With all the recent mania on capacitor limited power adapters at electroschematics.com, the Wall Wart seems to be the neglected power source. Wall warts are small, convenient and have transformer isolation. With line isolation they are free from ground fault shock hazard. The power level and output voltage is ideal for most experimentation, and difficult to specify or obtain “X” capacitors are not required. It has been suggested that wall transformers were coined “wall warts” because they often take up 2 or 3 positions in a power outlet strip, but I say it is simply because they protrude from the wall like a “wart.” The name is simply a humorous alliteration.
What is a wall wart?
It is an electrical outlet mounted transformer-isolated low voltage power supply adapter with power cord and barrel connector (now also USB connector). Virtually all types low power AC operated electronic equipment use them in lieu of batteries. They have no power switch and remain live even when the electronic equipment is turned off. In spite of this, power consumption is low. One with a quiescent power dissipation of 1W consumes 9KWH per year or approximately $1.00 per year, depending upon cost of power. While there is a cost of use, it is for a very important, necessary function-equipment safety!
Wall wart Specifications
Wall wart adapters come in various voltage and current ratings (typically 6, 9, 12 or 15V). Some have AC output, while most are DC. A few even have regulated output voltage. Usually, the specifications are molded into the plastic case. The power rating runs from about 4 to 25W. Most contain 50/60hZ transformers—these are relatively heavy. Others are small, lightweight off-line switchers with tiny 100kHZ+ transformers. Since low frequency transformers are relatively expensive due to both material and labor, the off-line switcher versions are gradually taking over. The switcher also lends itself well to the now popular USB power supply standard that requires a regulated 5V.
In order to get higher power for printers and computers, in-line power packs are often used. Anyone who owns an HP printer knows what I am referring to. These also may be used by experimenters.
There is no standard barrel connector—there are perhaps 10 different versions that look similar, but have various barrel diameters and internal pin dimensions. Also, there is no standard polarity—the center pin may be either positive or negative. To accommodate either polarity, it is not uncommon for DC equipment to have a 2nd bridge rectifier inside the unit. The next time you go to your electronics parts store, stock up on a few garden variety barrel connectors and receptacles—they are inexpensive and will make life easier in the future.
Never discard a wall wart
When discarding electronic equipment, save the wall-wart—it may be just what you need to power your next project. Should it be defective, save the cord and connector. Take the advice of a master pack-rat!
The North American Power Plug
While relatively crude by today’s international standards, the North American power plug has an interesting 100year history. Before its invention by Hubbell (about 1910), all lights, appliances and equipment were hard-wired to the electrical mains—rather unthinkable today.
Check out this interesting article—part 3 of a 4 part series:
Hubbell Electric continues to this day as a major electrical connector manufacturer.
While a brilliant inventor, I am sure that Harvey Hubbell never imagined the wall wart. In his era, there was no need for such a device as just about all equipment was big and klunky, and required lots of power.
Repairing the wall wart
Probably only the DC units are repairable. Do not be afraid to look inside—open the case with a hack saw—if you can repair it, glue the case back together with RTV silicon rubber compound. Some actually have cases that are held together with self-tapping screws.
The earliest wall wart
This bell transformer was (I believe) available in the 1960’s and was probably the first of the wall warts.
Out of my junk box
I found ten different wall warts—AC & DC and one regulated version. In the process of checking them out, I logged the specifications and did a simple voltage regulation test on each.
Specifications and regulation data
Voltage regulation (no load to full load) for AC adapters ranges from 12 to 34%. For DC adapters, it is roughly double that or 32 to 80%. This may be seen in the regulation data. The DC adapters have inferior regulation due to the peak detection effect of the input filter capacitor—this is not a serious issue because nothing really operates at no-load, not does anything actually work on AC without rectifiers. For frame of reference (because % regulation tends to be confusing), 0% is perfect and 100% indicates that the voltage drops to half when loaded. Just keep in mind that wall warts tend to be sloppy and the equipment to which it is connected is designed to accept double the specified input voltage safely.
For experimentation, I like the AC versions because I can connect a voltage doubler rectifier to get much higher voltages. Also, if connected to half-wave rectifiers, I can easily obtain ±voltages.
A look inside #9
This nice unit was held together with self-tapping screws—just inviting me to take a peek inside. Visible are the transformer, rectifiers and filter capacitor. Absent is a safety bleeder—after the unit is turned off the large capacitor stores a substantial charge for many minutes. An arc occurs if the barrel connector happens to short against conductive material.
Schematic for #9 wall wart
A look inside #8
Visible on the top: X capacitor (right), input rectifiers (2 of the 4 are visible), split bus capacitors, noise filter inductor, switching transistor (Q1), 145°C end of life thermal fuse behind (Q1), ferrite E-core transformer, opto-coupler for feedback (left foreground), schottky rectifier (left), output capacitor (left), and L-C filter (left).
Visible on the bottom: SOT23-5 op amp (right central) and many size 0603 SMD components. No schematic on this one—too complex to trace out for this exercise.
Bench test set-up
Folding the proper stranded wire size back and inserting it into the barrel connector hole is a simple means of making electrical connection for the purpose of evaluation or experimentation.
The regulation was worse than I expected especially the DC versions. But the real surprise came from my setup. Did you see the huge 225W Ohmite potentiometer I used for an adjustable load? Well, it took me the longest time to figure out why the pot settings were so different between the AC and DC tests. It turned out to be the reactance of the potentiometer when used as a low-voltage AC load. In fact, the inductive reactance was substantially higher than the resistance and had a profound effect upon the load. It is in effect a low Q air core toroid. I may have studied well and had tons of experience, but do I know it all? no way! Still learning…
ONE GLARING OVERSIGHT!
I hope that some influential person in the industry notices this and actually does something about it-while all wall warts have their specifications clearly indicated, no equipment anywhere (NONE-ZILCH) ever seems to indicate the specifications of the compatible wall wart. How must stuff we are unnecessarily adding to the waste stream each year due to this stupid oversight?
For the future
Off-line switcher schematic
Voltage doubler schematics