A Variable Capacitor Made From Junk


[Jezan] decided to introduce his son to electronics by building a small crystal radio. These crystal sets have been around for a long time, and make for a great beginner electronics project, but some of the required parts are a little hard to come by. The most difficult to source part for these radios is a variable capacitor, and not finding one in his parts bin, [Jezan] decided to make his own.

This variable capacitor comes directly from a piece of 1.5 mm thick aluminum sheet. Instead of fancy CNC machines, power tools, or even a pair of tin snips, [Jezan] cut the rotors and stators for his variable capacitors with a pair of scissors. The center hole was punched out with a piece of sharpened pipe, and all the pieces were filed down and sanded for a perfect finish.

Considering the variable caps you can get your hands on are either rare or very old, this looks like a great afternoon project for the budding electronics wizard or radio enthusiast. [Jezan]’s craftsmanship is incredible as well and the finished part looks like it came off an assembly line.

Building Air Variable Capacitors

In keeping with our opinion that radio operators were the original electronic hackers here’s a guide to building your own transmitting air variable capacitors. Using some roof flashing, Plexiglas, and various fasteners [David Hammack] was able to make it work. It’s not a perfect solution but he has some ideas to make the next one better. Give this a try after you’re done tweaking your crystals and building input devices.

[Thanks Rich]

Variable Super Capacitor Battery Provides Power On The Go


Instructable user [EngineeringShock] got sick of buying batteries for his devices all the time and has instead opted to build himself a super capacitor bank that can be used to power common household items.

His “forever” rechargeable capacitor bank is made of two large super capacitors rated at 400 farads apiece. It is charged through a LM317-based charging circuit that is adjustable to allow for slow or fast charging, the latter of which he admits, is slightly dangerous.

Since the super caps are only rated at 2.7 volts, they are wired through a DC-DC booster circuit that allows him to adjust the output voltage from 4.3 v to 34 v. The adjusted voltage is then passed through a digital display that allows him to see what the output voltage is at any time.

He says that the super cap bank can power his computer’s speakers for about two hours before requiring a recharge, which takes just a few short minutes, depending on how he is charging them.

While it’s not exactly cheap, the capacitor bank could be useful for those requiring quick portable power for relatively short periods of time. If we were to build one ourselves, we would likely fit all of the components into a small project box to protect the caps from accidental discharging, and top it off with a couple of solar cells to charge it for free during the day.

Keep reading to see a quick video demonstration of his super cap “battery” in action.

Continue reading “Variable Super Capacitor Battery Provides Power On The Go”

Build Your Own HV Capacitors

Finding high voltage capacitors can be tricky. Sure, you can buy these capacitors, but they are often expensive and hard to find exactly what you want. [RachelAnne] needed some low-value variable capacitors that would work at 100 kV. So she made some.

Instead of fabricating the plates directly, these capacitors use laminations from a scrap power transformer. These usually have two types of plates, one of which looks like a letter “E” and the other just like a straight bar. For dielectric, the capacitors use common transparency film.

Continue reading “Build Your Own HV Capacitors”

Make Your Own Variable Inductor

Inductors are not the most common component these days and variable ones seem even less common. However, with a ferrite rod and some 3D printing, [drjaynes] shows how to make your own variable inductor. You can see him show the device off in the video below.

The coil itself is just some wire, but the trick is moving the ferrite core in and out of the core. The first version used some very thick wire and produced an inductor that varied from 6 to 22 microhenrys. Switching to 22 gauge wire allowed more wire on the form. That pushed the value range to 2 to 12 millihenrys.

Continue reading “Make Your Own Variable Inductor”

DIY Tuning Capacitors From Washers And 3D-Printed Parts

The inside of classic radios holds wonders that the sterile chips and SMD components of today’s circuits can’t hold a candle to. Chunky resistors and capacitors, vacuum tubes with cathodes aglow, and seemingly free-form loops of wire forming inductors will all likely make an appearance. But the most fascinating bit of any old radio was connected to the tuning knob: the big variable capacitor with its interdigitating metal plates. Watching one at work, with its plates evenly and finely spaced, is still a joy to behold.

In an attempt to recapture a little of that magic, [Jeremy S. Cook] came up with this home-brew variable tuning capacitor. The frame is built mainly from 3D-printed parts, which supports a shaft made from a common bolt. Plates are fashioned from stainless steel fender washers cut in half; the fixed plates are press-fit into the frame while the rotary plates ride on the shaft. The spacing between the rotary plates is maintained by printed spacers, which also serve to lock the rotor into one solid unit. [Jeremy]’s prototype, for which he provides STL files, can be tuned between about 7 and 15 pF. Check out the build in the video below.

We love the look of this, and we can imagine custom tuning caps would come in handy for certain retro radio builds. The tuning range is a little narrow, but that could be fixed with more plates or closer spacing. That might be a tall order with thick steel washers, but we’ve seen really thin aluminum machined and closely spaced before, so this might be one approach to higher capacitance. Continue reading “DIY Tuning Capacitors From Washers And 3D-Printed Parts”

Building A Portable Solar Powered Spot Welder: Charging Supercapacitors

Before Lunar New Year, I had ordered two 3000 F, 2.7 V supercapacitors from China for about $4 each. I don’t actually remember why, but they arrived (unexpectedly) just before the holiday.

Supercapacitors (often called ultracapacitors) fill a niche somewhere between rechargeable lithium cells and ordinary capacitors. Ordinary capacitors have a low energy density, but a high power density: they can store and release energy very quickly. Lithium cells store a lot of energy, but charge and discharge at a comparatively low rate. By weight, supercapacitors store on the order of ten times less energy than lithium cells, and can deliver something like ten times lower power than capacitors.

Overall they’re an odd technology. Despite enthusiastic news coverage, they are a poor replacement for batteries or capacitors, but their long lifespan and moderate energy and power density make them suitable for some neat applications in their own right. Notably, they’re used in energy harvesting, regenerative braking, to extend the life of or replace automotive lead-acid batteries, and to retain data in some types of memory. You’re not likely to power your laptop with supercapacitors.

Anyway, I had a week-long holiday, and two large capacitors of dubious origin. Sometimes we live in the best of all possible worlds. Continue reading “Building A Portable Solar Powered Spot Welder: Charging Supercapacitors”