How Power Over Ethernet (PoE) Works

October 24, 2019 by Sharon Lin 24 Comments

A remote Ethernet device needs two things: power and Ethernet. You might think that this also means two cables, a beefy one to carry the current needed to run the thing, and thin little twisted pairs for the data. But no!

Power over Ethernet (PoE) allows you to transmit power and data over to network devices. It does this through a twisted pair Ethernet cabling, which allows a single cable to drive the two connections. The main advantage of using PoE as opposed to having separate lines for power and data is to simplify the process of installation – there’s fewer cables to keep track of and purchase. For smaller offices, the hassle of having to wire new circuits or a transformer for converted AC to DC can be annoying.

PoE can also be an advantage in cases where power is not easily accessible or where additional wiring simply is not an option. Ethernet cables are often run in the ceiling, while power runs near the floor. Furthermore, PoE is protected from overload, short circuiting, and delivers power safely. No additional power supplies are necessary since the power is supplied centrally, and scaling the power delivery becomes a lot easier.

Devices Using PoE

VoIP phones are becoming increasingly prevalent as offices are opting to provide power for phones from a central supply rather than hosting smaller power supplies to supply separate phones. Smart cameras – or IP cameras – already use Ethernet to deliver video data, so using PoE simplifies the installation process. Wireless access points can be easily connected to Ethernet through a main router, which is more convenient than seeking out separate power supplies.

Other devices that use PoE include RFID readers, IPTV decoders, access control systems, and occasionally even wall clocks. If it already uses Ethernet, and it doesn’t draw too much power, it’s a good candidate for PoE.

On the supply side, given that the majority of devices that use PoE are in some form networking devices, it makes sense that the main device to provide power to a PoE system would be the Ethernet switch. Another option is to use a PoE injector, which works with non-PoE switches to ensure that the device is able to receive power from another source than the switch.

How it Works

Historically, PoE was implemented by simply hooking extra lines up to a DC power supply. Early power injectors did not provide any intelligent protocol, simply injecting power into a system. The most common method was to power a pair of wires not utilized by 100Base-TX Ethernet. This could easily destroy devices not designed to accept power, however. The IEEE 802.3 working group started their first official PoE project in 1999, titled the IEE 802.3af.

This standard delivered up to 13 W to a powered device, utilizing two of the four twisted pairs in Ethernet cabling. This was adequate power for VoIP phones, IP cameras, door access control units, and other devices. In 2009, the IEEE 802.3 working group released the second PoE standard, IEEE 802.3at. This added a power class that could deliver up to 25.5 W, allowing for pan and tilt cameras to use the technology.

While further standards haven’t been released, proprietary technologies have used the PoE term to describe their methods of power delivery. A new project from the IEEE 802.3 working group was the 2018 released IEEE 802.3bt standard that utilizes all four twisted pairs to deliver up to 71 W to a powered device.

But this power comes at a cost: Ethernet cables simply don’t have the conductive cross-section that power cables do, and resistive losses are higher. Because power loss in a cable is proportional to the squared current, PoE systems minimize the current by using higher voltages, from 40 V to 60 V, which is then converted down in the receiving device. Even so, PoE specs allow for 15% power loss in the cable itself. For instance, your 12 W remote device might draw 14 W at the wall, with the remaining 2 W heating up your crawlspace. The proposed 70 W IEEE 802.3bt standard can put as much as 30 W of heat into the wires.

The bigger problem is typically insufficient power. The 802.4af PoE standard maximum power output is below 15.4 W (13 W delivered), which is enough to provide power for most networking devices. For higher power consumption devices, such as network PTZ cameras, this isn’t the case.

Although maximum power supply is specified in the standards, having a supply that supplied more power is necessary will not affect the performance of the device. The device will draw as much current as necessary to operate, so there is no risk of overload, just hot wires.

So PoE isn’t without its tradeoffs. Nevertheless, there’s certainly a lot of advantages to accepting PoE for devices, and of course we welcome a world with fewer wires. It’s fantastic for routers, phones, and their friends. But when your power-hungry devices are keeping you warm at night, it’s probably time to plug them into the wall.

Anatomy Of A Power Outage: Explaining The August Outage Affecting 5% Of Britain

September 7, 2019 by Jenny List 18 Comments

Without warning on an early August evening a significant proportion of the electricity grid in the UK went dark. It was still daylight so the disruption caused was not as large as it might have been, but it does highlight how we take a stable power grid for granted.

The story is a fascinating one of a 76-second chain of unexpected shutdown events in which individual systems reacted according to their programming, resulted in a partial grid load shedding — what we might refer to as a shutdown. [Mitch O’Neill] has provided an analysis of the official report which translates the timeline into easily accessible text.

It started with a lightning strike on a segment of the high-voltage National Grid, which triggered a transient surge and a consequent disconnect of about 500MW of small-scale generation such as solar farms. This in turn led to a large offshore wind farm deloading itself, and then a steam turbine at Little Barford power station. The grid responded by bringing emergency capacity online, presumably including the Dinorwig pumped-storage plant we visited back in 2017.

Perhaps the most interesting part followed is that the steam turbine was part of a combined cycle plant, processing the heat from a pair of gas turbine generators. As it came offline it caused the two gas turbines feeding it to experience high steam pressure, meaning that they too had to come offline. The grid had no further spare capacity at this point, and as its frequency dropped below a trigger point of 48.8 Hz an automatic deloading began, in effect a controlled shutdown of part of the grid to reduce load.

This is a hidden world that few outside the high-power generation and transmission industries will ever see, so it’s very much worth a read. It’s something we’ve touched on before with the South American grid shutdown back in June, and for entirely different reasons in 2018 when an international disagreement caused the entire European grid to slow down.

Header image: Little Barford combined-cycle power station against the sunset. Tony Foster, (CC BY-SA 2.0).

Spin Me Right Round, Baby: Generator Building Experiments For Mere Mortals

August 13, 2019 by Brian McEvoy 38 Comments

How many of you plan to build a wind-powered generator in the next year? Okay, both of you can put your hands down. Even if you don’t want to wind your coils manually, learning about the principles in an electric generator might spark your interest. There is a lot of math to engineering a commercial model, but if we approach a simple version by looking at the components one at a time, it’s much easier to understand.

For this adventure, [K&J Magnetics] start by dissect a commercial generator. They picked a simple version that might serve a campsite well, so there is no transmission or blade angle apparatus to complicate things. It’s the parts you’d expect, a rotor and a stator, one with permanent magnets and the other with coils of wire.

The fun of this project is copying the components found in the commercial hardware and varying the windings and coil count to see how it affects performance. If you have ever wound magnet wire around a nail to make an electromagnet, you know it is tedious work so check out their 3D printed coil holder with an embedded magnet to trigger a winding count and a socket to fit on a sewing machine bobbin winder. If you are going to make a bunch of coils, this is going to save headaches and wrist tendons.

They use an iterative process to demonstrate the effect of multiple coils on a generator. The first test run uses just three coils but doesn’t generate much power at all, even when spun by an electric drill. Six windings do better, but a dozen finally does the trick, even when turning the generator by hand. We don’t know about their use of cheap silicone diodes though, that seems like unintentional hobbling, but we digress.

Making turbine blades doesn’t have to be a sore chore either, and PVC may be the ticket there, you may also consider the vertical axis wind turbine which is safer at patio level. Now, you folks building generators, remember to tip us off!

Continue reading “Spin Me Right Round, Baby: Generator Building Experiments For Mere Mortals” →

Pop-Up Outlet Helps Make The Most Of A Tiny Shop

August 9, 2019 by Dan Maloney 8 Comments

You’ve got to admire the steps some people take to squeeze a shop into a small space. Finding ways to pack in ever more tools and to work on bigger and bigger projects become ends to themselves for some, and the neat little tricks they find to do so can be really instructive.

Take this workbench pop-up outlet strip for example. The shop that [Woodshop Junkies] occupies appears to be a single-car garage, on the smallish size in the first place, that is almost entirely filled with a multipurpose workbench. It provides tons of storage underneath and a massive work surface on top, but working with small power tools means stretching extension cords across the already limited floor space and creating a tripping hazard. So he claimed a little space on the benchtop for a clever trap door concealing a small tray holding an outlet strip.

The tray rides on short drawer glides and, thanks to a small pneumatic spring, pops up when the door is unlatched. There was a little trouble with some slop in the glides causing the tray to jam, but that was taken care of with a simple roller bearing. The video below shows its construction and how it stays entirely out of the way until needed.

As cool as this build is, it’s just icing on the small shop cake when compared to the workbench. [Woodshop Junkies] has a complete playlist covering the build which is worth watching. And you might want to refer to our tiny shop roundup for more tips on getting a lot done in a little space.

Continue reading “Pop-Up Outlet Helps Make The Most Of A Tiny Shop” →

Wireless Charging Without So Many Chargers

January 15, 2019 by Brian McEvoy 11 Comments

[Nikola Tesla] believed he could wirelessly supply power to the world, but his calculations were off. We can, in fact, supply power wirelessly and we are getting better but far from the dreams of the historical inventor. The mainstream version is the Qi chargers which are what phones use to charge when you lay them on a base. Magnetic coupling is what allows the power to move through the air. The transmitter and receiver are two halves of an air-core transformer, so the distance between the coils exponentially reduces efficiency and don’t even think of putting two phones on a single base. Well, you could but it would not do any good. [Chris Mi] at San Diego State University is working with colleagues to introduce receivers which feature a pass-through architecture so a whole stack of devices can be powered from a single base.

Efficiency across ten loads is recorded at 83.9% which is phenomenal considering the distance between each load is 6 cm. Traditional air-gap transformers are not designed for 6 cm, much less 60 cm. The trick is to include another transmitter coil alongside the receiving coil. By doing this, the coils are never more than 6 cm apart, even when the farthest unit is a long ways from the first supply. Another advantage to this configuration is that tuned groups continue to work even when a load changes in the system. For this reason, putting ten chargeables on a single system is a big deal because they don’t need to be retuned when one finishes charging.

We would love to see more of this convenient charging and hope that it catches on.

Via IEEE Spectrum.

Old Wattmeter Uses Magnetics To Do The Math

December 18, 2018 by Dan Maloney 19 Comments

Measuring power transfer through a circuit seems a simple task. Measure the current and voltage, do a little math courtesy of [Joule] and [Ohm], and you’ve got your answer. But what if you want to design an instrument that does the math automatically? And what if you had to do this strictly electromechanically?

That’s the question [Shahriar] tackles in his teardown of an old lab-grade wattmeter. The video is somewhat of a departure for him, honestly; we’re used to seeing instruments come across his bench that would punch a seven-figure hole in one’s wallet if acquired new. These wattmeters are from Weston Instruments and are beautiful examples of sturdy, mid-century industrial design, and seem to have been in service until at least 2013. The heavy bakelite cases and sturdy binding posts for current and voltage inputs make it seem like the meters could laugh off a tumble to the floor.

But as [Shahriar] discovers upon teardown of a sacrificial meter, the electromechanical movement behind the instrument is quite delicate. The wattmeter uses a moving coil meter much like any other panel meter, but replaces the permanent magnet stator with a pair of coils. The voltage binding posts are connected to the fine wire of the moving coil through a series resistance, while the current is passed through the heavier windings of the stator coils. The two magnetic fields act together, multiplying the voltage by the current, and deflect a needle against a spring preload to indicate the power. It’s quite clever, and the inner workings are a joy to behold.

We just love looking inside old electronics, and moving coil meters especially. They’re great gadgets, and fun to repurpose, too.

Continue reading “Old Wattmeter Uses Magnetics To Do The Math” →

Scott Swaaley On High Voltage

December 14, 2018 by Jenny List 29 Comments

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.

Continue reading “Scott Swaaley On High Voltage” →