Reusing Wall Warts

During a long and fruitful tinkerer's life, a lot of materials will accumulate, including a junk box full of AC/AC and AC/DC adapters. These colloquially are called wall warts.

It's understood that they are kept - you never know if and when one of these comes in handy... and you can be sure: Whenever you have a box with, say, ten wall warts, you will find at least eleven different output voltages available :-) And with some luck, one of them supplies the exact voltage you need for your purpose.

Since we all are ecology- and economy-minded, we replace batteries (i.e. dry cells, but not rechargeable batteries - or accumulators, as they call them in the UK) in portable devices by a wall wart when- and wherever it is possible. When you don't use batteries, they don't have to be disposed of - which is not only good for the environment but also for your quiet conscience as well as for your wallet - because powering a device from batteries is EXTREMELY more expensive than supplying it from the mains.

Some more details and background information on this subject can be found e.g. in Wikipedia under the title 'AC adapter' (or 'Steckernetzteil' in the German version).

Old...

There are the earlier ones with small, conventional mains transformers. They come in several different flavours:

1. With plain AC (alternating current) output (fig. 1)
2. With DC (direct current) output, simply the AC of the transformer rectified - without the (dashed) smoothing capacitor CS, resulting in a pulsed DC voltage (figs. 2-4)
3. With rectified DC output, smoothed by some capacitor CS (figs. 2-4)
4. With a variety of switch-selectable DC output voltages
5. With regulated DC output voltage

There exist different modes of rectification. The cheapest, rather inefficient version is the half-wave rectifier in fig. 2 above. They usually come with a smoothing capacitor.

Full-wave rectification is considerably better. This either needs one secondary transformer winding and four diodes (fig. 3), or only two diodes but two secondary transformer windings connected in series (fig. 4). Full-wave rectification is often complemented by a smoothing capacitor (CS), but from time to time you can also find un-smoothed, 'pulsating' DC.

Some of the wall warts with a slide or rotary switch for output voltage selection (no. 4 in the list) are the ones getting really warm or even hot during operation; these are the least efficient versions and not considered here.

The ones containing a linear voltage regulator (no. 5 in the list) are rarely found and also not considered here. BTW, 'regulated voltage' means that the voltage is kept constant regardless of the amount of current delivered (up to a certain limit, of course).

Any of these types can, of course, be converted into any of the others by adding or removing some components. However, if you need to add rectification and a smoothing capacitor, there has to be enough room within the case. When doing so, take care to always provide sufficient insulation!

In order to do this, the case needs to be opened. Some wall warts, particularly the earlier (or very early) ones can be opened easily, by a standard screwdriver of the slotted, Phillips/Pozidriv, Allen or Torx flavour. You might, however, stumble on the abominable, tamper-proof screws - such as tri-wing, or the Allen or Torx variety with the little centre pin... but you have of course matching screwdrivers or screwdriver bits in your tool kit.

(BTW, when checking https://en.wikipedia.org/wiki/List_of_screw_drive... I was absolutely amazed about the sheer amount of different screw head types existing!)

If the case is not screwed but glued together, you will need patience, fantasy, and creativity for opening it, sometimes even brute force. A utility knife or a hacksaw can be used for cutting the seams; these tools are best applied in the corners of the case only. Take care to only make very shallow cuts in order not to damage anything inside. The cuts can then be used for inserting e.g. a screwdriver as a wedge for prying/breaking the case shells apart.

If you should wish some more information and a little theory on rectification and smoothing, there is a nice little tutorial available under this link: https://www.elektormagazine.com/articles/starting... (only the circuit diagrams are rather ugly, unfortunately); since you never know how long links remain active, I felt free to copy/paste this article and attached it below in the 'Starting Out in Electronics - Rectifiers.pdf' file.

... and New

Then there are the more modern ones (e.g. for charging your smart phone) that do not use a mains transformer but a switch-mode, step-down voltage converter with, normally, regulated output voltage. They are much lighter than conventional ones because they do not feature a mains transformer; they usually produce less heat and thus are more efficient. The cases of most of them are glued together and are rather difficult to open.

How do you proceed when you want to power your, say, blood pressure monitor (or P-Touch label printer, or, god forbid, your antique walkman - or whatever) by a wall wart? First thing to do: Check whether there is a supply socket on the device you want to power by a wall wart (if not, this is not covered here, sorry!). If so, check what kind of and which supply voltage you need for your application. Usually this is indicated on the type label; if not, count the batteries in the battery compartment; if these are the standard, garden-variety dry cells, there is 1.5 Volt per cell; if you have, e.g. four of them, the sum of their voltages is 6 Volt. And if your unit is powered by batteries, you can be sure that you need rectified and smoothed DC. With some luck, even the current (or power) consumption is given on the label; if in doubt, you should measure the supply current from the batteries in operation.

On the wall wart's type label, you find information on which kind of output is provided. It gives the primary (input) voltage (normally either 115 or 230 Volt) and often the maximum current consumption from the mains. It also gives the secondary (output) voltage and current under nominal conditions, and whether the output is AC or DC. Most of them have a sort of coaxial plug at the end of the cable, and normally the centre hole of this plug is the positive output. In rare cases, however, the negative output is in the centre, so you need to check the corresponding pictogram on the type label, too. The standard garden variety (full-wave rectified and smoothed DC) is no. 3 in the list above, but the other variations can be found as well.

Please note that the device supplied by the wall wart, as well as the wall wart itself, can be damaged if voltage, power or polarity is wrong!

The VOLTAGE MUST BE THE SAME on both the wall wart and the device.

The CURRENT taken up by the device MUST BE EQUAL TO OR LOWER THAN the one delivered by the wall wart.

If no current indication is available, you should find a power indication in Watt (W) or milli-Watt (mW). Divide this number by the voltage to get the current available (or consumed) in Ampère (A) or milli-Ampère (mA), according to this simple formula:

Power P [W] = Voltage U [V] x Current I [A]

For example, 6 W at 12 V corresponds to 0.5 A (or 500 mA)

If in doubt, you should measure the voltage at the wall wart's plug. Don't forget that the no-load voltage will always be higher, sometimes considerably higher, than the voltage under nominal conditions! Refer to the type label - there, the plug's polarity should be given as well, usually by a pictogram.

The pictures above show

• a barrel-plug socket of a device, with voltage, current and polarity indication
• a type label of a P-Touch device, with battery type and power consumption indication
• a type label of a wall wart, with voltage, current and polarity indication
• and the standard polarity pictogram (with the positive terminal in the centre, in this case)

There is one more, large stumbling block on your path: The wall wart's plug has, of course, to match the socket in the device to be powered - but we will come to that.

A coaxial power plug is an electrical power connector used for attaching extra-low voltage devices, such as portable consumer electronics, to external electricity. Also known as barrel plugs, concentric barrel plugs or tip connectors, they come in an enormous variety of sizes. The picture above shows an adapter set, available e.g. from Amazon.

Usually, the sockets contain a normally closed switch that disconnects the internal batteries whenever the external power supply is connected.

You can also find 2.5 or 3.5 mm mini-jack plugs on a wall wart's output cable, but fortunately, rather rarely; their use is not really recommended here, since a momentary short circuit is produced upon connecting or disconnecting them. Even worse: Plugging one of them into, say, a headphones output of a portable device can produce considerable damage!

Standards

We love standards, therefore we have that many different ones...

There are standards provided by bodies such as the IEC, EIAJ in Japan and DIN in Germany. In addition, some manufacturers appear to have implemented their own system of voltages and plug sizes.

There appears to be a trend to standardize the negative pole on the barrel (or sleeve) of the barrel plug and the positive pole on its centre - but remember: Trust is good, control is better.

• IEC 60130-10:1971 defines five DC barrel plugs (outer / inner diameter):

1. Type A: 5.5 mm / 2.5 mm
2. Type B: 6.0 mm / 2.1 or 2.5 mm
3. Type C: 3.8 mm / 1.1 mm
4. Type D: 6.3 mm / 3.1 mm
5. Type E: 3.4 mm / 1.3 mm

• EIAJ power connectors (outer / inner diameter)

Five barrel plug and matching socket or jack designs are defined by the EIAJ standard RC-5320A. Each of these plugs is used with a specified voltage range. Most manufacturers use coloured insulating materials to distinguish these plugs from other similar-looking DC plugs, and for quick identification between EIAJ plugs of similar size.

1. EIAJ-01: 2.5 mm / 0.7 mm (0 to 3.15 V)
2. EIAJ-02: 4.0 mm / 1.7 mm (3.15 to 6.3 V)
3. EIAJ-03: 5.0 mm / 1.7 mm (6.3 to 10.5 V)
4. EIAJ-04 or JSBP 4: 5.5 mm / 3.4 mm (10.5 to 13.5 V)
5. EIAJ-05 or JSBP 5: 6.5 mm / 4.4 mm (13.5 to 18 V)

EIAJ-04 and 05 have an internal male pin in the plug. The 01 through 03 sizes do not, they are similar to the generic plugs in structure.

• DIN 45323 (outer / inner diameter)

The German national standards organization DIN (Deutsches Institut für Normung - German Institute for Standardization) issued DIN 45323, which defines two DC barrel plug/socket sizes:

1. 5.00 mm / 2.00 mm
2. 6.00 mm / 1.98 mm

And if the Plug Doesn't Match?

You might remove the connecting cables from all the wall warts in your collection and store them separately. Whenever you want to adapt one of them, you will with some luck find a cable that fits; this can then be attached to the wall wart of your discretion. Needless to say, you have to keep an eye, or even better, both your eyes on the polarity!