Introduction: Power Supply Tips by LOG

My cousin asked me, the Lazy Old Geek, how to buy a power supply for a laptop. Here are some quick tips.
The first two pictures are a laptop power supply and a ‘wall wort’ power supply (I have no idea what it was for).
Power supply labels typically specify:
Input: the electrical requirements coming from the AC outlet.
          Voltage: often a range
          Type: usually AC, sometimes AC is indicated by a (~) wavy line.
          Current: in Amps (this is the maximum required, may draw less)
          Sometimes Hz: 50 or 60Hz, for this type of supply
Output: the requirements coming out of the power supply
          Voltage: a fixed voltage
          Type: can be DC or AC sometimes indicated by a (~) wavy line.
          Current: in Amps
There are several pictures of various power supply labels.

Important Tip: The most important information is the output voltage and current (and type). The voltage is fixed. The current is maximum. What you connect has to require the same voltage (and type). The power supply will supply any current up to the maximum specified, e.g., you can hook up a 5VDC 0.5A wi-fi adapter to a 5VDC, 200A supply.
USE: I am an electronic hoarder, so I have probably 20 power supplies. This information will allow me to find a substitute power supply for a device or a new one for an Instructable project. Make sure the power supply voltage and the device voltages and types are the same. The power supply current can be the same or higher than the device. I was also able to supply my cousin the information she needed to get a replacement laptop supply.
Pictures: laptop label and router label. Devices connected to a power supply.
Input Voltage: fixed voltage
Input type: AC or DC
Input current: is the maximum current to power the device. It may use less.
TechnoTip: The last picture shows the measured voltage for a 19V power supply. Notice that it is higher (19.6Vdc). One of the reasons is that it is unloaded. There is no device connected to it. But it might be slightly higher or lower even when connected.
Less Important Tips:
TIP INPUT Voltage: I am fairly certain all countries have AC voltage outlets. This can be 100, 120, 220 or 240V. Just make sure your voltage is in the range.
TIP INPUT Current: This is the maximum the power supply will draw. Typical current will be less. This isn’t important unless the current exceeds the maximum for your AC outlet. U.S. residential outlets are rated at 15Amps.
TIP INPUT frequency: Every country I know of has either 50 or 60Hz. For power supplies, I would guess any should work any where. Where frequency is critical is for AC motors.
Tip Type: Some manufacturers label voltage as AC or DC, some will use the (~) wavy symbol for AC and the solid line over a dashed line for DC. I think thes symbols come from what the voltage looks like on an oscilloscope. AC is a sine wave. DC is a straight line above the ground level. Ground is represented by the dashed line.
Tip Connector: some labels have an additional diagram. This shows how the DC voltage is connected to the connector. The connector is in the middle. The dot shows the polarity of the center pin (positive). The C symbol is the outer connector (negative).


Gelfling6 (author)2013-12-28

Never trust a plug to indicate what voltage is present.. I accidentally plugged a 9V recharger supply for a Sylvania Netbook (7" ARM Based), into the plug to recharge a Magellan eXplorist-500 GPS. (same size coax plug, but 5V. as you can guess, it FRIED the GPS). I've also run into a few items designed to be run off 5V, with the same coaxial connector common with 12V. (scorch one network hub.) Also killed a old WiFi hub, which was +12V, but with a 12V wall-wart, that went to a cordless drill (guess what the coax polarity was? +-C- - )

banman11 (author)Gelfling62015-07-08

I know exactly what you are talking about. Did the same trick with a round 4 pin power connector from a LCD monitor into a an external DVD drive.

Nothing worth saving after that effort after

msuzuki777 (author)banman112015-07-08

About the only connectors that I know are tied to a specific voltage are (internal) computer power supply connectors. USB chargers are actually pretty standard. The only thing you have to be worried about is Apple products. USB chargers won't work with Apple products unless they're designed to.


msuzuki777 (author)Gelfling62013-12-28

Yes, you need to check the voltage and also the polarity. Reverse polarity can kill some devices and/or the power supply.
What I really hate is some supplies have a stated current but can't supply it.


TampaGeek (author)2012-08-16

Good stuff... I wish the labels told you the size of the darn connector, though (and the devices with the jack telling you what size connector to use). I've had to go to Radio Schmuck more times that I can recall to test fit connectors to see what was the right one...

msuzuki777 (author)TampaGeek2012-08-16

Oh, so true! Better yet, I wish manufacturers would agree on some standards. I looked at a 'universal' laptop power supply that came with 12 different adapters and I'm fairly certain none of them will fit my Lenovo laptop. Last year I looked all over trying to find a replacement connector and never could find one.


Gelfling6 (author)msuzuki7772013-12-28

See my most recent remark above.. I've killed too many things, where the plug size was right, but either the voltage or polarity was wrong. Nothing makes you angrier than the smell of burning electronics!

Gelfling6 (author)TampaGeek2013-08-10

I hear you on dealing with radio$hack. We have questions, they want to sell you something, ANYTHING! Except what we specifically asked for. (sorry, sarcastic.) They used to have a whole bunch of connectors held together like a cat-o-nine-tails, next to the adapt-a-plug devices.. Now, You're lucky if they even have the right one.. I couldn't find a replacement for an HP over-sized 3-conductor barrel connector, and even found out it needed a resistor to the center pin. (ended-up ripping apart the connector from the dead supply, and building it into a female jack socket, to adapt to a new 120W supply I got off ebay. Yes, the wattage is kind of minor, as the only thing you would need to worry about, is a dead short across the supply (then high amperage/wattage, would cause a problem, and some smoke.) the device will only use what it needs. IF it's more than the rated amperage/wattage, the supply will overheat and burn-out or fail quicker.

offtherails2010 (author)2012-08-15

Exactly what i needed !!!

Finally Deciphered Wall-Wart Symbols !

Thanks once again LOG !


I haven't been able to find an official list of these electronic symbols.


amandaghassaei (author)2012-08-14

great info, thanks for posting!


Hope it's useful info.