What you need to know to safely replace or substitute AC adapters

Last Updated on April 11, 2017 by Dave Farquhar

AC adapters inevitably break or get lost. That means you have to replace them to get your devices working again. But a lot of people don’t know how to do that safely. Here’s what you need to know when you need to substitute AC adapters.

This is important. Getting it wrong can damage your equipment, the adapter, or both. The damage can be immediate, or it can appear over time.

The specifications for your AC adapter (also not affectionately known as wall warts or power bricks) should be printed on the old one, and hopefully on the device too. If you lost the adapter and the specs aren’t on the device, try a web search on “ac adapter specifications” and the name of the product.

It used to be you could take the device to Radio Shack, and $25 and five minutes later you had a good replacement. Today you might try Batteries Plus for a similar experience. But if you’re willing to do the legwork yourself, often you can find a close-enough match for closer to $10.

So, with all that said, here are the handful of things you need to know when you’re shopping for an AC adapter. The specifications don’t have to match exactly, but you have to know when you can cheat and when you can’t.

AC or DC?

Alternating current reverses polarity 50 to 60 times per second, depending on where you are in the world. Direct current doesn’t. In rare instances you can substitute one for the other. To stay safe, always assume you cannot.

Polarity

Since polarity stays constant in DC, you have to pay attention to it. Most “universal” adapters let you change polarity; with less expensive adapters, the polarity is fixed to either positive or negative. Polarity is indicated by the symbols shown to the right.

Incorrect polarity is the most common cause of damage when substituting adapters. Always assume you have to match polarity.

Voltage

Think of voltage as the speed of the electrical current. Electrical devices are designed to tolerate small deviations in voltage, but you should match voltage as closely as possible. Too little voltage causes things to not work; too much voltage causes excess heat at best, and at worst, blows stuff up. Stay within 10 percent of the original.

Amperage

Think of amperage as the amount of electrical current that’s flowing. Think of a light bulb. A 20-watt bulb uses .18 amps. A brighter 100-watt bulb uses .91 amps.

If you use an AC adapter with too little amperage, you run the risk of damaging the adapter. Too much amperage is perfectly OK, because your device will only draw the amount of amperage it needs. The rest sits unused. If I offer 2 amps to both the 20-watt bulb and the 90-watt bulb, they’ll happily just pull .18 and .91 amps.

There is a caveat. If your device draws far too few amps, some cheaper, poorly regulated adapters start delivering more voltage.

Replacing an AC adapter with one that has double the amperage is fine. Replacing one with 20x the amperage is riskier.

Wattage

Wattage is volts x amps, or the total amount of power available. Often if an adapter tells you amperage, it will omit wattage, or vice versa. You can use one to figure out the other. Divide the number of watts by the number of volts to get amperage.

Again, think of light bulbs. A 100-watt bulb shines a lot brighter than a 20-watt bulb, because it uses five times the energy.

Just like amperage, too much wattage is perfectly OK, within reason, because your device will only draw the amount it needs. The rest sits unused. Replacing an AC adapter with one that has double the wattage is fine. Replacing one with 20x the wattage is riskier.

Tip size

No, tip sizes generally aren’t standard. Measure with a caliper or ruler before you go shopping, because if the adapter doesn’t fit, none of the other specs matter.

Price range

Expect to pay $10-$25 for an adapter for a small device like a phone, speakers, switch, or router. An adapter for a laptop or an LCD monitor can cost up to $40 or $50. You’ll usually pay more for a “universal” model with multiple tips and voltages than you will for a model with one hardwired tip and a fixed voltage and polarity. The versatility costs you, but if it means you can use one adapter to power or charge more than one device, it can be worth it. (Note the adapter will only work with one device at a time.)

It’s a good idea to note the tip, polarity, and voltage for each device you plan to use, and either write it on a label and stick it on the adapter, or write it on the adapter with a fine-tipped permanent marker or paint pen.