Learning electronics is a lot like peeling an onion. No matter how much you learn, there is always a deeper level of knowledge that can be uncovered and explored. It’s not just that new technologies are constantly being invented, it’s also that many components have become so mature that we simply take them for granted. Resistors and capacitors are perfect examples of this point. For the first few years of my college education, resistors and capacitors were, for me, simply defined by their schematic symbols. It wasn't until I got into the labs that they become tangible bits of metal, plastic, and various other materials. Still, in my then-inexperienced minds’ eye, a resistor was a resistor. A capacitor was, at most, polarized or non-polarized. But if we peel back the onion, there is a lot more there than meets the eye.
When you get to the point of designing your first project from scratch, after you’ve laid out the circuit it’s time to build the Bill of Materials (BOM). Naturally, you would go to mouser.com and start entering part names or numbers into the search bar. While you might be prepared to select the proper resistance value and/or power rating, what is probably less clear is the differences between the types of resistors and capacitors. Let’s take a look….
The number of choices is enough to make your head explode (Figure 1)! Obviously, the various resistor and capacitor types exist for good reasons. The various construction techniques yield different performance characteristics. Many are useful only in very high-end, specialized applications, and unit costs can range from a few pennies to over a hundred dollars for certain high power resistors. So what should most makers be looking for when selecting resistors and capacitors for their next project?
Figure 1: So many types of resistors! Where to start?
Depending on where you are in developing your soldering skills, the first thing you have to decide is your preference for through hole or surface mounted (SMD) components. Through hole components are much easier to hand solder but at the expense of taking up much more real estate on your printed circuit board (PCB).
Carbon resistors are the perhaps the most common resistors that you will use initially. They are included in many starter kits. They tend to be very cheap but also electrically “noisy,” especially as they get warm. For applications that don’t require tight tolerances, such as limiting current to LEDs, carbon resistors are just fine.
If you are willing to trade off cost, you can get metal film resistors that exhibit less noise and better stability as temperatures increase. This makes them better for high frequency applications such as RF circuits. In addition, very large resistor values are possible (measured in mega-ohms).
These resistors also have relatively higher unit costs. Their resistance values are only common up to a few kilo-ohms. However, they tend to be able to handle a lot of current and they are very precise, making them common in sensing applications.
These resistors offer a pretty good compromise between low cost carbon and higher performance film (smaller tolerance, more precision) and wirewound resistors (greater power handling ability). I would consider these the default type for most applications unless I know for sure another type is absolutely needed or if a cheaper variety will suffice.
Slightly less performance than thin film resistors but also tend to be less expensive.
Just like resistors, capacitors are also manufactured in a variety of ways, yielding different performance characteristics and costs. For the most part, the variation between capacitors is based on the techniques and materials used in their construction, chiefly the dielectric that separates the two plates of a capacitor.
These capacitors often take the form of the blue and silver vertical cylinders (though not always) that populated many a circuit board. They are very inexpensive, have very high capacitance per unit volume, and can handle high voltages. On the flip side, they are polarized, suffer from relatively high leakage currents, and have an Equivalent Series Resistance (ESR) that increases with frequency. In addition, the ESR in aluminum electrolytics gets worse with time. When aluminum electrolytics fail, an open circuit results. They are often used in the voltage regulation portion of a circuit.
These capacitors are also polarized and tend to be a bit more expensive than aluminum electrolytics. For that extra cost, tantalum caps exhibit lower leak currents and great stability of the capacitance value. Relative to aluminum electrolytics, they can’t handle the higher voltages but they do hold up better to a reverse voltage. Their best benefit is their size vs capacitance and they are typically used when circuit board real estate is at a premium. When they fail, they result in a short circuit.
The most widely used type of capacitor, ceramic disk capacitors are often those little orange colored circles (again, they do exist in other form factors and colors however) that are typically spread pretty liberally across many circuit boards. Often, they are used across the power and ground pins of an integrated circuit, which helps cut down on problems associated with voltage drops that can cause a chip to reset. They are also used in high speed signal coupling and decoupling applications. Ceramic disk caps typically range in capacitance values from a few picofarads to a few microfarads. They also have pretty low voltage ratings. When they fail, they result in a short circuit.
Film capacitors could warrant an entire vlog onto themselves given the wide variety of dielectric materials and construction techniques used for them. Typically more expensive than other capacitor types, but their parasitic losses are low and find use in applications with high currents. In addition, their tight tolerances make them appealing to use in in timing applications, such as motor speed controllers. When they fail they result in an open circuit, however they have life expectancies rated in decades.
Hopefully, you won’t look at capacitors and resistors the same again! This discussion was not meant to be an exhaustive review, but rather a collection of some of the more important rules of thumb that have been passed down to me over the years. There are lots of other nuances, so be sure to do your homework if this article has piqued your curiosity. Also, keep in mind that buying in bulk can help reduce unit cost. So if you need higher performance, it can be made more economical if you are manufacturing a product in bulk.
One last tip: When prototyping, I much prefer through hole components to keep things simple. If I intend to homebrew a circuit board with surface mount components I try to stick to the following packages: 0603, 0805, 1210, and 2010 in a thin or thick film variety.
Your turn, what are your experiences with using different types of caps and resistors? Let us know in the comments below.
Michael Parks, P.E. is the owner of Green Shoe Garage, a custom electronics design studio and technology consultancy located in Southern Maryland. He produces the S.T.E.A.M. Power podcast to help raise public awareness of technical and scientific matters. Michael is also a licensed Professional Engineer in the state of Maryland and holds a Master’s degree in systems engineering from Johns Hopkins University.
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