Power Supplies

One hundred forty-seven dollars and thirty-nine cents—that is the cost for replacing a power supply for an old MiniITX computer system I found in my office. Mind you, the entire unit cost about $199, and that was five years ago, but still, the cost for a replacement power supply is absurd.

Thankfully, a quick search on the Internet found a universal power adapter that fit my requirements for about $18. How can you find inexpensive replacements for your missing power supplies, without frying your vintage arcade cabinet computer? There are a few important things to watch for:

  1. Voltage: most universal power adapters have several voltage selectors; make sure that they match your needs. (For example, laptop power supplies generally require more voltage and, unfortunately, are more expensive.) The device should say near the power adapter how much voltage it requires. The network switch in Figure 1 shows a need for 7.5V of DC current. Some devices require AC voltage as opposed to DC, so be sure to look for “DC” on the device.

  2. Amperage: your device generally will say near its power port the amperage it requires along with the voltage. Amperage is a little different from voltage, and you want to make sure your power supply supplies at least as much amperage (usually measured in milliamps) as your device requires. The device will draw as much amperage as it requires, but there's no concern if the power supply gives more than it requires. The network switch in Figure 1 shows a 1 amp minimum requirement.

  3. Polarity: your device most likely will have a drawing that shows whether the tip of the plug is positive and the jacket negative, or vice versa. Most universal power supplies will have a selector that looks similar. Make sure polarity is lined up! It's just like putting batteries in backward if you flip the polarity.

  4. Plug: I wish there were a standard for the various types of plugs you might face, but sadly, there's not. Most universal adapters come with a selection of plugs that will fit most devices, but unfortunately, not all. It is possible, if you feel a bit adventurous, to cut the end off your old power adapter (assuming you have it) and solder or tape the correct plug on the wire of the universal adapter. Be warned, however, that it's easy to mess up polarity when you do that.

There are some other factors to weigh in as well. Some cheap universal power supplies are not regulated, which means they can vary in voltage depending on the load they're put under. If your device is particularly sensitive, you may want to watch for that. In the end, if you're worried you might mess up and ruin your prized powerless device, you always can shell out the $147.39 and get a new one. For me though, $18 was more in my budget.

Figure 1. This network switch shows a need for 7.5V of DC current.

Figure 2. Associate Editor Shawn Powers wrangles with his power supplies.


Shawn is Associate Editor here at Linux Journal, and has been around Linux since the beginning. He has a passion for open source, and he loves to teach. He also drinks too much coffee, which often shows in his writing.


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Multimeter Time

Cosmo's picture

If you are dinkering with power supplies, or even want to modify one make your own for special applications, it is not too hard. Either way you will want to take the opportunity to get a good multimeter and learn to test questionable power supplies before plugging em into your precious. Polarity you will want to test of course, but also you will find many supplies may put out more voltage than you expect - this is often the case in switching power supplies - and you will have to put the circuit on load to get a correct reading from the meter. You can build a simple test circuit in order to measure the supply rather than test it upon something important.

Some of your devices may have built in voltage regulators which can accommodate a little slop in the voltage but you would have to do some research to find out. To be safe you can add your own regulator, filtering capacitor, or rectifier to a cheaper supply to get a smoother voltage. Do some online reading it is a fun subject on basic electronics :)

The best place for cheap supplies I have found is goodwill you can buy a fistful of power supplies for $10 and cut them up to mix and match the connectors if necessary. Just please test them well and only play with the DC (low voltage) side of the supply!

You have a good point there

svenni's picture

Also consider the power efficiency (wattage out / wattage in) and the voltage delivered under the desired current (amperes). A multimeter helps here :)

A flaky power supply leads to weird hardware problems so perhaps steer away from the very cheapest ones (witch probably are manufactured under poor labor conditions anyways).

Beware of DELL power supplies

turgut's picture

Do NOT get an OEM power supplies for DELL laptops.
The sneaky Dell has added an "authentication" pin to their power supplies that talks to the laptop's BIOS, and if the BIOS determines that the power supply is not genuine, it will NOT let it charge the battery, and slows down the CPU!

Repeat after me: Will not get Dell again.

There are actually more than 4 things to watch...

Mr H's picture

There are actually more than 4 things to watch, even if for low-value devices it may be enough (if we add "signal frequency" to the list).

For more expensive or precious devices, we should, in my opinion, proceed as this:
1- What is the ideal expected power source? It includes: nominal voltage (or current, rare), frequency
2- What are the hardware requirements? Includes: plug, form factor
3- What are the acceptable value ranges of the different parameters of the real source? Includes: average AND maximal power (for hifi), signal quality (for hifi and computers), phase (for motors?), and many others I don't know...

Why? Because your devices are unlikely to have an linear power consumption nor accept all kind of little variations some cheap source is likely to produce (instead of a perfect sinusoid at anytime).

try an xbox power supply

henry eight's picture

There are several various 360 power supplies wired differently. Some will work with the ultra small form factor Dells. Good if you need one, as your local used game store probably has a bin of them behind the counter and may barter if you're broke....

Some other symbols to watch out for

Frank L.'s picture

Some devices don't say AC or DC, they have a little symbol for each. The symbol for AC is a sine wave (similar to ~) and the DC symbol is a straight line with a dashed line underneath it.

Box of old stuff

eric.john.miller's picture

This is one reason why I never toss out old power supplies. You never know when you'll need one.

Wall warts

cwsnyder's picture

Universal wall-wart supplies are also notorious for having insufficient power capacity (limited current). Think of this: You put in a new whiz-bang video adapter in your (cheap, big-box-store) computer and suddenly start having all kinds of mysterious problems, with maybe the computer refusing to even start properly, your hard drive refusing to spin up, etc. All of this caused by an undersized power supply.

I have also had problems with external hard drive enclosures whose supplies are unable to properly source sufficient power for the old IDE drives which I have around. New drives would work, but I can't use the enclosure to access data on the old drives.


Anonymous's picture

Isn't "current" the more current (pun intended) word for "amperage"?

Although I could understand "amperage", I can't recall having seen it anywhere else in an English text. Please, I am not being sarcastic or ironic, nothing like that. It just so happens that, coming from Ampère, it is very used in French (and Portuguese).

Yes, it's archaic and current

Anonymous's picture

Yes, it's archaic and current is (and has been for some time) a more appropriate term.

In short, no. The Ampere (A)

Anonymous's picture

In short, no. The Ampere (A) is the SI unit of electrical current.

On a power supply you will generally find some unit in mA or A. This is much the same as using Watt (W) as for electical power.

Um, minor points

Rickb's picture

You wrote "This network switch shows a need for 7.5V of DC current"

Voltage is NOT current. The example you were writing about might better be described as:

"This network switch shows a need for 7.5V of DC voltage, and 1 amp or more of available current".

When I first read this article, I realized that the topic was external power supplies, or 'wall warts'. Matching up voltages and current capacities is actually the easy part, but matching the plug/jack and polarity is always an issue. There are normally no standards here, so matching these up is a nuisance. But there is almost always (99.44% of the time)

It's not easy to describe these problems and solutions, and your article is very helpful to others in offering alternatives and avoiding the amazingly high prices of 'original equipment' replacements.


Old Post

metalx2000's picture

Wasn't this already posted a while back?

Everything you ever need to know about Free Software.


Shawn Powers's picture

It was in the magazine. And then published online with the whole issue. Katherine often takes the "UpFront" pieces and publishes them individually.

Shawn is Associate Editor here at Linux Journal, and has been around Linux since the beginning. He has a passion for open source, and he loves to teach. He also drinks too much coffee, which often shows in his writing.

Ah, I new I had read it

metalx2000's picture

Ah, I new I had read it before. Thought for a second there that I might be psychic :)

Everything you ever need to know about Free Software.


fest3er8's picture

Or having a Yogi Berra moment. :)