Tripping Out: A Field Guide to Circuit Protection

My introduction to circuit protection came at the tender age of eight. Being a curious lad with an inventive – and apparently self-destructive – bent, I decided to make my mother a lamp. I put a hose clamp around the base of a small light bulb, stripped the insulation off an old extension cord, and jammed both ends of the wires under the clamp. When I plugged my invention into an outlet in the den, I saw the insulation flash off the cord just before the whole house went dark. Somehow the circuit breaker on the branch circuit failed and I tripped the main breaker on a 200 amp panel. My mother has never been anywhere near as impressed with this feat as I was, especially now that I know a little bit more about how electricity works and how close to I came to being a Darwin Award laureate.

To help you avoid a similar fate, I’d like to take you on a trip (tee-hee!) through the typical household power panel and look at some of the devices that stand at the ready every day, waiting for a chance to save us from ourselves. As a North American, I’ll be focusing on the residential power system standards most common around here. And although there is a lot of technology that’s designed to keep you safe as a last resort, the electricity in your wall can still kill you. Don’t become casual with mains current!

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Upcycle An Isolation Transformer

There are several reasons you should have an isolation transformer. They can prevent ground loops and also prevent a device under test from having a DC path to ground (or isolate an oscilloscope from DC ground, which can be dangerous in its own right, but that’s another discussion). [Tanner_tech] noticed that finding ballast transformers for sodium vapor street lights is getting easier as more street lights move to LED technology. What to do with these transformers? Build an isolation transformer, of course.

Of course, your dumpster transformer might be a little different than the one shown in the post (and the video, below). [Tanner] shows how to work out the leads you need. A little wood work and a PC power supply case finished the project.

Judging from the comments, some people take [Tanner’s] talk about safety as an implication that a transformer makes working on mains safe. It doesn’t. It makes it safer if you know what you are doing. Working with high voltage isn’t a place to learn by doing.

If you want some practical advice, [Jenny List] has a good read for you. You probably also ought to invest an hour in watching this video that has a lot of practical advice.

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EPROM Timer

[glitch] had a cheap EPROM eraser with very few features. Actually, that might be giving it too much credit: it’s barely more than a UV light that turns on when it’s plugged in and turns off when it’s plugged out unplugged. Of course it would be nice to implement some safety features, so he decided he’d hook it up to a software-controlled power outlet.

Of course, controlling a relay that’s wired to mains is old hat around here, and in fact, we’ve covered [glitch]’s optoisolated mains switch already. He’s gone a little beyond the normal mains relay project with this one, though. Rather than use a microcontroller to run the relay, [glitch] wrote a simple Ruby script on his computer to turn the EPROM eraser on for the precise amount of time that is required to erase the memory.The Ruby script drives the relay control directly over a USB to serial adapter’s RTS handshake pin.

[glitch]’s hack reminds us that if you just need a quick couple bits of slow output, a USB-serial converter might be just the ticket. You could imagine driving everything from standard lamps to your 3D printer’s bed heater (provided you use similar hardware), but it’s especially helpful for [glitch] who claims to forget to turn off the eraser when it’s done its job, which leaves a potentially dangerous UV source just lying about. It’s always a good idea to add safety features to a dangerous piece of equipment!

Controlling Mains Power Rube Goldberg Style

[g3gg0] has some nice radio equipment including an AOR AR-5000 receiver and a HiQ SDR. They are so nice that it appears they lack an on/off switch. [g3gg0] grew tired of unplugging the things, and decided to nerdify his desk with a switch that would turn his setup on and off for him. He decided to accomplish this task by emulating the Scroll, Number and Caps Lock LEDs on his keyboard via a Digispark board. He uses the LEDs to issue commands to the Digispark allowing him to control a 5V relay, which sits between it and the AC.

Starting off with some USB keyboard emulation code on the Digispark, he tweaked it so he could use the Scroll Lock LED as sort of a Chip Select. Once this is pressed, he can use the Caps Lock and the Number Lock LED to issue commands to the Digispark.

It’s programmed to only stay on for a total of 5 hours in case he forgets to turn it off. Let us know what you think about this interesting approach.

Non-Invasive Smart Electricity Meter

There are a lot of ways to measure energy usage in the home, but most of them involve handling mains voltage. Not only that, but sometimes they require handling mains voltage before it gets through a breaker panel or fuse box, meaning that if you make a mistake there are a lot of bad things that can happen. [Yonas] has been working on this problem, and has come up with a non-invasive, safer way to monitor electricity consumption without having to work directly on live wires.

Please note that you should still not be working on mains voltage without proper training, but if you have the required know-how then the installation should be pretty straightforward. The project is based on the Spark Core, and uses clamp-on current sensors to measure energy use. The sensors wrap around the mains cable, meaning you don’t have to disconnect anything to hook them up. The backend runs on a LAMP server which could be a Raspberry Pi if you have one. [Yonas] runs it on a hosted server as a matter of preference.

All of the source code for this is available, and assuming you can get your hands on the current sensors this could be a great way to get started monitoring your energy usage in the house. Be sure to check out the video below for a demonstration of the operation of this device. Of course, if you have a gas line you’ll need this energy monitoring setup too.

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Portable 120V to 240V Converter; or How to Fast Charge at Your In-Law’s House

[Nick Sayer] falls into the “would rather build it than buy it” category. This particular project is a clone of a fast electric vehicle charger. There are commercially available versions sold under the Quick 220 brand name. The idea is that for fast charging, some electric vehicles call for a 240V outlet and Americans without electric cars often don’t have one. If they do it’s for an appliance like a stove or clothes dryer and probably not found in the garage.

The device uses two hot and one ground to supply the 240V output which is, in some business where there is three phase power this will be closer to 208V but should still work. Obviously you shouldn’t be doing this unless you know exactly how it works, and we applaud [Nick] for airing these hazards while at the same time supplying the knowledge behind the concerns.

Two inputs for the beefy converter are supplied from outlets not just on separate circuits, but on two circuits whose hot lines are 180 degrees out of phase. That means identifying where there are two plugs, not protected by GFCI outlets or breakers, which are on two separate hot lines of split phase power. To protect the user, [Nick] designed in a set of relays which kill the circuit when one of the two supplies is unplugged. A system that didn’t have these protections would have mains voltage on the prongs of the disconnected plug.

We’ve seen very few car charging hacks. If you know of one, or have been working on your own, let us know!

Mains Power Detector For A Thing For Internet

inductor The Internet of Things is fast approaching, and although no one can tell us what that actually is, we do know it has something to do with being able to control appliances and lights or something. Being able to control something is nice, but being able to tell if a mains-connected appliance is on or not is just as valuable. [Shane] has a really simple circuit he’s been working on to do just that: tell if something connected to mains is on or not, and relay that information over a wireless link.

There are two basic parts of [Shane]’s circuit – an RLC circuit that detects current flowing through a wire, This circuit is then fed into an instrumentation amplifier constructed from three op-amps. The output of this goes through a diode and straight to the ADC of a microcontroller, ready for transmission to whatever radio setup your local thingnet will have.

It’s an extremely simple circuit and something that could probably be made with less than a dollar’s worth of parts you could find in a component drawer. [Shane] has a great demo of this circuit connected to a microcontroller, you can check that out below.

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