Wallwart

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Wallwart fe 153-3.jpg

How to choose a wallwart for your small appliance, and how to get rid of them.


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

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
  • Some items containing logic chips will die if not fed with the correct regulated 5v

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 a wallwart giving 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, as they can't power anything other than an appliance designed for an ac wart.

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 equal to 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: = or Recdc2.gif

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).

Appliances that accept rectfied ac may be marked with either a dc or 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 before use.


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 most things, but can destroy some more sensitive appliances.


Polarity

The majority of small appliances use the outer connection of the power connector for +ve, and the centre pin is -ve. However there are plenty that want a wart connected the other way round. Check first to avoid destruction.

A polarity diagram is usually moulded into the plastic surface near the connector socket (on the appliance). Often its only visible when the light catches it at a suitable angle.

A wart with the right specs except for wrong polarity is easily corrected: cut the dc power lead and rejoin it, swapping the 2 wires over. Insulate each wire separately to avoid a short. Multipurpose warts normally have user selectable polarity.

Electromagnetic and electronic

Electronic wallwart

There are 2 types of wallwart. Many 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, higher price and the existence of warts with unsafe insulation.

Unsafe wallwarts

There are many electronic wallwarts in current use that make no attempt to conform to legally required insulation standards. The usual failings are:

  • less than 3mm separation between mains and low voltage PCB tracks
  • No insulating bobbin on the power transformer, leaving only a thin paint coating on the wire to insulate. These are a greater risk than the former.

Testing

Wallwarts may be tested for this safety deficiency by internal inspection, or by electrical test with a megger or PAT tester. Internal inspection consistes of checking there is at least a 3mm band of separation between mains and low voltage side on the PCB, and checking the transformer has a plastic bobbin.

Unbranded charger warts are the most common culprits. Unknown 3rd party brands are very much more likely to have a problem than OEM warts - OEMs check their warts for conformance.

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

Wallwarts are rarely attractive, and there are a few ways to reduce the number of warts in use.

Sometimes its possible to run 2 or more appliances off one wart to reduce clutter. The wart needs to be the right voltage for both appliances, and its current rating must be at least the current requirements of both appliances added together. 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. However there are significant issues with this option:

  • Such supplies are too big to be built in wart format
  • They also aren't very common
  • low voltage appliances tend to be spread about the house rather than concentrated in one place

Its also possible to use installed low voltage wiring to distribute low voltage, removing the need for most warts. This is generally supplied by a single centrally located regulated power supply, often a standalone PC supply. (Other options also exist, but aren't usually chosen).

Finally, some appliances can be run off a desktop computer's built in power supply. These provide relatively high current at 12 V, 5 V and 3.3 V, and often also lower currents at 24v (+12 to -12) & 17v (5v to -12v or 12v to -5v)). In the past 7v has also been used by connecting from 12v to 5v lines, but this is problematic with a computer that sleeps, and can't be generally recommended. The big downside to PC supplies is that many PCs aren't on 24/7.

Computer supplies will happily run most small appliances, the one exception being audio appliances with an audio link to the PC, which sometimes suffer from noise when run this way. This method can be made to work well by using a filter in the low voltage lead. Possible filter topologies include CRC, CLC, CDC, and some others.

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.

  • Don't overlook that 78 series regs need a minimum 4v drop across them.
  • There are low dropout (LDO) regulators available that are much more versatile than the old 78 series. These can work with just a fraction of a volt drop.

Regulators can be used to provide multiple voltages, and thus run multiple appliances off one supply. Droppers can do the same to a limited degree - they cause a lot of deregulation.

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 benefits from 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. This is found with a multimeter, or if necessary by taking covers off.

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.

Regulator

A minority of appliances contain a regulator to provide a stable clean supply voltage from a basic wallwart. The most common regulators are 7805 (5v), 7809 (9v), 7812 (12v), 78L05 (5v). These generally require their output voltage plus 4v, and can accept a little more.

IC

Appliances using an IC: each IC has its own specific supply voltage rating. Googling the right markings on the chip can usually find this.

Some chips can work on a wide range of voltages, so this approach doesn't solve the mystery in every case, but it often does.

Occasionally applying the right voltage doesn't work, if so check the voltage on the chip supply pins when you're applying power - 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

DC plug

Most low voltage appliances use these DC plugs, but they come in various sizes, and there is no standardisation on polarity.

USB connectors for 5v are gradually gaining market share, providing power compatibility with computers.

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.

Various other non-standard connectors are seen on some appliances, especially mobile phones.

Wart hate

Wart hate is fairly common, but as well as being ugly and bulky they do have their good sides. If the power supply were built into the appliance instead, the appliance would be bigger, heavier, and cost significantly more.


The future

Its fairly likely that we will soon be seeing:

  • the gradual move toward higher efficiency electronic warts
  • 1 or more widely adopted power standard, making warts for most appliances compatible. This might well be the 5v USB standard, which is already gaining some ground with portable appliance supplies.

and futher in the future maybe we'll see:

  • standardisation of low voltage connectors, with each connector type easily distinguishable and used for just one supply spec, thus eliminating use of the wrong supply.
  • and perhaps the increasing use of warts with more than one power output (once standardised warts are common)

See Also

Unusual wallwart