Scott Swaaley On High Voltage

If you were to invent a time machine and transport a typical hardware hacker of the 1970s into 2018 and sit them at a bench alongside their modern counterpart, you’d expect them to be faced with a pile of new things, novel experiences, and exciting possibilities. The Internet for all, desktop computing fulfilling its potential, cheap single-board computers, even ubiquitous surface-mount components.

What you might not expect though is that the 2018 hacker might discover a whole field of equivalent unfamiliarity while being very relevant from their grizzled guest. It’s something Scott Swaaley touches upon in his Superconference talk:  “Lessons Learned in Designing High Power Line Voltage Circuits” in which he describes his quest for an electronic motor brake, and how his experiences had left him with a gap in his knowledge when it came to working with AC mains voltage.

When Did You Last Handle AC Line voltages?

If you think about it, the AC supply has become something we rarely encounter for several reasons. Our 1970s hacker would have been used to wiring in mains transformers, to repairing tube-driven equipment or CRT televisions with live chassis’,  and to working with lighting that was almost exclusively provided by mains-driven incandescent bulbs. A common project of the day would have been a lighting dimmer with a triac, by contrast we work in a world of microcontroller-PWM-driven LEDs and off-the-shelf switch-mode power supplies in which we have no need to see the high voltages. It may be no bad thing that we are rarely exposed to high-voltage risk, but along the way we may have lost a part of our collective skillset.

Scott’s path to gaining his mains voltage experience started in a school workshop, with a bandsaw. Inertia in the saw kept the blade moving after the power had been withdrawn, and while that might be something many of us are used to it was inappropriate in that setting as kids are better remaining attached to their fingers. He looked at brakes and electrical loads as the solution to stopping the motor, but finally settled on something far simpler. An induction motor can be stopped very quickly indeed by applying a DC voltage to it, and his quest to achieve this led along the path of working with the AC supply. Eventually he had a working prototype, which he further developed to become the MakeSafe power tool brake.

Get Your AC Switching Right First Time

The full talk is embedded below the break, and gives a very good introduction to the topic of switching AC power. If you’ve never encountered a thryristor, a triac, or even a diac, these once-ubiquitous components make an entrance. We learn about relays and contactors, and how back EMF can destroy them, and about the different strategies to protect them. Our 1970s hacker would recognise some of these, but even here there are components that have reached the market since their time that they would probably give anything to have. We see the genesis of the MakeSafe brake as a panel with a bunch of relays and an electronic fan controller with a rectifier to produce the DC, and we hear about adequate safety precautions. This is music to our ears, as it’s a subject we’ve touched on before both in terms of handling mains on your bench and inside live equipment.

So if you’ve never dealt with AC line voltages, give this talk a look. The days of wiring up transformers to power projects might be largely behind us, but the skills and principles contained within it are still valid.

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Sidney Darlington

In a field where components and systems are often known by sterile strings of characters that manufacturers assign or by cutesy names that are clearly products of the marketing department and their focus groups, having your name attached to an innovation is rare. Rarer still is the case where the mere mention of an otherwise obscure inventor’s name brings up a complete schematic in the listener’s mind.

Given how rarely such an honor is bestowed, we’d be forgiven to think that Sidney Darlington’s only contribution to electronics is the paired transistor he invented in the 1950s that bears his name to this day. His long career yielded so much more, from network synthesis theory to rocket guidance systems that would eventually take us to the Moon. The irony is that the Darlington pair that made his name known to generations of engineers and hobbyists was almost an afterthought, developed after a weekend of tinkering.

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Hybrid Lab Power Supply From Broken Audio Amp

The lab power supply is an essential part of any respectable electronics workbench. However, the cost of buying a unit that has all the features required can be eye-wateringly high for such a seemingly simple device. [The Post Apocalyptic Inventor] has showed us how to build a quality bench power supply from the guts of an old audio amplifier.

We’ve covered our fair share of DIY power supplies here at Hackaday, and despite this one being a year old, it goes the extra mile for a number of reasons. Firstly, many of the expensive and key components are salvaged from a faulty audio amp: the transformer, large heatsink and chassis, as well as miscellaneous capacitors, pots, power resistors and relays. Secondly, this power supply is a hybrid. As well as two outputs from off-the-shelf buck and boost converters, there is also a linear supply. The efficiency of the switching supplies is great for general purpose work, but having a low-ripple linear output on tap for testing RF and audio projects is really handy.

The addition of the linear regulator is covered in a second video, and it’s impressively technically comprehensive. [TPAI] does a great job of explaining the function of all the parts which comprise his linear supply, and builds it up manually from discrete components. To monitor the voltage and current on the front panel, two vintage dial voltmeters are used, after one is converted to an ammeter. It’s these small auxiliary hacks which make this project stand out – another example is the rewiring of the transformer secondary and bridge rectifier to obtain a 38V rail rated for twice the original current.

The Chinese DC-DC switching converters at the heart of this build are pretty popular these days, in fact we’re even seeing open source firmware being developed for them. If you want to find out more about how they operate on a basic level, here’s how a buck converter works, and also the science behind boost converters.

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Class-D audio amplifier makes it from breadboard to PCB

[Ben Laskowski’s] been working on a Class-D audio amplifier for several months. What you see above is the most recent version of the amp. A class-D amplifier uses transistor switching (or in this case MOSFET switching) to generate the pulse-width-modulated signal that drives the speaker. This is different from common amplifiers as it doesn’t generate the kind of heat that traditional amplifiers do, making it much more efficient.

After the break you can hear it demonstrated. It’s operating off of a single-supply laptop brick and we do hear a bit of a hum coming through the system. Still, we’re quite pleased at the power and quality the small board can put out. Take a look at a post from November to get a handle on what went into development. If you still hunger for more details, [Ben’s] shared the bulk of his prototyping materials in the github repository.

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Dummy loads and heat sinks

In [Dave’s] latest episode of the EEVblog he takes a look at constant current dummy loads. These are used to test power supply designs and instead of just chaining resistive loads together every time he’s decided to look into building a tool for the job. What he ends up with is a reliable constant current load that can be dialed anywhere from 1.5 mA up to just over 1A. There’s even an onboard meter so you don’t have to probe the setting before use.

It may look like he sent his design off to the board house for production but that’s actually a re-purposed PCB. In walking though his junk-box assembled dummy load [Dave] shares some great tips, like using multiple 1% resistors instead of shelling our for one large and accurate power resistor. But our favorite part comes at about 12:00 when he takes us through some rough math in calculating heat sinks. We’ve always just guessed, but like any good teacher, [Dave] explains the theory and then measures the actual performance taking the guesswork out of the design. See for yourself after the break. Continue reading “Dummy loads and heat sinks”

120v switching

[Kenneth] built a 5v controlled power outlet inside of a junction box. We’ve seen plenty of projects that can switch 120v outlets using 5v logic for refrigerator controllers, lighting controllers, or grow systems, but they almost always use solid state relays to facilitate the switching. This iteration uses mechanical relays along with the necessary protection circuitry. The project is housed in an extra deep single-gang box and allows for individual switching of the two outlets. You can see this connected to an Arduino switching two lamps after the break.

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