Measuring Capacitors Over Their Working Voltage

Ceramic capacitors are small, they don’t leak, they’re convenient, but they are downright strange. Certain types of caps will lose their capacitance depending on the voltage they’re operating at. If you’re using ceramic caps for filters, DC to DC power supplies, bypass caps, or anything where you need an exact capacitance in a circuit, this can be a problem.

[Mathieu] has come up with a tool that’s able to measure the capacitance of a cap over its entire working range. He’s calling it the OpenCVMeter, and although the name might be slightly confusing, the functionality is not. This little box will measure the capacitance of a part over a voltage range from 1.3 to 15.5V.

By attaching the SMD tweezers or test clips to a capacitor, the OpenCVMeter ramps up the voltage and measures the capacitance of the part through the test cycle. This data is then dumped to a Chrome app – a surprisingly popular platform for test equipment apps – and a determination of the cap’s ability will to work in a circuit is displayed on the screen

If you’ve ever tooled around with antique electronic equipment, you’ll know the first thing to go bad in any piece of equipment are caps. Either caps had extremely loose manufacturing tolerances back in the day or the values really were that critical, but a dodgy cap can bring down everything from tube amps to computers. It’s a very neat tool, and something that doesn’t really exist in a single dedicated device.

The Most Powerful DIY Railgun

The US Navy is working on a few railgun projects that will eventually replace the largest guns on the fleet’s cruisers and destroyers. These rail guns will fire a projectile away from the ship at around Mach 7 on a ballistic trajectory to a target one hundred miles away. It’s an even more impressive piece of artillery than a gun with a nuclear warhead, and someday, it will be real.

most-powerful-non-military-railgunUntil then, we’ll have to settle with [Zebralemur]’s DIY mobile railgun. He built this railgun capable of firing aluminum projectiles through pumpkins, cellphones, and into car doors and blocks of ballistics gelatin.

All rail guns need a place to store energy, and in all cases this is a gigantic bank of capacitors. For this project, [Zebralemur] is using fifty-six, 400 Volt, 6000 microfarad caps. The MSRP for these caps would be about $50,000 total, but somehow – probably a surplus store – [Zebralemur] picked them up for $2,400.

These caps are just the power supply for the rail gun, and aren’t part of the structure of this already large, 250 pound gun. Luckily, with the seats down in [Zebralemur]’s car, they fit in the back of his hatchback.

These caps are charged by a bunch of 9V batteries stuck end to end. When the caps are charged, all the power is dumped into two copper bars in the gun, accelerating the aluminum projectile to speeds fast enough to kill. It’s an incredible build, but something that should not be attempted by anyone. Although this does seem to be the year that all danger seekers are busting out their electromagnetic projection flingers.

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Hackaday Links: October 18, 2015

We have our featured speakers lined up for the Hackaday Supercon, one of which is [Fran Blanche]. We’ve seen a lot of her work, from playing with pocket watches to not having the funding to build an Apollo Guidance Computer DSKY. In her spare time, she builds guitar pedals, and there’s a biopic of her in She Shreds magazine.

Halloween is coming, and that means dressing children up as pirates, fairies, characters from the latest Marvel and Disney movies, and electrolytic capacitors.

There’s a new movie on [Steve Jobs]. It’s called the Jobs S. It’s a major upgrade of the previous release, featuring a faster processor and more retinas. One more thing. Someone is trying to cash in on [Woz]’s work. This time it’s an auction for a complete Apple I that’s expected to go for $770,000 USD.

Hackaday community member [John McLear] is giving away the factory seconds of his original NFC ring (think jewelry). These still work but failed QA for small reasons and will be fun to hack around on. You pay shipping which starts at £60 for 50 rings. We’ve grabbed enough of them to include in the goody bags for the Hackaday Superconference. If you have an event coming up, getting everyone hacking on NFC is an interesting activity. If you don’t want 50+, [John] is also in the middle of a Kickstarter for an improved version.

Your 3D printed parts will rarely come out perfectly. There will always be some strings or scars from removing them from the bed. There’s a solution to these problems: use a hot air gun.

Everyone has a plumbus in their home, but how do they do it? First, they take the dinglebop, and smooth it out with a bunch of schleem. The schleem is then repurposed for later batches.

Pulse Density Modulation

[esot.eric] was trying to drive a motor and naturally thought of using pulse width modulation (PWM) to control the motor speed. However, he found that even with a large capacitor, his underpowered power supply would droop before the PWM cycles were complete. So instead of PWM he decided to experiment with pulse density modulation.

The idea is to use smaller pulses over a longer period of time and make the average power equal to the percentage motor speed desired. With a PWM system, for example, if the time period is T, a 50% PWM drive would have the  drive high for T/2 and low for the other half of the cycle. With pulse density, each pulse might be T/10 (as an example) and then the output would be on for 1/10, off for 1/10, on for 1/10 and so on, until by time T you’d still get to 50%. The advantage is the output capacitor gets a kick more often and has less opportunity to droop.

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Homemade High Voltage Caps

Do you happen to have any 15,000 volt capacitors sitting around? [Ludic Science] didn’t so he did the next best thing. He built some.

If you understand the physics behind a capacitor (two parallel conductors separated by a dielectric) you won’t find the build process very surprising. [Ludic] uses transparency film as an insulator and aluminum foil for the conductive plates. Then he wraps them into a tube. He did throw in a few interesting tips about keeping the sheets smooth and how to attach the wires to the foil. The brown paper wrapper reminded us of old caps you might find in an antique radio.

The best part by far, though, was the demonstration of drawing an arc from a high voltage power supply with and without the capacitor in the circuit. As you might expect, playing with a few thousand volts charged into a capacitor requires a certain amount of caution, so be careful!

[Ludic] measured the capacitance value with a standard meter, but it wasn’t clear where the 15,000 volt rating came from. Maybe it was the power supply he used in the video and the capacitor could actually go higher.

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Remove Security Issues From Untrusted USB Connections

USB has become pretty “universal” nowadays, handling everything from high-speed data transfer to charging phones. There are even USB-powered lava lamps. This ubiquity doesn’t come without some costs, though. There have been many attacks on smartphones and computers which exploit the fact that USB is found pretty much everywhere, and if you want to avoid these attacks you can either give up using USB or do what [Jason] did and block the data lines on the USB port.

USB typically uses four wires: two for power and two for data. If you simply disconnect the data lines, though, the peripheral can’t negotiate with the host for more power and will limp along at 0.5 watts. However, [Jason] discovered that this negotiation takes place at a much lower data rate than normal data transfer, and was able to put a type of filter in between the host and the peripheral. The filter allows the low-frequency data transfer pass through but when a high-frequency data transfer occurs the filter blocks the communication.

[Jason] now has a device that can allow his peripherals to charge at the increased rate without having to worry about untrusted USB ports (at an airport or coffee shop, for example). This simple device could stop things like BadUSB from doing their dirty work, although whether or not it could stop something this nasty is still up in the air.

Adding A Backlight To A Cheap Multimeter

We don’t all need super high quality electronic testing gear. Sometimes second-hand or inexpensive equipment is accurate enough to get the job done. Though it can be a bit annoying to miss out on some of those “luxury” features. [Ekriirke] had this problem with his cheap multimeter. He wished the LCD screen had a backlight for easier visibility, so rather than upgrade to a more expensive unit he just added one himself.

After opening up the multimeter [Ekriirke] found that it ran on a single 12V battery. He realized that the simplest thing to do would be to wire up four white LEDs in series. The four LEDs were arranged within the case off to each side of the LCD, one in each corner. The leads were bent at 90 degree angles and soldered together “dead bug” style. Thin strips of copper foil tape were attached to the PCB in such a way that the anode and cathode from the LEDs would make contact when the case was closed back up.

The tape wraps around to the other side of the PCB where there was more room for the next piece of the circuit. A capacitor, resistor, and transistor are used in conjunction with a momentary switch. This circuit allows [Ekriirke] to turn on the light for about ten seconds by pressing the button one time. The circuit also runs through the meter’s dial switch, preventing the LEDs from being turned on while the meter itself is turned off.

[via Reddit]