[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.
Continue reading “Pulse Density Modulation”
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.
Continue reading “Homemade High Voltage Caps”
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.
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.
It only takes one mistake to realize electrolytic capacitors have a polarity, but if you’re working with old tube gear, tube amps, or any old equipment with those old orange dip, brown dip, or green dip foil capacitors you also have to watch your polarity. These old caps were constructed with a foil shielding, and there’s always one side of these caps that should always be connected to the chassis ground. If you don’t, you’re going to get interference – not something you want in an amplifier circuit.
Old caps that have long since given up the ghost usually have a black band designating whatever side of the cap the ‘foil ground’ is. This is the side that should be connected to ground. If you look at modern foil caps, you might also see a black band on one side of the cap, which should – if we lived in a just world – also designate the foil ground. This is not always the case.
To properly test foil caps and determine which side should be closer to ground, you can construct a small tester box that’s more or less an h-bridge with a single switch and a pair of alligator clips in the middle. Connect the cap to the clips, put the output of the circuit in your scope, and flick the switch: the direction that has the least amount of interference is the denotes the foil ground of the cap. Replace those old caps in your vintage equipment with a new, correctly oriented cap, and you’re well on your way to having a great sounding amplifier.
Continue reading “How To Tell If You’re Installing Foil Capacitors Backwards”
[xsdb] had a real problem. His JBL L8400P 600 watt subwoofer went up in flames – literally. Four of the large capacitors on the board had bulged and leaked. The electrolyte then caused a short in the mains AC section of the board, resulting in a flare up. Thankfully the flames were contained to the amplifier board. [xsdb’s] house, possessions, and subwoofer enclosure were all safe. The amplifier board however, had seen better days. Most of us would have cut our losses and bought a new setup. Not [xsdb] he took on the most extreme PCB repair we’ve seen in a long time.
After removing the offending caps and a few other components, [xsdb] got a good look at the damage. the PCB was burned through. Charred PCB is conductive, so anything black had to be cut out. The result was a rather large hole in the middle of an otherwise serviceable board. [xsdb] had the service manual for the JBL sub. Amazingly, the manual included a board layout with traces. Some careful Photoshop work resulted in an image of the section of PCB to be repaired. [Xsdb] used this image to etch a small patch board.
The amplifier and patch were milled and sanded to match up nearly perfectly. Incredibly, all the traces aligned. [Xdsb] soldered the traces across the join with small sections of wire and solder wick. After soldering in some new high quality capacitors, the amplifier was back in action!
If you’re a big fan of burned PCB’s, check out Hackaday Prize Judge Dave Jones latest EEVblog video, where he works on a Ness home alarm panel with a similarly cooked section of FR4.
[Thanks for the link JohnS_AZ!]
Lithium ion supercapacitors. No, not lithium ion batteries, and yes, they’re a real thing. While they’re astonishingly expensive per Farad, they are extremely small and used as the first line of defense in some seriously expensive heavy-duty UPS installations. Here’s a Kickstarter using these supercaps to replace the common AA, C, and D cell batteries. Even better, they can be recharged in seconds.
For each size battery, the caps used actually have a slightly higher energy density than a similarly sized dollar store battery. By adding a little bit of circuitry to drop the 3.8 Volts out of the cap down to the 1.5 V you expect from a battery, this supercap becomes a very expensive rechargeable battery, but one that can be recharged in seconds.
This is one of those crowdfunding campaigns we really like: an interesting tool, but something we just can’t figure out what the use case would be. These lithium ion supercaps are too expensive to be practical in anything we would build (save for a Gauss pistol), but the tech is just too cool to ignore. If you have a use case for these caps in mind, please leave a note in the comments.
Somewhat relevant Mouser link.