Some of the most enjoyable projects tend to have the terrible drawback of also having the most potential to cause bodily harm, like getting zapped by the capacitor when digging into a disposable camera. But often — if you’re careful — this curiosity pays off and you wind up learning how to make something cool like this coil gun from a camera flash’s capacitor. This handheld launches a small nail, and is packed in a handheld form factor with a light switch trigger.
[LabRatMatt] dispels any illusions of potential harm upfront and then repeatedly urges caution throughout his detailed guide. He breaks down the physics at work while maintaining a lighthearted tone. This coil gun uses a capacitor and charging circuit ripped from a disposable camera — [LabRatMatt] decided to double up with another capacitor that he had on hand from a previous project. The coil was repurposed from an old doorbell, but make sure to use a few hundred windings if you make your own coil. A light switch ended up being suitable for a trigger since it is able to handle the voltage spikes.
When assembled, it almost looks like something you’d expect to see in a post-apocalyptic wasteland, but it works!
Got a bunch of questionable electrolytic caps sitting in your junk bin? Looking to recap a vintage radio chassis? Then you might need to measure the equivalent series resistance of the capacitors, in which case this simple five-transistor ESR meter might come in handy.
Even if you have no need for an ESR meter, [W2AEW]’s video below is a solid introduction to how ESR is determined. The circuit itself comes from EEVBlog forum user [Jay-Diddy_B] and is about as simple as such a circuit can get. Two transistors form an oscillator that generates a square wave that drives a resistor bridge network. The two legs of the bridge feed matched common-emitter amps, one leg through the device under test. The difference in voltage between the two legs is read on a meter, and you have a quick and simple way to sort through the caps in your junk bin. [Jay-Diddy_B]’s circuit is only presented in breadboard form; no attempt was made to field a practical instrument. Indeed, [W2AEW] already built a home-brew ESR meter using hex inverters and op amps to which he compares the five-transistor circuit’s results. His intention here seems to be to clarify the technique of ESR measurement and evaluate an even simpler circuit than his. We think he’s done a good job on both counts.
How many integrated circuits do you need to build up a power supply that’ll convert mains AC into a stable DC voltage? Would you believe, none? We just watched this video by [The Current Source] (embedded below), where he builds exactly that. If you’re in the mood for a very well done review of diode bridges as well as half- and full-wave rectifiers, you should check it out.
First off, [TCS] goes through the basics of rectification, and demonstrates very nicely on the oscilloscope how increasing capacitance on the output smooths out the ripple. (Hint: more is better.) And then it’s off to build. The end result is a very simple unregulated power supply — just a diode bridge with some capacitors on the output — but by using really big capacitors he gets down into the few-millivolt range for ripple into a constant load.
The output voltage of this circuit will depend on the average current drawn, but for basically static loads this circuit should work well enough, and the simplicity of just tossing gigantic capacitors at the problem is alluring. (We would toss in a linear regulator somewhere.)
Quibbling over circuit designs isn’t why you’re watching this video, though. It’s because you want to learn something. Check out the rest of his videos as well. [TCS] has only been at it a little while, but it looks like this is going to be a channel to watch.
Every now and then you need to make your own capacitor. That includes choosing a dielectric for it, the insulating material that goes between the plates. One dielectric material that I use a lot is paraffin wax which can be found in art stores and is normally used for making candles. Another is resin, the easiest to find being automotive resin used for automotive body repairs.
The problem is that you sometimes need to do the calculations for the capacitor dimensions ahead of time, rather than just throwing something together. And that means you need to know the dielectric constant of the dielectric material. That’s something that the manufacturer of the paraffin wax that makes it for art stores won’t know, nor will the manufacturers of automotive body repair resin. The intended customers just don’t care.
It’s therefore left up to you to measure the dielectric constant yourself, and here I’ll talk about the method I use for doing that.
Once upon a time I was a real mad scientist. I was into non-conventional propulsion with the idea of somehow interacting with the quantum vacuum fluctuations, the zero point energy field. I was into it despite having only a vague understanding of what that was and without regard for how unlikely or impossible anyone said it was to interact with on a macro scale. But we all had to come from somewhere, and that was my introduction to the world of high voltages and homemade capacitors.
And along the way I made some pretty interesting, or different, capacitors which I’ll talk about here.
Large Wax Cylindrical Capacitor
As the photos show, this capacitor is fairly large, appearing like a thick chunk of paraffin wax sandwiched between two wood disks. Inside, the lead wires go to two aluminum flashing disks that are the capacitor plates spaced 2.5cm (1 inch) apart. But in between them the dielectric consists of seven more aluminum flashing disks separated by plain cotton sheets immersed in more paraffin wax. See, I told you these capacitors were different.
Big wax cylindrical capacitor
Exposed wax of the capacitor
The experiment and the capacitor’s interior
I won’t go into the reasoning behind the construction — it was all shot-in-the-dark ideas, backed by hope, unicorn hairs, and practically no theory. The interesting thing here was the experiment itself. It worked!
I sat the capacitor on top of a tall 4″ diameter ABS pipe which in turn sat on a digital scale on the floor. High voltage in the tens of kilovolts was put across the capacitor through thickly insulated wires. The power supply contained a flyback transformer and Cockcroft-Walton voltage multiplier at the HV side. As I dialed up the voltage, the scale showed a reducing weight. I had weight-loss!
But after a few hours of reversing polarities and flipping the capacitor the other way around and taking plenty of notes, I found the cause. The weight-loss happened only when the feed wires were oriented with the top one feeding downward as shown in the diagram, but there was no weight change when the top wire was oriented horizontally. I’d seen high voltage wires moving before and here it was again, producing what looked like weight-loss on the scale.
But that’s only one of the interesting capacitors I’ve made. After the break we get into gravitators, polysulfide and even barium titanate.
If you enjoy building radio projects you may have noticed something slightly worrying over the last few years in your component supply. Variable capacitors are no longer as plentiful as they used to be. There was a time when all radio receivers contained at least one, now with the advent of the varicap diode and the frequency synthesiser the traditional tuning capacitor is a rare breed. They are still made, but they’re not cheap and they won’t appear so readily in your junk box any more.
Fortunately a variable capacitor is a surprisingly simple device, and one you can make yourself if you are of a mind to do so. [Patrick] did just that with his home-made capacitor, in this case of a few tens of pF and suitable as a low-power trimmer capacitor or in a single-chip FM radio.
Rather than make a set of interlocking vanes as you’d find in a commercial design, he has gone for a screw in a tube. The capacitance is set by the length by which the screw is inserted into the tube. And his tube is not a tube in the traditional sense, instead he has used a coil of enamelled copper wire wound on the screw thread, whose insulation forms the dielectric. It looks wrong to use a coil in this way as you’d expect a similar coil to form the inductive part of a tuned circuit, but this coil is shorted out to prevent its inductance becoming a factor at the frequency in question.
It’s evidently not the answer to all variable capacitor problems, but it’s a neat piece of lateral thinking and it will make a simple working capacitor from readily available parts.
The pioneering years in the history of capacitors was a time when capacitors were used primarily for gaining an early understanding of electricity, predating the discovery even of the electron. It was also a time for doing parlor demonstrations, such as having a line of people holding hands and discharging a capacitor through them. The modern era of capacitors begins in the late 1800s with the dawning of the age of the practical application of electricity, requiring reliable capacitors with specific properties.
One such practical use was in Marconi’s wireless spark-gap transmitters starting just before 1900 and into the first and second decade. The transmitters built up a high voltage for discharging across a spark gap and so used porcelain capacitors to withstand that voltage. High frequency was also required. These were basically Leyden jars and to get the required capacitances took a lot of space.
In 1909, William Dubilier invented smaller mica capacitors which were then used on the receiving side for the resonant circuits in wireless hardware.
Early mica capacitors were basically layers of mica and copper foils clamped together as what were called “clamped mica capacitors”. These capacitors weren’t very reliable though. Being just mica sheets pressed against metal foils, there were air gaps between the mica and foils. Those gap allowed for oxidation and corrosion, and meant that the distance between plates was subject to change, altering the capacitance.
In the 1920s silver mica capacitors were developed, ones where the mica is coated on both sides with the metal, eliminating the air gaps. With a thin metal coating instead of thicker foils, the capacitors could also be made smaller. These were very reliable. Of course we didn’t stop there. The modern era of capacitors has been marked by one breakthrough after another for a fascinating story. Let’s take a look.