Compass Rose logo for the Compass DeRose guidesThe Compass DeRose Guide to Building a Universal Power Supply

This page was written by Steven J. DeRose in early 2003, and was last updated on 2003-04-14.

It's a pain to carry lots of power supplies for the various devices a road warrior may want on the road. Targus™ makes a universal power supply with the voltage automatically set by the tip, but the range of available tips is narrow. Radio Shack makes a manually switchable supply and a much wider range of tips. But you can also just build such a device yourself, and probably for a lot less money. This page describes how to do that.

You can build or buy a power supply that either

  1. lets you set the voltage manually, or
  2. builds voltage-control into plugs in a different (and simpler) way than the (patented) Targus approach).

Depending on the approach you take, you can gain advantages including lower cost, greater choice of tips, ability to use varying power sources (including not frying your supply if you plug it in in Europe), and fewer tips to carry. You can carry fewer tips because with the Targus supply, if you have 3 devices that all use the same shape plug, but differ in any of voltage, polarity, and current limit, you have to buy 3 tips (hmmm.... bug or feature?). By separating out voltage control and polarity (and ignoring current limiting), you may end up with far fewer tips, thus less space and cost.

Option 1: Manual voltage-setting supplies from Radio Shack

Radio shack power supplies nearly all terminate at a simple 2-pin socket connector . You then buy an "AdaptaPlug" of the right shape for your device(s), and plug it into the supply's socket. That socket is symmetrical, so any AdaptPlug can be used with plus and minus swapped (I find it shameful that there isn't an industry standard for plug polarity).

AdaptaPlugs cost about $5 (versus about $15 for a Targus PowerTip). There is a wide variety of Adaptaplugs (see chart) -- mostly the common cylinder types. And you only need one AdaptaPlug per shape: not one for each distinct combination of shape, polarity, and voltage. So if your cell phone and your MP3 player both have the same shape power socket but use different voltages, that would mean two PowerTips but only one AdaptaPlug.

The drawbacks the Radio Shack solution are that you have to set the voltage manually (either via a switch, or by having multiple power supplies); and that you have to plug the AdaptaPlug in the right way to get the polarity right.

If setting voltage and polarity manually are ok with you, you can get a very cheap power supply from Radio Shack, (model 273-1680) with a little voltage switch on the front for 3, 4.5, 6, 9 or 12V ($40). It's also a switching supply, so should operate on 220V as well as 120 (the Targus requires you to buy a separate 220V supply connector).

Radio Shack #273-1827 is a little more expensive, but lets you dial any integer voltage from 12-27 volts at up to 60 watts ($80). To cover lower voltages like 3-12V, they have item 273-1817 ($20), and you're done.

Both of these take power from a cigarette-lighter connection, so you can combine them with a 110V-12V adaptor like their item #22-505. I think that may be the ideal solution.

To run directly from mains power (anywhere from 100-240V), they have item #273-1687. It switches from 12-24V at 80 watts and plugs in to 110V ($120). It comes with Adaptaplugs to fit most laptops, so can go head-to-head against the Targus device.

Gripe: Seems to me it would be more useful to bag some oddball voltages (I've never seen a device that wanted 13, 14, 15, 17, 18, 19, 21, or 23 volts), and replace them with some others like 1.5, 3, 4.5, 5, 6, 7.5, 9 and 14.4 (or just go in 1.5V increments up to 12V, then 2V increments from there). No harder to make, but you could get an awful lot more use out of it. If they really wanted to get me excited, they could provide two switches and 2 AdaptaPlug sockets, so I could run 2 things at a time (subject to the same total wattage limit).

In my opinion, the Targus has the advantage of auto-switching, and the disadvantages of needing a separate ($) tip for everything: to use 220V, to get a different output voltage on the same type tip, etc. The Radio Shack version has the advantage of handling 100-220V, and of letting you use any tip at any voltage and polarity, and of having many more tips available; but the disadvantage of being manual (though if you (manually) select the wrong Targus tip you're in just as much trouble).

Gripe: The most severe problem with the Radio Shack supplies is that they don't come with adaptors for Apple Powerbooks™_. However, you can work around this because (except for a non-essential big outer ring), the PowerBook G4 connector is a standard 2.5mm (3/32") stereo plug (according to my measurements the tip is unused, middle is ground, base is +24VDC). Same for Powerbook G3 except it's a a 3.5mm (1/8") stereo plug. See here for instructions on making the right AdaptaPlug for your PowerBook. Item #274-373 will convert a 3.5 to a 2.5, so add that and you've got both.

Radio Shack doesn't seem to sell AdaptaPlugs for most laptops and cell phones except bundled with a power supply. If you need a weird-shaped tip that Radio Shack doesn't have, you could just buy that tip elsewhere (either from some of the sources I link below, or by buying the appropriate Targus PowerTip without necessarily buying the Targus Power Supply), and wire it to the common AdaptaPlug connector so you can plug it into the Radio Shack suppl. As shown to the right, you can get that plug on a short piece of wire as item #273-1742) for $1.99. Or you can just solder directly to the pins on the tips.

You could also make a PowerTip-to-AdaptaPlug convertor by combining the item just shown with half of a Targus supply-to-tip cable; but I can't find where anyone sells such a replacement or extension Targus cable.

I checked what it would cost to have an AdaptaPlug-to-Powertip convertor manufactured: Tooling for a simple plastic connector like this runs a few thousand dollars; the actual parts then cost a few dollars each. I'm not keen on laying out the tooling costs, but I'm suprised Radio Shack isn't (and they'd surely get better prices than I would, too).

Option 2: Standardize on 12 volts

Buy a car cigarette-lighter charging cord for any device that you can't get a Radio Shack Adaptaplug for; most device manufacturers make these, since they're so valuable. This is useful in its own right, since you can run your devices in the car.

The big plus, though, is that you then wire a cigarrette-lighter socket to a connector that fits any power supply that can put out 12 volts. Then your car cords plug into your new 12-volt socket, regardless of where the 12 volts are coming from. Radio Shack makes just the ticket for this: a little box that pugs into the wall and has a cigarette lighter socket on the other side. Product #22-505.

To go one step better, wire a 12-volt fixed positive voltage regulator into a little box, with an adaptaplug socket or cord on each end. The output end can then use an AdaptaPlug to connect to any 12V device you want (or just to a cigarette-lighter socket, out of which you'll get much cleaner 12V power than otherwise); the input end can be plugged into a cigarette lighter via a Radio Shack adapter, or into most any power supply that puts out a typical voltage (say, 3 to 40 volts or so). Higher-power voltage regulators can be found, up to about 5-6 Amps.

Option 3: USB power cords

You can do the same thing with USB charging cords, which are available for many devices, though not as many as car cords. This makes your laptop the "hub" of the recharging strategy: you carry a supply for it, and charge everything else from that. One thing to test: make sure the USB ports still supply power even when your laptop is asleep (or perhaps off), otherwise charging your cell phone or PDA overnight will be a pain.

Targus doesn't seem to have a tip for my Handspring Visor Pro, so I bought a "sync-and-charge" cable, which uses USB power to recharge the VIsor and lets me sync it without having to take the cradle along either. Targus and Radio Shack both supply such USB-powered cables for many cell phones. I believe the power rating for USB is a maximum of 5.25V at 500 mA.

You can also generate a stable 5V with a positive fixed voltage regulator, as described above for 12V -- then you can draw your power from any old supply. It's hard to find panel-mount USB sockets, though sockets mounted on cables are easy enough....

Option 4: Building your own universal power supply: manual voltage setting

The key to building your own supply is a little gadget called a voltage regulator. You put in any voltage (within certain limits) on the input side, and it puts out a precise selected voltage on the output side.

Doing this takes a little soldering and wiring skill, but gets you a much more flexible supply. The beauty of voltage regulators is that you can run them from just about any DC source, such as:

If you build this kind of supply and carry cords to plug it into some of these various sources, you can use just about any DC power available, including borrowing almost anybody else's supply.

For more information on how to do it, and for links to relevant connector suppliers, see the following page.

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