You are undoubtedly familiar with supercapacitors (supercaps), which pack farads of capacitance into a very small volume. They truly are amazing little bundles for storage vessels energy and thus power. The formal name for their most-common version is electrochemical (or simply electric) double layer capacitor (EDLC).

Back in the 1950s and 1960s, the “conventional wisdom” was that even a 1 farad capacitor would never be practical for general applications as it would occupy an entire desktop or cabinet, given the size of a conventional 1000-µF high-density electrolytic unit, on the order of about 200 cm3 for a low-voltage unit. Of course, you should never say "never” when it comes to technology. Research at General Electric (GE) in the late 1950s started the supercap effort, but it did not become a viable, standard component until the 1990s, when advances in materials and manufacturing technique made them both practical and affordable.

Supercaps store and release energy by reversible adsorption and desorption of ions at the interfaces between electrode materials and electrolytes. Unlike the chemical reaction of batteries (both one-time primary and rechargeable secondary units), they use physical-charge storage and so can be charged and discharged very quickly (milliseconds to seconds). Also unlike secondary batteries with their charge/discharge limit of several thousand cycles, the supercap cycle life is far greater due to the absence of chemical reactions.

Despite their undeniable differences, supercaps are often compared to rechargeable batteries as their top-level function as replenishable energy reservoirs is the same. Whether a given design should use a rechargeable battery or a supercap depends on many factors, including the magnitude, duration, and duty cycle of the application’s power demands. In general, standard (non-super) capacitors can deliver large amounts of power but store only relatively small amounts of energy per unit volume. In contrast, batteries can store larger amounts of energy, but have lower power ratings. Supercapacitors are between the two with respect to energy versus power balance; Table 1 calls out a few comparative attributes.

batteries vs capacitors key attributesTable 1 This is one perspective on the key attributers of NiCd and Li-ion rechargeable batteries, as well as aluminum electrolytic capacitors and supercapacitors. (Source: KEMET Electronics Corporation)

One of the things I find fascinating about supercaps is the many interesting niche applications where they solve a small but nonetheless annoying problem. I saw an article in the October 2019 issue of Model Railroader (behind a paywall, sorry) where the monthly “electronics” columnist explained how supercaps are used in commercially available stay-alive circuits to provide power across rail gaps.

Here’s a brief explanation of the scenario: today’s model railroads no longer use the simple, obvious method of supplying DC power to the locomotive motor by applying a variable voltage to the track rails (these are literally power rails). Instead, most models have shifted over to a networked approach called digital command control (DCC), where a fixed voltage is applied to the rails with superimposed digital codes on the rails. Each locomotive has an internal decoder that controls the motor based on the digital code, telling it how fast to go, direction, initiating realistic sounds, controlling head/trailing lights, and much more. DCC has completely changed the wiring and operation of model railroads, as you no longer need isolated, switchable rail sections, called blocks, to run multiple trains on the same electrical and physical track.

That’s the good news. The less-good news is as a networked system, DCC has the usual complications in set up and troubleshooting. Further, any gap in the rails or break in the track-to-engine electrical path means that motive power flow and network connectivity are lost. It might seem that the track is a solid physical rail without any such breaks, but that’s not the case. There are two dominant problems: intermittent connection between the wheels and their power pick-off wipers (dirt, vibration) and connection loss in the rail at the “frog” of the track switch (also called a “turnout” to minimize confusion with electrical switches), as shown in Figure 1.

track swith anatomyFigure 1 Unavoidable gaps in the rail at a track switch (turnout) make it a less-than-perfect continuity path for power and signal despite its apparently rugged structure. (Source: Thomas M. Tuerke)

Supercaps come to the rescue here. Using three of four supercaps in series (each with nominal voltage 2.5 to 3 V), they can power a keep-alive circuit that provides motive power over the gap period, which is typically only a fraction of a second. While network connectivity is also lost at these gaps, that is not an issue, as that is only needed to institute a change in engine function. Standard keep-alive modules are available from multiple vendors, such as Electronic Solutions Ulm, Digitrax, Soundtraxx, NCE, and TCS.

It’s a nice solution at modest cost, but there is a downside: the space in most locomotives (in the smaller scales of HO, N, and Z) is very limited, and it is often hard to find room for just the DCC decoder IC itself, and even harder for the supercap package (the larger O and G scales generally have room). Interestingly, models of generally obsolete steam locomotives usually do have room for the decoder and supercaps stay-alive due to their associated coal/wood tender, while model diesels are very cramped.

Ironically, the need for the supercap may soon shrink due to advances in battery capacity, especially at the larger scales. Some modelers avoid using the physical rails as power rails despite their availability and low cost (They are there, you might as well use them, right?). Instead, they are using locomotives with on-board rechargeable batteries. This completely solves the rail-gap problem, of course, and eliminates the need for insulated wheelsets and trucks on the locomotive and all cars.

It also eliminates another commonplace, unavoidable headache called the reverse-loop problem, where the track loops back on itself, thus reversing the physical rail connection and creating an instant short circuit (Figure 2). The usual solution is to cut a tiny gap in the rail, and then use a manual or automatic switch to reverse the polarity of the loop track while the train is in the loop. But that can be a real pain or source of mistakes.

short circuit loopFigure 2 Another problem using a rail for power and data occurs when the track curves back on itself as a loop resulting in a short circuit, unless gaps and rail-switching are added. (Source: Azatrax LLC)

Have you used supercapacitors for any unusual or clever applications? Have there been cases where they nicely solved an otherwise small but nasty problem?

Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.

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