Fundamentally, you need to know four basic things when selecting a new power supply for a broken electronics device. The voltage coming from the socket at the wall (Europe/Japan/US), the voltage output, the current, and the type of connector (USB Type-B mini, barrel, USB-C). What most of us don’t know is that there are some other areas of importance. Buying a cheap power supply (or “wall wart”) can cause problems (interference with the normal operation of other devices) and even destroy your device.

Not everything is listed on the label as in (Figure 1). Here’s what you need to know:

Input voltage range and line frequency – The AC line voltage and operating frequency are different in some countries. In Figure 1 shows that the power supply can accept anything between 100 and 240 volts at 50 or 60Hz. The regulator inside this “brick” type power supply will put out a DC voltage of 19.5V at 3.34A.

Output voltage and current– This should be listed on the label and is what an electronic device needs to operate. In Figure 1, I can tell that if I have any other power supply that can output 19.5V and 3.34A, then the device will operate. The connector might not match, but an adapter could work around the miss-match. The voltage level is a primary consideration, not the wattage.

Although watts and voltage are related, due to internal regulators, it’s more important that you match the voltage as close as possible. Current can be a little higher without danger. Here’s why:

Variance in the output voltage by 5%, either way, may not be an issue if the replacement actually does provide the voltage as advertised. A crate full of the same power supplies will still have power supplies that vary slightly from one another (unless it’s for something that needs a precision voltage level). So if you lost the power supply in Figure 1, a replacement that outputs 19.4V or 19.6V probably won’t matter, especially if the electronic device that needs power has internal regulators of its own. But you can only push it so far. It’s more important to match the output voltage as accurately as possible rather than worry about a current source that’s too high.

In replacing the power supply in Figure 1, getting a power supply that delivers over 3.34A by half an amp is still safe. (Keep it to 20% over at most, because the power supply you’re replacing is likely already oversized by 25 – 30%.)

Think of electricity as a hose. The flow coming out is current, and the voltage is the available pressure. Your electronic device is a precise hose nozzle. If your hose pressure is too low, the nozzle won’t deliver the amount of water at the distance you need to squirt a plant 5 yards away. (Stick with me, this makes sense if you’ve ever watered a garden by hand.) The flow out of the nozzle is adjustable by the trigger on the nozzle, so if you have the water spigot turned up full blast, you can still control the flow out of the nozzle to a trickle if that’s what you need. The water flow (volume) out of the hose correlates to current flowing to your device. Even so, it’s not good to go too high on the current with your replacement power supply.

Selecting a replacement switching power supply to get as close to the original values as possible is wise because you don’t know the component ratings in the device’s battery charging circuit. For example, you can overheat a laptop, which can damage components. Heat-damaged components causes them to permanently operate at lower performance levels unless your laptop is designed with circuit protection devices (that cost extra). Your battery’s life could get reduced to 1 hour before needing recharging, versus the 4 – 6 hours that it used to hold. I know, because I did this once. I needed a power supply for my laptop immediately and used the one from my old laptop for a few minutes to keep it from shutting down. Bad idea, because after using a power supply with an output voltage that wasn’t a close match, my laptop didn’t hold a charge for more than 5 minutes after that.

A lot of people probably have misplaced a laptop power supply, and it’s a hassle to try and find one quickly. Nowadays, it’s much easier to find replacement power supplies for USB-charging devices. About a decade ago, European standards groups agreed to make smartphones that only use standard USB chargers with USB-micro Type-B connectors. With the new USB Type-C (connector) and USB-Power Delivery (USB-PD) standards, hopefully, that harmonization of standards for powering smartphones will be applied to laptops and any other device that needs up to 100W, which together, USB-C and USB-PD can deliver. Until then, get a good backup power supply or, if you have several laptops of varying ages, at least label power supplies to match corresponding laptops, especially if you tend to buy the same brand. It’s really easy to grab that black laptop power supply off the library table when 4 other people have a similar type of laptop. At the very least, put a dot of matching nail polish on your laptop and it’s power supply, which can go a long way.

What if my device needs current that’s too low for the power supply?

A minimum current rating may also be mentioned in your electronic device’s specifications. A switching power supply delivers power in tiny pulses when it’s supplying current to a very low load. The minimum energy level of the pulses can be more than the external load can use, which can cause a pulse width modulation IC in the electronic device to shut down and restart in a cycle akin to experiencing hiccups. A power supply’s minimum load specification offers a margin that ensures enough load to run without initiating hiccups. Learn more about minimum load requirements in the application note Minimum – Why is it required and What Happens if I Don’t Have Enough? By SL Power (PDF)

Operating temperature

If you’re selecting a power supply for a different application, perhaps something that’s supposed to operate well in a cabinet in the desert, you will need to check the operating temperature range. High temperatures will elicit different responses from almost all electronic components and devices (unless they’re rated for space travel or military exercises). You can bake cookies inside a car in the summer in some places, and if your power supply and device are not rated for operating in the heat, then you will get erratic behavior at best and catastrophic failure at worst. Even storing a sensitive device in extreme heat—heat that it’s not rated at for in storage—can cause performance loss. E.g., try not to leave your devices in a hot car, and at the least, turn them off.

Regulations

Agency safety approvals are a requirement for a reason. RoHS, CE, FCC, and additional compliance requirements based on life-safety applications (e.g., medical, aviation) are meant to safeguard people and the environment, protect other devices from interference, and provide certified levels of efficiency, among other things. You can ensure a higher level of genuine parts if you buy only from authorized distributors. Counterfeit or cheap power supplies from overseas often display certification symbols without compliance or safety considerations. Is it worth it to buy a cheap power supply for a $1,000 laptop? Online retail outlets can sell counterfeit power supplies that look like the original equipment.

Enclosures

Another consideration when selecting a power supply is the enclosure’s type and size. Enclosures might need to be water-resistant or perhaps small enough to fit in a certain spot in your application.

Efficiency levels

Power supplies have proliferated as electronics devices have flooded the world. Efficiency became a concern, and higher levels of efficiency became requirements. The latest level of efficiency is Level VI. To learn more, see Energy Efficiency Becomes Critical for External Power Supplies.

Thermal management

Some power supplies might require a cooling system such as a temperature-controlled fan or more complex cooling systems such as liquid-cooled systems. If you plan to operate your device where the power supply will need to be in a hotter environment, consider getting one that has an integrated fan. Maintain your devices by removing the periodic buildup of dust (which can act like that fluffy pink stuff they shoot into your attic to retain the heat/cooling in your house). Laptops tend to have small fans inside. Keep the vents clear so airflow can be maintained over the internal components. For example, don’t put the laptop on a comforter or a pillow for long, because it can overheat your laptop. If your power supply has a fan, keep it clean, out of direct sunlight, and don’t let it get blocked from airflow.

Let’s hope the above tips help. It’s good to be aware of whether your device is getting hot while you’re using it. It’s even possible for a software update to cause your device to overheat, so keep an eye on overheating after allowing updates. Once an electronic device experiences high heat for too long, performance and reliability are changed forever. As soon as you notice that your device is hotter than normal, make sure that it’s not just operating in a bad place (sunlight, vents blocked, etc.) or shut it down.