How-To

Complete Guide to Using the Correct Charger or Power Adapter (and What Happens If You Don’t)

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The other weekend, I sat down and sorted through all my random electronics junk. As part of that process, I took all my power supplies and adapters and threw them into a box. It ended up being a pretty big box. I’m willing to bet that any given household has a dozen or more different types of cell phone chargers, AC/DC adapters, power bricks, power cables, and charger plugs.

Having so many chargers can be pretty frustrating. It’s easy to get them separated from the phone, laptop, tablet, or router. And once that happens, it can be complicated to figure out which goes with which. The default solution is to try random plugs until you find one that fits into your device. However, this is a big gamble. If you grab an incompatible power adapter, your best-case scenario is that it works, albeit not the way the manufacturer intended. The second worst-case scenario is that you fry the gadget you are trying to power up. The worst-case scenario is that you burn down your house.

In this article, I’m going to walk you through the process of digging through your junk drawer and finding the right power adapter for your device. Then, I’ll tell you why it’s so important to do so.

In a nutshell:

• The following will cause damage to your device:
  • Reverse polarity
  • Higher voltage adapter than device rating
• The following will cause harm to your power cord or adapter:
  • Reverse polarity
  • Lower current adapter than device rating
• The following might not cause damage, but the device will not work properly:
  • Lower voltage adapter than device rating
  • Higher current adapter than device rating

A Very Brief Introduction to Electrical Terminology

Each AC/DC power adapter is specifically designed to accept a certain AC input (usually the standard output from a 120 V AC outlet in your home) and convert it to a particular DC output. Likewise, each electronic device is specifically designed to accept a certain DC input. The key is to match the DC output of the adapter to the DC input of your device. Determining the outputs and inputs of your adapters and devices is the hard part.

Power adapters are a bit like canned food. Some manufacturers put a lot of information on the label. Others put just a few details. And if there is no information on the label, proceed with extreme caution.

The most important details for you and your delicate electronics are the voltage and the current. Voltage is measured in volts (V), and current is measured in amps (A).  (You’ve probably also heard about resistance (Ω), but this doesn’t usually show up on power adapters.)

To understand what these three terms mean, it helps to think of electricity as water flowing through a pipe. In this analogy, the voltage would be the water pressure. Current, as the term implies, refers to the flow rate. And resistance relates to the size of the pipe. Tweaking any of these three variables increases or decreases the amount of electrical power sent to your device. It’s important because too little power means your device won’t charge or operate correctly. Too much power generates excess heat, which is the bane of sensitive electronics.

The other important term to know is polarity. There is a positive pole (+) and a negative pole (-). For an adapter to work, the positive plug must mate with a negative receptacle or vice versa. By nature, direct current is a one-way street, and things won’t work if you try to go up the downspout.

If you multiply the voltage by the current, you get the wattage. But the number of watts alone won’t tell you if the adapter is right for your device.

Reading an AC/DC Adapter Label

If the manufacturer was smart enough (or compelled by law) to include the DC output on the label, you are in luck. Look at the “brick” part of the adapter for the word OUTPUT. Here, you’ll see the volts followed by the direct current symbol and the current.

The DC symbol looks like this:

To check the polarity, look for a + or – sign next to the voltage. Or, look for a diagram showing the polarity. It usually consists of three circles, with a plus or minus on either side and a solid circle or C in the middle. If the + sign is on the right, then the adapter has positive polarity:

If there is a – sign on the right, then it has negative polarity:

Next, you want to look at your device for the DC input. You’ll usually see at least the voltage near the DC plug receptacle. But you also want to make sure the current matches, too.

You might find both the voltage and the current elsewhere on the device, on the bottom or inside a battery compartment cover, or in the manual. Again, look for the polarity by either noting a + or – symbol or the polarity diagram.

Remember: the input of the device should be the same as the output of the adapter. This includes polarity. If the device has a DC input of +12V / 5.4A, get an adapter with a DC output of +12V / 5.4A. If you have a universal adapter, ensure it has the proper current rating and choose the correct voltage and polarity.

Fudging It: What Happens if You Use the Wrong Adapter?

Ideally, you’ll have the same voltage, current, and polarity on your adapter and device.

But what if you accidentally (or purposefully) use the wrong adapter? In some cases, the plug won’t fit. But there are many instances where an incompatible power adapter will plug into your device. Here’s what you can expect in each scenario:

• The wrong polarity – If you reverse the polarity, a few things can happen. If you’re lucky, nothing will happen, and no damage will occur. If you are unlucky, your device will be damaged. There’s a middle ground, too. Some laptops and other devices include polarity protection, which is essentially a fuse that burns out if you use the wrong polarity. If this happens, you might hear a pop and see smoke. But the device may still work on battery power. However, your DC input will be toast. To fix this, either replace the polarity protection fuse or get it serviced. The good news is that the main circuitry wasn’t fried.
• Voltage too low – If the voltage on an adapter is lower than the device, but the current is the same, then the device may work, albeit erratically. If we think back to our analogy of voltage being water pressure, it would mean that the  device has “low blood pressure.”  The effect of low voltage depends on the complexity of the device. A speaker, for example, might be okay, but it won’t get as loud. More sophisticated devices will falter and may even shut themselves off when they detect an under-voltage condition. Usually, an under-voltage condition won’t cause damage or shorten the life of your device.
• Voltage too high – If the adapter has a higher voltage, but the current is the same, then the device will likely shut itself off when it detects an overvoltage. If it doesn’t, it may run hotter than normal, which can shorten the device’s life or cause immediate damage.
• Current too high – If the adapter has the correct voltage, but the current is greater than what the device input requires, you shouldn’t see any problems. For example, if you have a laptop that calls for a 19V / 5A DC input, but you use a 19V / 8A DC adapter, your laptop will still get the 19V voltage it requires, but it will only draw 5A of current. As far as current goes, the device calls the shots, and the adapter will have to do less work.
• Current too Low – If the adapter has the correct voltage, but the adapter’s rated current is lower than what the device input, then a few things might happen. The device could power on and draw more current from the adapter than it’s designed for. This could cause the adapter to overheat or fail. Or, the device may power on, but the adapter may not be able to keep up, causing the voltage to drop (see voltage too low above). For laptops running on undercurrent adapters, you might see the battery charge, but the laptop is not powering on, or it may run on power, but the battery won’t charge. Bottom line: it’s a bad idea to use a lower current rating adapter since it could cause excess heat.

You would expect to see all of the above based on a simple understanding of polarity, voltage, and current. What these outlooks don’t take into account is the various protections and versatility of adapters and devices. Manufacturers may also build a bit of a cushion into their ratings. For example, your laptop may be rated for an 8A draw, but in reality, it only draws around 5A. Conversely, an adapter may be rated at 5A but can withstand currents up to 8A. Also, some adapters and devices will have voltage and current switching or detecting features that will adjust the output/draw depending on what’s needed. And, as mentioned above, many devices will automatically shut down before they cause damage—not that I promise this, however.

That being said, I don’t recommend fudging the margin under the assumption that you can do the equivalent of driving 5 MPH over the speed limit with your electronic devices. The margin is there for a reason, and the more complicated the device, the more potential for something to go wrong.

Do you have any cautionary tales about using the wrong AC/DC adapter? Warn us in the comments!

P.S. Final Thoughts

Wall adapters that give you a USB port for charging aren’t nearly as tricky. Standard USB devices have a 5 V DC voltage and a current of up to .5 A or 500 mA for charging only. This is what allows them to play nice with the USB ports on your computer. Most USB wall adapters will be 5 V adapters and have a current rating well over .5 A. The iPhone USB wall adapter I’m holding in my hand right now is 5 V / 1 A. You also don’t have to worry about polarity with USB. A USB plug is a USB plug, and all you usually have to worry about is a form factor (e.g., micro, mini, or standard).

Furthermore, USB devices are smart enough to shut things down if something isn’t right. Hence, the oft-encountered “Charging is not supported with this accessory” message.

If you’re like most people, you probably have a collection of old power adapters and chargers that you’ve held onto, even though the devices they used to power are long gone. You probably thought that someday, one of them might come in handy:

This article will help you figure out if an adapter is compatible with a given device.

A little terminology

Before we begin, let’s make sure we’re using the same words to refer to the different “plugs” on an adapter or charger…

• By plug, I mean the part of the adapter or charger that you plug into the wall.
• By connector, I mean the part of the adapter or charger that you plug into the device.

With that out of the way, let’s begin!

How to tell if a power adapter or charger is right for your device

Step 1: Is the adapter’s polarity correct for your device?

Although you could do steps 1 and 2 in either order, I prefer to get the “device killer” question out of the way first. That question is: Does the connector’s polarity match the device’s polarity? Simply put, you want to find out which part of the connector is positive and which part is negative.

In DC current, which is the kind of current that an adapter provides, the polarity determines the direction in which current will flow through the device. You do not want current to flow into your device in the reverse direction.

Here’s a connector and its parts. The sleeve is the outer metal part, while the tip is the inner metal part:

Both your adapter and device should have some kind of label or tag that indicates their polarity. It should be either negative sleeve/positive tip, which is indicated by this symbol…

…or positive sleeve/negative tip, which is indicated by this symbol:

Are the polarity markings on both the adapter and the device are the same?

• Yes:

  If the polarity markings on both are the same, you can proceed to the next step.

• No:

  If the polarity markings are different, DO NOT proceed to the next step, and definitely DO NOT plug the connector into the device.

• If the are no polarity markings on the adapter:

  See the SPECIAL BONUS SECTION at the end of this article.

Step 2: Does the adapter’s connector fit into your device?

With the adapter’s plug NOT plugged into an outlet, can you plug the connector into the device?

• Yes:

  If the connector fits, you can proceed to the next step.

• No:

  If the connector doesn’t even fit, you can be pretty certain that this adapter isn’t going to work for the device.

Step 3: Do the voltage and current coming from the adapter match the voltage and current required by the device?

If you’ve reached this step, you’ve now taken care of the simple matches: The adapter will push current into your device in the right direction, and the connector fits.

Now it’s time to look at the numbers, namely voltage and current.

1. Look at the voltage (measured in volts, or V for short) and current (measured in amperes, or amps or A for short) marked on the adapter.
2. Look at the same values marked on the device.

Do the voltage and current values on the adapter and device match?

• Yes:

  If the numbers match, you’re good! You can use the adapter to power the device.

• No, both numbers don’t match:

  Don’t use the adapter to power the device.

• No, one of the numbers matches, and one doesn’t:

  If only one of the numbers doesn’t match, don’t write off the adapter as incompatible yet. Consult the table below:

If the voltage (V) doesn’t match…

MMMMAYBE.

Your device might work, but it also might work unreliably.

Simpler devices, where electricity is converted directly into some kind of result (such as a light, or a speaker) are more likely to work than more complex ones (such as a hard drive, or anything with a processor).

NO! WILL PROBABLY RUIN YOUR DEVICE.

Your device might work. The additional voltage may overheat and damage your device.

If the current (A) doesn’t match…

NO! WILL PROBABLY RUIN YOUR ADAPTER.

Your device might work. Your device will attempt to draw more current than the adapter is rated for, which may overheat and damage the adapter.

GO FOR IT!

The adapter’s current rating states the maximum that it’s capable of delivering.

Your device will work. It will draw only the current it needs from the adapter.

SPECIAL BONUS SECTION:
What if the adapter doesn’t have polarity markings?

Believe it or not, it happens. In fact, I have one such adapter, pictured below:

As you can see, its label section lists a lot of information, but not the polarity. This means you’ll have to determine the polarity yourself, or you can take a leap of faith.

If you want to determine the adapter’s polarity yourself

If you want to determine the polarity yourself, you’ll need a voltmeter. Set it up to read DC voltage in the range of the adapter. In the case of the adapter above, it’s rated to output 12 volts (V), so I set my meter to read a maximum of 20 V. I put the positive probe inside the connector so that it made contact with the tip, and touched the negative probe to the sleeve. A positive number appeared on the display:

With the positive probe touching the tip and the negative probe touching the sleeve, a positive voltage means that current is flowing from the tip to the sleeve, which in turn means that the tip is positive and the sleeve is negative.

If the number were negative, it would means that current was flowing from the sleeve to the tip, which in turn means that the sleeve is positive and the tip is negative.

In fact, when I put the positive probe on the sleeve and the negative probe on the tip of the same adapter, this is what happened:

Note that the voltage reported is negative. In other words, the current appears to be flowing backwards  — from the negative probe to the positive probe —because I had the probes backwards. Once again, this indicates that current is flowing from the tip to the sleeve, which means that the tip is positive and the sleeve is negative.

If you want to take a leap of faith

If you don’t have a multimeter handy, you can always take a leap of faith and assume that your adapter has a positive tip and a negative sleeve, which is how most adapters are designed. The tip is well-protected and difficult to touch by accident. Since current flows from positive to negative, you prevent accidental shorts and electrocution by making the hard-to-reach tip positive and the easy-to-reach sleeve negative.