Reverse engineer a PSU to change its output voltage


[Semicolo] has a bunch of old PSUs on hand which he pulled out of some Lexmark dot matrix printers. In their stock form they put out 40V, which is close to the 35V max he needs to run the stepper motors on a 3D printer he’s been building. So he reverse engineered the PSU to change its output.

On the left you can see the top of the PCB. [Semicolo] flipped it over and snapped a picture of the traces on the bottom of the board. With a bit of work in The Gimp (FOSS image editing software) he was able to convert the traces to black and white. Overlaying the picture of the top with a 50% transparency of the traces made it rather easy see the connections and generate a schematic for the hardware. That’s a really cool trick!

Figuring out how it’s supposed to work is a big step in achieving his goal. The next step was to see if he could bend the circuit to his will. He had previously run across ATX PSU hacks which changed the reference voltage in order to alter the output. He grabbed a datasheet for the HA17431 variable shunt regulator. It lays out how to tune the output based on values of a few external components. He dropped in one resistor and the output measured 31V, well within his target range.

Add external power to any USB hub

[Andrew] was getting some poor performance from a couple of USB devices he had connected through an unpowered hub. This is a problem because the hub prevents devices from negotiating with the host controller for more current. He fixed it by adding an external power supply to his USB hub.

In this case the PCB already had a footprint for a power connector. The manufacturer uses one board for several different models and just leaves the supply components unpopulated. [Andrew] managed to find a barrel jack in his parts bin that matched the footprint.

One important thing to do before hooking up the source is to disconnect the 5v wire from the incoming cable from the computer. The other tip we can give you is to use a good regulated 5v source to ensure you don’t damage the stuff you’re trying to power. That means avoiding deals that are too good to be true.

RPi control your server PSU over the Internet


Here’s an interesting use of a Raspberry Pi to control the PSU on a server. [Martin Peres] is going to be away for a few months and still wants access to his PC. This isn’t really all that tough… it’s what SSH is made for. But he also wants lower-level access to the hardware. Specifically he needs to control and get feedback on what the PSU is doing, and even wanted to have access to the serial console without having to go through the computer’s NIC.

The image above shows one part of his solution. This is a custom Ethernet port that connects to his Rasberry Pi header breakout board. Inside the computer the jack is wired to the motherboard power LED to give feedback about the current state of the power supply. It also patches into the green wire on the PSU, which lets him turn on the power by pulling it to ground. After working out the cable routing he developed a web interface that makes it easy to interact with the setup.

As with other hacks along these lines letting an embedded computer run 24/7 is a lot less wasteful than leaving a PC on. That’s a concept we can really get behind.

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ATX Raspi is a smart power source for Raspberry Pi

One aspect of the Raspberry Pi that has always challenged us is the power supply. It was a great idea to power the board from a standard micro-USB port because economy of scale makes phone chargers (even in the 1A range necessary for stable operation of the RPi) cheap and easy to acquire. The thing we miss is the ability to power the device on and off using the built-in hardware. The quandary has given rise to many different solutions, and the ATX Raspbi smart PSU is one of the better ones we’ve come across. It’s a nicely packaged take on the PIC-based version we saw earlier in the year.

The device is a small PCB that acts bridge between the micro-USB power supply and the RPi board. It offers several breakout headers, one of which is used for a power button. The button is monitored by a microcontroller that switches the on-board relay accordingly. But it won’t just kill the power when you want to shut down. It first signals one of the RPi GPIO pins, causing the OS to execute a shutdown script. It then monitors the RPi for the shutdown tasks to finish before cutting the power.

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Apple MagSafe cord repair

[Tommy Ward] had a big problem with the cord for his laptop power supply. This thing’s not cheap so he figured out a way to fix the frayed cord on his Apple MagSafe. He asserts that the shortened rubber collar on the plug end of the cord is to blame for this type of damage. We think rough use may have something to do with it too, but having had to repair our own feline-damaged power cords we’re not about to start pointing fingers.

To pull off an appropriate fix [Tommy] pries apart the case housing the power converter. This lets him get at the solder connections of the cord. After removing it from the circuit board he clips off the damaged portion of the cable. To reuse the strain relief grommet he drilled out the old portion of wire and insulation, making room for the undamaged cable to pass through, adding a cable tie on the inside to aide in strain relief. The last part of the fix involves gluing everything back together.

If your power supply problems have to do with the computer connector itself there’s a fix for that too.

Ammo box PSU


Sometimes you need a power supply that can be thrown into the back of a car and taken into the field. [BadWolf] didn’t want to take his bench supply, so he whipped up this very portable power supply made from a computer PSU. To ruggedize his build a little, he put it in a 50 caliber ammo can making it more than able to handle the roughest field work.

While not a proper adjustable power supply, this ammo can is more than capable of delivering a whole lot of current in a number of different voltages. There are a few bells and whistles – a ‘plugged in’ and ‘on’ light, as well as a few very cool looking toggle switches that are sure to arouse the suspicions of unsuspecting bystanders.

[BadWolf] kept all the safety features built-in to the computer PSU, so this ammo box power supply is still protected from short circuits, and over-current, making it much safer than its appearance belies. It’s also a great example of what can be done if you don’t have a proper bench supply, so we’ve got to tip our hat to [BadWolf] for that.

Mr. Tea is a hot plate and magnetic stirrer in the same enclosure


Not being a coffee drinker [Hunter Scott] wanted a way to make tea while lurking in his workshop. Well it’s not exactly rocket science, as all you need is water at the right temperature and a vessel in which the tea can be steeped. But we do commend him on not only building a nice little hot plate enclosure, but rolling a magnetic stirrer into the other side of the box.

You heard us right, the stirrer is not combined with the plate, but resides on the underside of the same PSU enclosure. The plate itself is from a unit he bought at the store and cannibalized. The light switch dimmer lets him adjust the heat it puts out. When not hot, he can flip it over and use the stir plate. This consists of a hard drive magnet attached to a PC fan. For the stirrer itself he encased a neodymium magnet in some thermoplastic. The magnetic combination works well together with a demonstration which shows it stirring water through the base of a tea-cup.