image source: Ohmite.
Power and thermal considerations
Even resistors of the same type of technology may have different power ratings based on their construction. These are typically given in watts. Ohm’s law is useful for figuring out how much power the resistor needs to handle. Depending on how exact a project’s demands are, choosing a resistor power value higher than the maximum power expected is often advisable. As stray transients can cause substantial voltage or current spikes, leaving a wide margin of power can prevent resistor failure, which often leads to an open circuit at the resistor. Other failure modes include smoke, fire, and, in some cases, even de-soldering or metal-bridging creating a short circuit.
Moreover, resistor value also changes with temperature as a result of external factors or internal heating from power dissipation. Depending on the power requirements of the project and how precise the resistance needs to be controlled, choosing a resistor with a low thermal coefficient may be necessary for the resistance to stay within an acceptable margin over temperature and power extremes.
Important electrical specifications
Obviously, the resistance value is an important specification for a resistor. However, there are many other specifications that could be important to consider.
For transducers, sensors, and anything wireless, the noise produced by a resistor may be of concern. Certain resistor types, such as wire-wound resistors, tend to have lower noise than other common resistors. Noise is also a function of temperature, so choosing a resistor of a low noise variety that has a power handling much higher than required may keep its noise generation to a minimum.
Another consideration is the frequency behavior. Not all resistors have the same resistance value over frequency. SMD resistors are often used for RF and wireless applications for high-frequency circuits, as some SMD resistor technologies have low variation of resistance over frequency.
Quality
Resistors also come in many quality grades. For some common carbon composition, thin-film, and thick-film resistors, there is an additional band, sometimes metallic in color, indicating the accuracy grade of the resistor. This color often equates to a percent range of accuracy, typically ±1%, ±5%. Depending on the tolerances for resistance, which often means voltage or current tolerances, the accuracy of the resistor can be a significant consideration.
Reliability is another concern, especially for critical medical devices and automotive, aerospace, and military applications. Hi-reliability resistors typically follow a standard or rating that is referenced to a low statistical chance of failure over a lifetime. Some resistors are also built to withstand environmental conditions and even radiation or g-forces.
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