A reader weighs in on the technical issues one may want to consider first
Regular readers may recall that about a month ago I posted an item about a piece-of-crap reading lamp that was only interesting because it draws power not from an electrical socket but rather an unused telephone jack.
Questions were raised about both the wisdom and legality of such a device. Public relations professionals from Verizon and the FCC told me they would get a reaction/explanation from their organizations, but neither came through with anything meaningful.
(2010’s 25 Geekiest 25th Anniversaries)
So, up steps reader John Crout with insight about the technical issues involved that he’s agreed to share here:
Any power engineer can tell you that power quality is a critical concern … and should be to anybody considering one of these lights. Even power meters don’t work right when the power quality is sub-standard — they measure less than they should.
From college physics, P = V x I. Learn a little more info and it looks like this: P = V x I x PF, where P is power, V is voltage, I is current and PF is the Power Factor. PF is always less than 1 and greater than 0. Without thinking about PF, all we need to think about is power — which may be why someone thought about running a light from phone power. When the PF isn’t perfect, you are likely going to find noise on the power line, too.
FCC interest addresses the size of the load, not power quality. Causes for low quality can cause early, permanent failure of devices exposed to them. Too big of a load and nothing on the line will work.
You are better off with a reading light that runs on batteries, or that plugs into an electrical socket. Rechargeable batteries can save money and avoid adding nasty stuff to a land-fill. A USB-powered light might also be an option if there is a USB bus nearby (say, from a laptop). Be warned though, a less expensive add-on hub can cause power quality problems on the USB bus just as these phone-jack lights can cause problems with phone equipment.
Let’s take a deeper dive into these details and find out why.
Noise and Power:
1) V x I = Volt-Amps; it doesn’t really equal power except when you are learning about DC theory.
2) Volt-Amps x PF = Power (Watts).
3) In DC circuits, current maximizes with voltage.
4) Phone lines are AC, so it is important to analyze what happens instantaneously. A capacitor looks like a short circuit at the very moment power is applied. As a result, current is at its maximum at power-on, and voltage is 0 — at its minimum. Current is said to “lead” voltage.
5) When one leads or lags (as inductor I lags V) they are out of phase.
6) An out-of-phase signal usually exhibits other anomalies — e.g. spikes and dropouts. This is noise. Notice that a lamp dims when a vacuum cleaner plugged in close to it, is turned on. This occurs when noise is introduced onto the power line by one device and affects another.
Noise can also make its way through a power supply, to become part of the voltage that runs the CPU.
Wall warts, LED and electroluminescent lights use switching power supplies and these types of devices can mess with power quality.
Related to this are the electric fields emitted from PCs, CRTs and wall warts. These fields are the reason the FCC regulates them. The same sources of radiated interference can cause a device to conduct noise out a power line just as it can cause the device to radiate noise as EMI (electromagnetic interference). Another device acting like an antenna can pick it up and radiate it as conducted noise. Same goes for a wire (or a pc board trace) inside the device that radiates the noise.
AC lines are driven by a relatively huge supply of current. Not so, for telephone lines. Short telephone lines together and nothing connected to it works correctly but nothing smokes, either. Not so with AC (fuses blow or fires start).
Why aren’t telephone lines fused? The maximum power it can deliver is not likely to cause death or to start a fire because it is limited by the phone company. This means that once you’ve connected the maximum load, adding more can cause everything to fail. The FCC came up with the Ringer Equivalency Number (REN) for use as a device rating. Phone companies set a maximum load (measured in REN) that can be connected to your phone line if they are expected to guarantee service. I do recall addressing excessive loads with some of the earlier electronic phones. Devices from this century draw noticeably less than those from earlier periods, so exceeding the REN isn’t as likely.
(By the way, I found a more “practical” use for this information as a high school student. To keep the school from calling my home if I skipped the day — just once, I swear! I figured out that the right load would make the line seem “busy,” rather than appearing to the phone company as a trouble spot. I wasn’t very proficient at planning back then, so neglected to consider the ramifications of my handy work. I came home later and found out my mom had used a neighbor’s phone to report a line problem because she couldn’t dial out. The phone company never found my work and I was just “so smart” for being able to fix what the company couldn’t.)
Historically, power issues (load issues) have justified FCC involvement. We should be so lucky that concerns about this idea are still as innocuous as this. The greater concern is preventing undesirable behavior. To avoid the possibility of your phone wiring working as a transmitter of interference with your Wi-Fi (or your neighbor’s Wi-Fi) you might want to consider an alternate light source.
Also, this might be a good time to realize the phone lines of the 1970s have been replaced by a home network. When you connect something you didn’t build to a network, you better trust it.
Thanks, John.
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