Wallwart
How to choose a wallwart for your small appliance.
What's a wallwart?
A wallwart is a small plug-in power supply commonly used to run small appliances at low voltage. The power supply and mains plug are one and the same item.
How to choose
When picking a wallwart for an appliance, 4 things matter:
- voltage
- AC, DC or rectified AC
- current
- regulated or unregulated
Voltage
Wart voltage needs to match appliance voltage. Sometimes it must be precise, sometimes small variation is ok.
- Lighting needs correct voltage
- Motors will run ok with a bit less, although a bit slower. They may overheat and fry with a bit higher voltage
- Computer speakers are fine with a bit less voltage, though max power output may be reduced a bit
The Voltage gotcha
There is one thing to beware of. Warts with switchable voltage will store power at the highest voltage setting in their reservoir capacitor, so if you switch a wart from say 12 V off load (which will be anything from 15 V - 19 V) to 5 V then connect up an appliance containing logic circuitry, your appliance will be hit with over triple rated voltage, and probably die instantly.
When switching voltage down on an adjustable wart, unplug it and short the output to discharge it before using it on a lower voltage appliance.
AC, DC or rectified AC
For an explanation of AC, DC & rectified AC, see here.
AC
Appliances requiring AC warts can be run happily from AC, DC or rectified AC. Neither AC warts nor appliances expecting AC warts are common, they are most often telecoms equipment. AC warts have few uses.
If an appliance expecting AC is powered by a DC wart,
- lightbulbs should use the same voltage DC as the expected AC voltage.
- electronic appliances should use a DC wart of voltage: AC wanted voltage x 1.4
AC symbol: ~
DC
DC warts will happily run all small appliances, regardless of whether the appliance expects AC, DC or rectified AC.
DC symbol: =
Rectified AC
Most small appliances will run happily on rectified AC, but not all. Most likely to object are audio equipment (severe hum), motors (death by overheating) and miniature equipment (no room for a reservoir capacitor, major malfunctions).
Rectified AC symbol: 
Current
Wart current rating must be the same as or more than the appliance current consumption.
There's just one situation in which higher current rating can cause a problem. If the wart current rating is a lot higher than the appliance (say 3x as high), and the wart is unregulated, it will put out above specified voltage, and some appliances can object to this. If you're using an unregulated wart of excessive current rating, best check the output voltage with a multimeter.
Regulated or unregulated
Regulated warts stay at the same voltage output under all conditions. Unregulated warts give the rated voltage under full load, but voltage will be quite a lot higher with light load or no load.
Also regulated warts give a smooth flat output free of ripple. (Ripple causes hum in some audio equipment.) Unregulated warts on the other hand usually produce a lot of ripple.
Some electronics must have a regulated supply, and some doesn't mind unregulated. Generally speaking,
- anything with logic ICs requires regulated, and is likely to let the magic smoke out if given an unregulated supply (or even 1v more than rated voltage)
- Motors and lamps are usually fine with unregulated, though there are occasional exceptions.
Regulated supplies will run anything, if their other specs are right for the job. The more common unregulated supplies will run a lot of things, but can destroy some more sensitive appliances.
Electromagnetic and electronic
There are 2 types of wallwart. Most use an iron core transformer, and these are several times the size of a standard plug.
Some are a similar size to an ordinary plug, and these are electronic warts. Electronic warts are small and light, and more or less always regulated. The downsides of electronic warts are lower reliability and higher price.
Multipurpose warts
Multipurpose warts are available to suit many appliances. These typically have:
- several selectable voltages, eg 3 V, 4.5 V, 6 V, 9 V, 12 V.
- switchable polarity
- an assortment of power connectors
Cheap unregulated 300mA warts of this type are common. The number of appliances they can run increases if the wart is also regulated. Regulated supplies do cost more.
1 A 12 V variable regulated supplies are used where its desired to maximise the variety of appliances that can be run.
Wart reduction
Sometimes its possible to run 2 or more appliances off one wart to reduce clutter. The wart will need to be the right voltage for both appliances, and its current rating must be at least the current requierments of both appliances added. Ways to connect 2 appliances include:
- make a plug-in adaptor from 3 connectors
- Join a 2nd DC lead to the first
Another approach to wart reduction is to use a power supply with multiple simultaneously usable output voltages. Then almost everything can be run off the one supply. Such supplies are too big to be built in wart format, and not often seen.
Its also possible to use installed low voltage wiring to distribute low voltage, removing the need for most warts.
Finally, some appliances can be run off a computer's built in power supply. These provide 12 V, 5 V and 3.3 V. In some cases one can obtain other voltages from them, eg 7 V, but 7 V comes with issues.
Extension leads
As well as the usual bulky mains extension leads, it's also possible to extend the low voltage side of a wart. The result is far less visually intrusive. Speaker wire is good to extend the output of almost all warts. Such leads can help run multiple appliances from one wart, as long as the necessary specs are met.
Ampacity
Most speaker wire is 1 A rated, which is more than enough for pretty much any wart.
Voltage drop
The voltage drop for 0.5 mm wire is 90 mV per metre per amp. Thus a 300 mA supply would drop 27 mV or 0.027 V per metre of flex. If a drop of 0.3 V is acceptable, an 11 m extension is fine.
Invisible wire
When low voltage extension leads are wanted to be invisible, enamelled copper wire may be used. This is great for tucking into corners and cracks. Its available in reels from any electronic component supplier. 0.3 mm is good for most wallwart extensions, and is as good as invisible to people standing up if its at floor level.
Enamelled copper wire has very thin varnish for insulation, and the insulation deteriorates if moved. Thus the wire needs to be either fixed completely immobile, or else treated as if uninsulated, and the 2 wires kept apart all the way.
Uninsulated wire or wire with fragile insulation should be protected from pets if over 6 V is used. Many animals are much more prone to shock than humans.
Changing the voltage
The output voltage of a wart can be reduced either with dropper diodes or a regulator. Use of regulators is a bit beyond DIY territory, but is fairly simple if you have basic electronic knowledge, and use a 78 series regulator or a variable LM317.
Droppers and regulators can be used to provide multiple voltages, and thus run multiple appliances off one supply.
When an appliance has no markings
Usually appliances have the supply requirement marked on them. Sometimes the marking is only shallow embossment in the plastic, and needs light at an angle to be visible.
When there are no markings, the fun begins. Following these steps will usually get it working, but is not risk free, and a percentage of dead appliances will occur. Experiment at your own risk.
Its better to use a DC wart, but rectified AC will work in the majority of cases.
Batteries
If it takes batteries, the same voltage wart should work fine. Occasionally an appliance needs a higher voltage wart, either due to an inbuilt regulator, diode, or a desire to reduce power consumption when on batteries.
Diode
In electronic appliances, usually the first thing the power input encounters is a protection diode. Usually this is wired across the supply so that the wrong polarity causes a short. Thus the diode polarity will tell you the required power input polarity.
Less often the diode is series wired.
Even less often a bridge rectifier is used instead. The presence of one of these means the appliance will work on DC, rectified AC or even AC, and that polarity doesn't matter.
Reservoir capacitor
In electronic appliances the power input usually goes to a relatively large cylindrical electrolytic capacitor with a voltage marking. The power supply will be below the capacitor voltage rating at all times. The reservoir cap rating can't pin down the voltage exactly, since sometimes the cap rating is well above working voltage, usually to increase reliability.
IC
Appliances using an IC will have a specific voltage rating for the IC. Googling the right markings on the chip can usually find this.
Occasionally applying the right voltage doesn't work, if so check the chip supply pins voltage - if its much lower than the wart voltage, there may be a regulator in the way. If so the appliance will want a higher voltage supply. Measure the voltage drop between wart and IC, and your wart will then need to be IC rated voltage + 20% + measured Vdrop.
Final option
When all else fails, start with 1.5 V and increase voltage until the appliance works properly. You can use batteries for this voltage test if a variable wart isn't available.
Connectors
Most low voltage appliances use these DC plugs, but they come in various sizes, and there is no standardisation on polarity.
1970s appliances often use 3.5 mm TRS plugs to deliver power. These momentarily short during insertion, so the supply must tolerate this. Most do, but certainly some wallwarts don't. With iron warts, the smaller it is the more likely it is to be short tolerant.
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