Whether you’re an electrical engineer or someone who likes to tinker on the weekends, you (hopefully) know that capacitors can store a large amount of charge and release it at a moment’s notice. This makes them extremely useful, yet storing a large amount of voltage means that this stored charge can be released through human beings, causing a painful shock… or worse.
What is a Supercapacitor
A supercapacitor is a high-capacity capacitor with capacitance values much higher than other capacitors (but lower voltage limits) that bridge the gap between electrolytic capacitors and rechargeable batteries. Supercapacitors, however, are less well-known and are likely avoided by some out of fear or unfamiliarity, when compared to their standard counterparts. While supercapacitors can store a much greater charge in coulombs per volt (farads) than normal capacitors, their breakdown voltage is generally in the single digits. Additionally, while they can release current very fast when compared to batteries, current flow is much slower than normal capacitors.
How do Supercapacitors Work
If you do need higher voltage, supercapacitors can be connected in series in the same way batteries would be, but unless quite a few are chained together, there is little risk of electric shock to humans from them. The potential danger comes from shorting the leads together, which causes a massive amount of current flow. This can destroy your wiring, and even lead to a fire in the right (err…wrong) circumstances.
So, with a very brief primer on the pros and cons of these charge carriers, we'll walk you through an easy experiment to familiarize yourself with these devices; how to receive energy from a solar panel to power an LED.
Charging Supercapacitors Using Alternative Power
Fig 1: Supercapacitor Diagram
Once the circuit shown directly above (and demonstrated in the first image) is hooked up, you’ll notice—or not notice—that nothing seems to happen. In fact, the solar panel won’t read nearly the rated voltage that was advertised when the input to the capacitor is measured. After a few seconds, the voltage reading does begin to slowly creep up, eventually reaching the 2-3 volts required to power your LED. This works similar to how a traditional capacitor must first be charged before releasing energy to restive components. Instead of things happening extremely quickly, as you might expect with traditional capacitors—generally specified in micorfarads, or millionths of a farad—supercapacitors, which come in multiples of entire farads or manageable fractions, take much longer to charge.
If fact, if you decide to duplicate this experiment, I’d recommend using a very small supercapacitor, in the fractional farad range. This will allow a solar panel to charge the supercapacitor to a reasonable level in a few minutes (or less, depending on your panel), at which time you can turn off the light and still operate the LED for some time. Alternatively, you can let a larger cap charge for much longer, giving a corresponding discharge time. If you do let a supercapacitor charge for a long period of time, be sure not to exceed the capacitor’s breakdown voltage level if your panel voltage rating is higher than the capacitor.
Fig 2: A DC motor can also be used to charge a supercapacitor
While powering an LED with solar panels is interesting, practically speaking, it’s not going to do you any good if there is already light available. Another way to charge a supercap is by using a DC gearmotor instead of a panel. The circuit is the same—just sub in a DC motor from your electronics collection as the voltage source. Ideally, you’ll want something with a geared output so that you can spin the motor at a relatively high speed by hand. You’ll need to spin the motor in the correct direction in order to generate power, but the diode that’s already there to keep power flowing in the correct direction with the solar panel will do the same job with your motor.
Of course, there are many other experiments you can do with supercapacitors to get familiar with them. Putting a few in series to increase voltage capacity is another step to take, or you could even build your own lawn decorations that remain on when the sun goes down. Depending on your use case, you could attach a regulator to keep voltage constant as the supercapacitor’s energy is depleted. Unlike batteries, voltage is linearly related to the stored charge.
While still expensive when compared to rechargeable batteries, supercapacitors do have some excellent advantages, such as their high charge/discharge rate and the fact that they last much longer. As technology improves, there’s a very good chance we’ll be seeing more and more of this tech in the future—perhaps one or more will even be appropriate for your next project.