Bright Lightbulb Saves Old Radios

If you work on old equipment, you know that there’s always that tense moment when you first plug it in and turn it on. No matter how careful you have been, there’s some chance your garage sale find is going to go up in smoke. [BasinStreetDesign] built a little box that can help. On one side is a variac and the device you want to test goes into the other side.

In the middle? A lightbulb, a few switches, and a meter to monitor the current. The magic happens because the lightbulb will stay relatively cool and only light dimly if the device under test is drawing an appropriate amount of current. You match the bulb wattage with the approximate watts you expect the load to draw. If the device’s power is shorted to ground, though, the bulb will light brightly and this causes the lightbulb’s resistance to increase, thus helping to protect the device.

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Custom Isolated Variac Is Truly One Of A Kind

It’s no surprise that many hardware hackers avoid working with AC, and frankly, we can’t blame them. The potential consequences of making a mistake when working with mains voltages are far greater than anything that can happen when you’re fiddling with a 3.3 V circuit. But if you do ever find yourself leaning towards the sparky side, you’d be wise to outfit your bench with the appropriate equipment.

Take for example this absolutely gorgeous variable isolation transformer built by [Lajt]. It might look like a  high-end piece of professional test equipment, but as the extensive write-up and build photographs can attest, this is a completely custom job. The downside is that this particular machine will probably never be duplicated, especially given the fact its isolation transformer was built on commission by a local company, but at least we can look at it and dream.

This device combines two functions which are particularly useful when repairing or testing AC hardware. As a variable transformer, often referred to as a variac, it lets [Lajt] select how much voltage is passed through to the output side. There’s a school of thought that says slowly ramping up the voltage when testing an older or potentially damaged device is better than simply plugging it into the wall and hoping for the best. Or if you’re like Eddie Van Halen, you can use it to control the volume of your over-sized Marshall amplifiers when playing in bars.

Image of the device's internal components.Secondly, the unit isolates the output side. That way if you manage to cross the wrong wire, you’re not going to pop a breaker and plunge your workshop into darkness. It also prevents you from accidentally blowing up any AC powered test equipment you might employ while poking around, such as that expensive oscilloscope, since the devices won’t share a common ground.

Additional safety features have been implemented using an Arduino Uno R3 clone, a current sensor, and several relays. The system will automatically cut off power to the device under test should the current hit a predetermined threshold, and will refuse to re-enable the main relay until the issue has been resolved. The code has been written in such a way that whenever the user makes a configuration change, power will be cut and must be reestablished manually; giving the user ample time to decide if its really what they want to do.

[Lajt] makes it clear that the write-up isn’t meant as a tutorial for building your own, but that shouldn’t stop you from reading through it and getting some ideas. Whether you’re in the market for custom variac tips or just want to get inspired by an impeccably well engineered piece of equipment, this project is a high-water mark for sure.

Protect Yourself — And Your Project — While Working With Mains Power

When debugging ordinary low-voltage circuitry, you’re pretty safe: unless you have some really power-hungry devices that need a ton of current, there aren’t that many truly bad things that can happen, so you can take a lot of liberties with electrical-safety rules. With mains-powered devices, you don’t have this luxury, and a lack of knowledge, sloppy work practices, or simple mistakes can cost you — and your project — dearly. While you still need to know what you’re doing and use the requisite caution, [Yann Guidon]’s latest project — and entry in the 2019 Hackaday Prize —  a mains protection box, might keep simple mistakes from becoming a disaster.

There are a number of precautions you can take when working with mains power. We’ve all used the simple in-line power strip so you can quickly switch off the current, but [Yann] has included a number of devices that can be configured in different ways to experiment with mains-powered devices safely. Built into a sturdy open-topped wooden box with carry handles, the project evokes the traditional breadboard in appearance and functionality. A number of different devices are included, which could be re-configured into different topologies if needed.

[Yann] included an isolation transformer, which can be useful not only for protection against shock in case of accidentally grounding, but also for noise suppression. There is also a variac, which allows the output voltage to be adjusted over a wide range for testing. Of course, circuit breakers are a must, and current and voltage meters keep you informed about what’s going on. A big, easy-to-access switch cuts the power quickly when needed.

The (maybe) final touch is an adjustable output current limit, which is still a work in progress. Built around a current-monitoring relay and a DPDT relay wired as a latch, this allows the output to be disconnected if it draws more than a specified current, equivalent to between 10 W and 100 W. This is the perfect thing for initial testing of new projects.

So, if you’re thinking of working on mains-powered projects, have a close look at what [Yann] has assembled, and learn proper safety procedures before you begin. One place to start is with a great series by our own Jenny List about mains safety: part one and part two. Stay safe out there!

A Dramatic Demo Of AC Versus DC Switching

Switches seem to be the simplest of electrical components – just two pieces of metal that can be positioned to either touch each other or not. As such it would seem that it shouldn’t matter whether a switch is used for AC or DC. While that’s an easy and understandable assumption, it can also be a dangerous one, as this demo of AC and DC switching dramatically reveals.

Using a very simple test setup, consisting of an electric heater for a load, a variac to control the voltage, and a homemade switch, [John Ward] walks us through the details of what happens when those contacts get together. With low-voltage AC, the switch contacts exhibit very little arcing, and even with the voltage cranked up all the way, little more than a brief spark can be seen on either make or break. Then [John] built a simple DC supply with a big rectifier and a couple of capacitors to smooth things out and went through the same tests. Even at a low DC voltage, the arc across the switch contacts was dramatic, particularly upon break. With the voltage cranked up to the full 240-volts of the UK mains, [John]’s switch was essentially a miniature arc welder, with predictable results as the plastic holding the contacts melted. Don your welding helmet and check out the video below.

As dramatic as the demo is, it doesn’t mean we won’t ever be seeing DC in the home. It just means that a little extra engineering is needed to make sure that all the components are up to snuff.

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DIY Variacs Get ESP8266 Upgrades

If you’be been hacking and making long enough, you’ve probably run into a situation where you realize that a previous project could be improved with the addition of technology that simply wasn’t available when you built it. Sometimes it means starting over from scratch, but occasionally you luck out and can shoehorn in some new gear without having to go back to the drawing board.

The two isolated variacs that [nop head] built were already impressive, but with the addition of the ESP8266 he was able to add some very slick additional features which really took them to the next level. He’s done an exceptional job detailing the new modifications, including providing all the source for anyone who might be walking down a similar path.

His variacs have digital energy meters right in the front panel which give voltage, amps, and a real-time calculation of watts. After reading an article by [Thomas Scherrer] about sniffing the SPI data out of one of these meters with an Arduino, [nop head] reasoned he could do the same thing with an ESP8266. The advantage being that he could then pull that data out over the network to graph or analyze however he wishes.

For his older variac, he decided to automate the device by adding a stepper and belt to turn the knob. The stepper is controlled by a Pololu stepper driver, which in turn get’s its marching orders from another ESP8266. He even came up with a simple web interface which allows you to monitor and control the variac from your smart device.

We don’t often see many variacs around these parts, and even fewer attempts at building custom ones. It’s one of those pieces of equipment you either can’t live without, or have never even heard of.

Things Learned From Hot Wire Cutting A Droid’s Body

One of [Bithead]’s passions is making Star Wars droids, and in the process of building the outer shell for one of them he decided to use hot wire foam cutting and make his own tools. Having the necessary parts on hand and having seen some YouTube videos demonstrating the technique, [Bithead] dove right in. Things didn’t go exactly to plan but happily he decided to share what did and didn’t work, and in the end the results were serviceable.

[Bithead] built two hot wire cutters with nichrome wire. The first was small, but the second was larger and incorporated some design refinements. He also got an important safety reminder when he first powered on with his power supply turned up too high; the wire instantly turned red and snapped with an audible bang. He belatedly realized he was foolishly wearing neither gloves nor eye protection.

When it came to use his self-made tools, one of the biggest discoveries was that not all foam is equal in the eyes of a hot wire cutter. This is one of those things that’s common knowledge to experienced people, but isn’t necessarily obvious to a newcomer. A hot wire cutter that made clean and effortless cuts in styrofoam did no such thing with the foam he was using to cast his droid’s outer shell. Still, he powered through it and got serviceable results. [Bithead]’s blog post may not have anything new to people who have worked with foam and hot wire cutters before, but if you’re new to such things you can use it to learn from his experiences. And speaking of improving experiences, [Bithead] most recently snazzed up the presentation of his R2-D2 build by getting tricky with how he hides his remote control.

Taming A Variac With A Thermistor

The folks at TOG, Dublin Hackerspace, have a large variac. A variac is a useful device for testing some fault conditions with AC mains powered equipment, it allows an operator to dial in any AC output voltage between zero, and in the case of TOG’s variac, 250V.

Their problem was with such a magnificent device capable of handling nearly 3KW, it presented an inductive load with a huge inrush current at power-on that would always take out the circuit breakers. Breakers come with different surge current handling capabilities, evidently their building is fitted with the domestic rather than the industrial variants.

Their solution was a simple one, they fitted an NTC surge limiter in series with the variac input. This is a thermistor whose resistance falls with temperature. Thus on start-up it presented an extra 12 ohm load which was enough to keep the breaker happy, but soon dropped to a resistance which left the variac with enough juice.

This is a simple fix to a problem that has faced more than one hackerspace whose imperfect lodgings are wired to domestic-grade spec. In a way it ties in neatly with our recent feature on mains safety; making the transformer no longer a pain to use means that it is more likely to be used when it is needed.

Via: TOG, Dublin Hackerspace.