Wall warts - Embedded.com

Wall warts

Click here for reader response to this article

A couple of weeks ago I was pounding away at the computer when a shrill whistle sounded for a few seconds. The frequency rapidly shifted higher till it was inaudible. Seeing no obvious causes I resumed work. Then that ineffable electrical smell, signaling something seriously smoked, wafted through the room.

Alarmed, I searched for causes but found none in the jumble of computers and test equipment. The odor increased and for a moment I deliberated: act like a hero and try to isolate the problem, or call the fire department? Discretion beats seeing my place turn to toast, so 911 quickly dispatched a truck.

The firemen were courteous but admonished me about the proliferation of outlet strips along the floor. We don't pull many amps here, and the number of plugs isn't all that huge. But those annoying wall wart power supplies cover more outlets than they use. Outlet strips plug into outlet strips ad infinitum.

Afterwards I bought a bunch of APC power strips optimized for wall transformers. But those are Band-Aids. Wall warts are simply a stupid idea.

Sure, making the power supply an external brick shrinks electronics; a 2 pound Vaio notebook doesn't have much room for a transformer. And yes, putting a hot component out in the air does make a certain amount of sense.

But most are dumb linear supplies that waste enormous amounts of energy. Even when the equipment isn't in use these vampires secretly suck power, creating plenty of useless waste heat. The air conditioning works harder, burning even more power. I googled somewhat unsuccessfully for estimates of the amount of power wasted by these supplies; there's little data but some sites suggested they uselessly suck 5% of the nation's grid. Expensive–and ever scarcer–energy shouldn't be dissipated so uselessly.

So here's my idea: design a wall wart that fits entirely within the expanded grip on a power cord that goes into the wall–that molded bit of plastic from which the metal prongs sprout. Use switching technology to get 90% efficiency or more. Since most electronic devices take only an amp or two (sometimes far less) at 5 volts or so, practically no heat will be generated.

In Britain the plugs are larger than here in the USA so there's more room for parts or for a higher capacity power supply.

The unit will be an off-line switcher, one that connects directly to the AC mains, since the small form factor won't allow a transformer. I have no idea what the regulatory requirements are for such a device, but off-line switchers are hardly rare. Somehow people manage to get approval for the technology today.

Check out the datasheets at Supertex for one possible design. Power Integrations at www.powerint.com

In volume the costs would be low and customer satisfaction high. Everyone hates wall warts. Sell a product with the supply integrated invisibly into the cord and you'll have a competitive advantage over the dinosaurs pushing antique power bricks. And the reduced energy waste is a huge benefit in itself.

The firemen never did find the cause of the problem, as the smell stopped as soon as the engine arrived. But seeing the expression on my wife's face as she drove up to find the fire truck parked outside and three burly firefighters through the window was priceless.

What do you think? Why do we persist in building annoying and wasteful wall warts?

Jack G. Ganssle is a lecturer and consultant on embedded development issues. He conducts seminars on embedded systems and helps companies with their embedded challenges. Contact him at . His website is .

Reader Response

Jack, You have made an interesting suggestion regarding the integration of switch mode power supplies into line cords. However the problem I have with this suggestion comes from my exprience as an amateur radio operator. I was recently horrified to find that most switch mode power supplies used with under counter low-voltage Halogen light fixtures generate massive RFI across the HF spectrum. Just ask my neighbours who's lights I voluntarily replaced!

You admit that you have no idea of the regulatory requirements for these devices. I suspect that you are not alone. If dirt cheap switching supplies are to be unleashed upon the world, let's start by putting in place suitable emission standards and enforcement procedures. I understand that the EU standards ensure that this is not a problem in Europe. Why can't we treat the spectral environment on this side of the pond with similar respect?

– Dave – VE3TLY

I just read your article on wall warts and I think that you are a little late. My understanding is that California has required that all new wall warts be switchers, for exactly the reason that you mentioned, three wasted Watts times twenty million wall warts is a LOT of power. I noticed as I started collecting equipment for an upcoming tour of Scotland where I'm taking a lot of electronics that all of my equipment using wall warts is rated for 100 to 240 Volts, and they are a fraction of the size and weight of the older transformers. Progress is a wonderful thing.

I teach electronics and the smell that you described is what we refer to as the “smell of money” for a technician.

– Hugh Tomlinson

As usual, I found your column interesting and thought provoking.

I identify with the problem, even though I have never heard them called”wall warts” before. As I write this, I can see the mess of wires andmulti-ways that get power around this small room and, even with ourbigger plugs, sockets don't get used efficiently.

I recall reading that house fires resulting from electrical faults aremuch more common in the US than here. The lower voltage, althoughsuperficially safer, results in a higher current and, thus, more heat.Does that make sense?

It seems to me that a solution would be a low voltage power distributionsystem. I could imagine a good size 12v [say] transformer providing thepower for a whole load of devices. Why can't we do this? I guess theanswer is every device has slightly different needs – connector,polarity, voltage. But does it need to be this way?

I like devices that use a USB socket to get power. Sometimes this is aside-effect of their connectivity; in others, the only thing they getfrom the USB port is power. I have an adaptor that plugs into the poweroutlets [they used to be cigarette lighters in the old days] in my carand presents a USB socket that can [obviously] deliver nothing exceptpower.

So, what's needed is a box, which plugs into the mains and provides acouple of dozen “power only” USB sockets. Some devices could use thisimmediately [my cellphone, Palm, Blackberry …]. Others would need asimple [I guess] conversion cable – USB one end and specific power-inconnector on the other.

Would this work? Shall I start a company to make these things rightaway? I could use the odd $1m …

– Colin Walls

Along with wall warts sucking vast amounts of energy even when turned”off”. don't forget anything with a remote control: stereos, cable boxes,VCRs, CD/DVD players, and maybe the worst offender, the “instant on” TV.Some of these TVs go 10-15 watts when “off” and if you truly turn them offwith an external switch (or pull the plug) they require as much as 30minutes to reacquire their channel programming. Not real convenient.

– Dennis L. Darling

I remember the time when the industry found a way to dump the power transformer in AC/DC type radios. They even began before that with so called line cord resistors, the object being to eliminate the iron and weight of the transformer and replace it with a device that dropped the line voltage to power the filaments in the vacumm tubes. They reduced cost and quality but sold millions. It was a step in the right direction toward conservation. There were quite a few substandard line cord resistors and subsequent fires so they were banned.

Now have you not been exposed to the power saving light bulbs, including halogen and Xenon( for those that can't take the UV from Halogen)? These low voltage lamps and many many cousins found in Walmart and Home Depot generate all sorts of radio noise because they are made for a price but don't consider the radio and TV listeners locally. I'm a short wave listener and all the spectral effluent I hear generated by switch mode power supplies ought to be banned for its degrading use of valuable frequency spectrum.

All manufacturers know how to make these devices to be quiet becuase they are mandated to do so if they want to export to Europe. Some treadmillsdo the same thing and you can hear them a block away.

You may have a valid point with a smps built into the power line but please get the noise limits specified to something reasonable so it doesn't constitute a nuisance.

Why not a central power source that you could plug your device to and derive power from it. Eliminate the wall wart.

– Ralph Cameron

A couple of notes about wall warts…

The transformer in most of these provides some isolation albeit at the expense of efficiency. And it helps to prevent some fatal shock hazards that can arise from not being correctly polarized with respect to the line. Ironically the polarization problem is primarily due to the bulkiness of the wall wart itself, which causes the user to thwart the electrical code in drastic attempts to 'plug it in'.

Also, the use of a wall wart (or any external supply)can greatly facilitate the ability of a product to pass UL / CSA requirements. No small expense when developinga product.

– Steven J. Ackerman

My Sony Ericson P900 phone has just such a supply. It fits in a standard size (UK) plug, doesn't warm up when not charging the phone, and does a good job of charging it quickly. Someone beat you to it!

– Paul Tiplady
Principal Software Engineer
TRW Automotive

“What do you think? Why do we persist in building annoying and wasteful wall warts?”

Let me answer you with a question: Why do we buy cheap T-shirts from China, Korea, etc for a 5 Euros instead investing 30 Euros for a quality one that will outlast the previous by tenfold?

Can you see, as I sometimes don't . . .

– jussi vnsk

You only tangentially touched on what I believe is the principal factor in the wall-wart's longevity: the transformer serves a major safety function by providing galvanic isolation between the power mains and the powered device.

Those of us who first learned electronics on radios that glowed in the dark have to admit that the 5-tube AC/DC broadcast receivers that littered the inhabited world were accidents waiting to happen, due to their use of what is essentially an “off-line power supply.” The wall-wart confines the potentially lethal zone to within a couple inches of the actual mains outlet. While I don't recall the specifics of safety requirements for off-line supplies, I suspect requirements for conductor separation, reinforced insulation, user inaccessibility, etc., amount to a greater economic burden than the cost of energy wasted in the wall-wart, even over a 5 or 10 year product lifetime.

The electronic wall-wart you propose (can I coin the term “Smart-Wart”? “Cool-Wart”? “Joule-Saver”?) will probably have to provide that safety isolation to be widely accepted. Yes, it can be done with switching supplies, though not as cheaply as the off-line topologies. And providing transformer isolation certainly increases the bulk. But using the bulky, hospital-grade line plug as a guideline for an acceptable package size I suspect that 10 watts should be obtainable with off-the-shelf components. That's enough to drive a lot of answering machines, GameBoys, Walkmans, etc.

Now if I could only find somebody to fund the agency approval process, assemble a design team, and include a gray-haired analog artisan on it . . .

– Dale Chisholm

I seem to recall AT&T producing an 8-pin DIP that could supply 5V @ 100mA directly from the mains.

Another idea would be to include an enable/disable signal to the brick. It would only be enabled when power is needed.

Last, but certainly not least, build one with multiple, interchangeable connectors so it can supply more than one device! That should lance more than a few warts.

– BBtheEE

The answer is quite simple.

Those ugly wall warts are not part of the core competency of the mfr. that makes the XYZ laptop and such. It is for this reason why the mfrs typically go with the ugliest, cheapest ,and least-engineered wart-looking thing possible.

Plus, I very much doubt one can create a wall-wart type product in general. From what I can see, wall warts by themselves do not sell very well, nor do they command very reasonable, (to the vendor), prices.

– Ken Wada
Sr Embedded Systems Consultant
Aurium Technologies Inc

Wall warts use proven designs (no NRE needed), are available through a variety of Asian manufacturers cheaply, and are a known electrical interface by the majority of the populace.

Easy, cheap, and accepted. That's the marketing mantra.

OK, let's look at the technical aspects. If you're talking about replacing the wall wart with a “wartless cord” I assume you still want the barrel or ring/tip DC interface. I believe that UL (and other safety people) state that you can not have access to any electrical connection that is NOT isolated from the AC mains. Case in point: those ubiquitous IEC cords that plug into your monitor and PC (among others) that use a shielded connection to prevent you from contacting line voltage.

So now you're either putting in isolation, making a non-detachable cord, or creating a new DC interface. The latter two solutions cost excess money and will probably cause the customer hassles.

The former solution is a wall wart.

– John Patrick
Sr. Electrical Engineer
L-3 Communications/Electrodynamics, Inc.

I think wall warts are here to stay. Here's why: Off the shelf wall warts already have all agency approvals for safety and radiated emissions, and have a proven track record of reliability. Since they're being mass-produced, they're already cheap. That allows me to spend my time designing my product and not designing some mundane supply with many risks.

As for Linear vs. switchers: Due to FCC regulations, all off-line switching power supplies need some moderately expensive input conditioning circuitry to prevent high-frequency pulse from travelling back into the power line. That makes them more expensive to produce in output sizes less than 10W. Additionally, switchers' two main advantages (efficiency and weight) are less noticeable or unimportant at lower output levels. Finally, linear supplies tend to be more reliable due to both the lower parts count and the lesser stress levels on those parts.

– Bryan Mills
Senior Design Engineer
Fireye, Inc.


I have to admit I'm impressed with the inline power supplies (“floor warts”?) that come with most laptop computers these days. I had one from an old '386 lapwarmer that would provide a quiet 13.8 volts at 2+ amps. It was half the size of a thick paperback book. I ended up using it to power a portable ham radio transceiver long after the computer died. If they can fit that many watts into a case that size, they can surely fit an isolated 2 watts into a candy bar.

I wonder, though, if wall-wart manufacturers are sticking to analog circuits to dodge FCC RF emission limits? Meeting Class B limits for unintentional radiators is getting to be more and more of a chore.

– Dave Hinerman

A number of years ago the Zip 100 drives came with a switcher wall wart that is only a little larger that a regular plug. It's about 2 inches wide, just over an inch thick, and 2.25 inches long, not including the prongs. We found similar units in the DigiKey catalog, at about US$10 for a 5v, 1a unit (in small quantities). They do exist, they just cost a little more. Oh, by the way, the units we used are nicely regulated, unlike most linear wall warts. Like you, we hate regular wall warts, and refused to inflict them on our customers.

– Phil Olynyk

It's certainly possible to create the type of wall wart you wrote about. Here's one from Radio Shack (www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=273-1760) that's pretty cheap. Unfortunately, not cheap enough. Nearly everyone outsources these adapters to China, which knows that the cheapest bidder wins. And why not, who besides a few concerned granola engineers care about the power efficiency of their wall warts? You can now get transformer-based warts that fit standard sockets as well, for a few cents more than the normally sized one. They don't sell either. They're a few cents more.

The hams who have already written also have a valid point about switchers pouring out the EMI. And offline switchers are death in waiting. The personal injury lawyers would just love for a few of those to get on the market. The reason the tube radios of the 1950s could run off line is because they were carefully designed to deny the user any chance to touch metal (plastic knobs, recessed screws, etc.) Otherwise, a few listeners would have surely been fried while tuning in “The Lone Ranger” or “The Shadow.”

-Steven Leibson
Technology Evangelist

I believe Leviton makes a multi-connection power block for their structured wiring panels. When you add a new device to the panel you just connect the DC power cable to the block. Sure, its for products they sell, but the concept is good. From my experience, it seems that most 5 VDC devices will accept anything between 7 and 15 volts from their wall-warts. It should be easy to standardize around 9 to 12 volts and a common connector.


Wall warts are cheap and avoid issues of power-supply design and agency approvals. The one problem with them is that they are at the wrong end of the power cord! Overcrowding the power bars and staying on when the device is off are not the only problems. The wall wart tends to get lost when the device is moved.

Why can't I get a similarly-priced small power supply that fits inside the case of the device, with an IEC power jack (to use standard power cords) and switch protruding through a hole in the case? I build test fixtures, mostly one-of-a-kind units that are often moved around. To put the power supply inside the case, typically I pay more for an open-frame supply with solder terminals than a wall-wart would cost, plus the power cord, jack, fuseholder, switch, and assembly labor.

– markm

I too dislike wallwarts. They tend to take a lot of space, are very inefficient. It is, of course, very easy for a manufacturer to use them–just accept 9V DC supply and regulate it down to the required voltages. I'm just looking at some PC- based supplies (takes a standard [hard disc] power connector to a board and then supplies wallwart type connections–they seem to be described as “PC Power Supply Cards.” I wish people would spend more time on power supplies–in my experience they are always the thing that goes wrong.

– A L Jackson

1 thought on “Wall warts

  1. “Wall warts scare me more than obvious external fires that can be easily extinguished. It does not matter even if you are well-prepared or even trained to put out fires, but if you cannot find where the source of that burnt smell is, then you are no good.

    Log in to Reply

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.