by Erik T. Heidt | January 8, 2018 | Comments Off on IoT Lab | Maker Safety, Part 2
Part 1 focused on understanding that risk is different in the workplace than it is in the enthusiast or hobby world, and tried to explore resources for understanding and addressing that gap. Here in part 2, I want to identify some of the safety measures I use. Note, as with all Gartner blogs these are my personal options and not a substitute for professional advice.
My interest in electronics started early in my childhood, and I was fortunate to have parents that were supportive of experimental kids. I will refrain from recounting actual incidents or close calls, but keep in mind:
- Small electronics will burst and smoke when polarity, voltage or current mistakes are made. This is mostly a hazard to eyes as small plastic bits fly around.
- Any motor or actuator with non-trivial torque or force can be dangerous.
- Fumes from solder or other materials are not good for you.
Some videos of failures include: EEVblog #1035 – Flaming DIY Power Supply! and Switch & Lever Catastrophic Failure. If you prototype is powered via USB or small batteries, generally these issues don’t come up. When projects start using mains AC or more powerful DC sources increased caution is warranted.
Electrical Safety: Mains AC Power
Any project that involves mains power deserves respect. The scope of AC power safety is too large to be handled here, and I recommend getting direct guidance from someone with experience in mains power management in electrical and electronics (they are different) safety. Keep in mind that electrical wiring safety is only part of the story. Mains powered equipment can contain filtering capacitors and other components that can store substantial amounts of energy long after the device is powered off.
First, if all your project needs is simple mains power switching considering using a device such as the IoT Power Relay. This device takes as its input a low voltage, nominal current input and allows the switching of up to 12 amps of 120V AC mains. The entire unit is sealed, well-constructed and contains an internal fuse. Paul DeBeasi leverages one of these units in the POC build documented in “A Guidance Framework for Developing an IoT Proof of Concept”.
If including the entire IoT Power Relay isn’t practical for your prototype (due to cost or size), I recommend examining the “SparkFun Beefcake Relay Control Kit”. The kit itself is well engineered and the full schematic, Eagle files, etc. are available enabling you to design this into your own projects (check the license info).
My favorite piece of mains power safety equipment is this foot switch:
While often called a dead man’s switch, I think of this as a life protecting device!
This is a 15 amp rated momentary switch that is controlled by foot. I use this anytime I want to be able to fail-safe to power off. If I am debugging a piece of equipment that is mains powered and must be energized to troubleshoot it – in additional to other precautions – I use this switch. It is also handy for controlling power tools and as a teaching aid. It is an excellent tool for being able to cut power hands free while teaching someone else how to use a power tool.
What about Ground Fault Circuit Interrupter (GFCI aka CFI)? These devices are designed to switch off power when electricity is finding an alternative pathway back to source (aka ground). If all the power to your project is returning on the neutral the GFCI won’t see a problem with it. That same power could be melting something due to a connection or part placement problem but as long as the electrons are moving from the hot to the neutral the GFCI won’t trip. The GFCI is helpful when an unplanned return circuit is occurring, such as from project to you to metal desk and so on. GFCIs save lives every day, and a GFCI in your working area is a great idea – but don’t count on them to solve problems that aren’t related to ground faults.
Electrical Safety: DC Power
Here are four power supplies from my office. Each is source of DC power, let take a quick look at them.
Bottom Right: 9V Transistor battery adaptor. These 9V sources are very low current, and are used as the power source in many an electronics kit. They avoid a range of problems and often the currents are low enough that if I am bending a circuit into being – trying different options on a breadboard – that a misconfiguration doesn’t usually destroy a component. As long as there is enough current, extra current capacity often just results in prototyping frustration. (One of the nice things about breadboard prototyping is the ease of changing components and reconfiguring the circuit to try alternatives approaches – this is the fail “fast agile approach”, it does require some care to keep from turning into the “destroy all ICs” approach.)
Bottom Left: This is a repurposed power supply. It originally powered an external hard disk that failed. Note the fact that it is labeled Arduino Safe. This involved measuring (not trusting the label) the volts out, confirming polarity and then plugging it into an test Arduino. This supply is ground isolated, meaning that all of the parts that plug into the wall are isolated from the outputs. Some DC power supplies have the negative rail connected to either the neutral or ground. When I repurpose supplies I generally discard the ones that are not ground isolated (more on that in a moment).
Top Left: This is a 32 Amp voltage adjustable supply I use to power radio equipment and various projects. This is a great supply for powering an on-going project, but has two issues that must be managed. The only current limits on this supply kick in at 32 amps. This is great for powering equipment, but as a bench supply this is a challenge because any short, polarity, or other connection mix up can result in dumping 32 amps into the wrong places. Great care needs to be taken when using high current supplies. An often overlooked second issue, is the fact that the mains ground is connected to the negative return on this DC supply (and this is not uncommon). If you project has multiple power sources or you connect equipment to your project that is also ground referenced high current events (i.e. sparks, pops, smoke) can result. (FYI, Dave Jones of touches on a related and similar problem to this in EEVblog #279 – How NOT To Blow Up Your Oscilloscope!.
Top Right: This is a “lab power supply”. I currently consider this to be one of my most important pieces of safety equipment. Beware that there are many articles about repurposing or building lab power suppliers. There is a difference between a “power supply used in the lab” and a proper “lab power supply”. That 9V transistor battery adaptor can be a “power supply used in the lab”.
Important characteristics for a lab power supply:
- Voltage Control
- Ability to set maximum current output
- Ability to operate either ground isolated or ground interconnected
- Tolerance to momentary abuse, such as shorting
The thing that everyone does when they first sit down with one of these supplies is play with the voltage control, and this is a tremendously useful function. That said I think the most important function is being able to set the maximum current at that voltage. If I am working on a project that has low current needs, I set the supply to 250ma and not worry about it. If I draw the full 250ma the current meter will let me know and I can creep it up (or figure out why more current than expected is being used). I find this allows me to have faster learning cycles with breadboard prototyping (fail fast but safe), without having to worry about a minor error causing a component failures (at least as often).
The fact that you can control the outputs relationship to ground is also valuable. Functionally, this allows you to be able to use the supply ground isolated and to team the supply with similar supplies to provide power when multiple voltages are required in a single project.
The rosin in solder generates most of the fumes from solder. Lead is still commonly used in solder and present in almost everything with solder made before 2006. A label of “lead free” should not be read as “harmless”. None of these fumes are good for you.
Recently I started use a fume extractor whenever I am soldering. If you are in a shared environment, be courteous to others and don’t share your solder (or other) fumes. It is a common practice to use a fan to blow away these fumes, but keep in mind they are going someplace and usually this means they are going to be coating everything else in the room (think about how cigarette smoke coats everything).
Here are the rules of thumb I would recommend: If you are soldering in a shared space where smoking is prohibited, use a fume extractor. If you are going to solder more than a 10 hours a year, use a fume extractor. If I was teaching a 90 min how to solder class would I require extractors? Probably not, I would just use good ventilation – but I would encourage everyone to add a fume extractor to their kit. (The budget for a solid soldering station and proper accessories without a fume extractor is going to be $120-$150, and a fume extractor can be added for $20.)
Fume extractors are loud enough to be annoying. Extractors have to be close to your work to operate properly, and this usually means they will be pretty close to you – so the noise from the fan is right in front of you not far from your fingertips. I use a momentary foot switch (like the one featured above) with mine to turn it on when I have the iron in my hand and then to be able to turn it off while preparing for the next soldering task.
One last thought, I selected this particular unit because it can be configured in both of these positions (the 2nd provides a downdraft), so far it has worked out pretty well. That said, it was more expensive. The unit is basically a fan with an active carbon filter attached. Do consider the less expensive units. There are units available for $19.95 shipped. If you have a muffin fan, used plastic container and a piece of active carbon filter foam you can Maker-up and make one yourself!
Question: Should there be a part 3?
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