Look on the back of your laptop charger and you’ll find a mess of symbols and numbers. We’d bet you’ve looked at them before and gleaned little or no understanding from what they’re telling you.
These symbols are as complicated as the label on the tag of your shirt that have never taught you anything about doing laundry. They’re the marks of standardization and bureaucracy, and dozens of countries basking in the glow of money made from issuing certificates.
The switching power supply is the foundation of many household electronics — obviously not just laptops — and thus they’re a necessity worldwide. If you can make a power supply that’s certified in most countries, your market is enormous and you only have to make a single device, possibly with an interchangeable AC cord for different plug types. And of course, symbols that have meaning in just about any jurisdiction.
In short, these symbols tell you everything important about your power supply. Here’s what they mean.
How did every power supply end up plastered with hieroglyphics? It works like this; Acme Corp wants to sell a Thingamajig in Benchoffistan, so the company sends a pallet of Thingamajigs there. The Customs officer in Benchoffistan looks at this pile of goods and says “how will I know this thing is safe for my citizens to use? You must have appropriate certificates that say this product is allowed to be imported.” And just like that, an industry called “Market Access” is born.
Market Access deals with all kinds of problems: logistics, politics, taxes and tariffs, labels and user manuals, materials, timing, and even occasionally palm greasing. Every country has their own nuances, and there are some companies who specialize in helping negotiate this minefield. Russia requires special testing if a device uses encryption or connects to telecommunications equipment (BLE and WiFi both count). Many countries require in-country testing. Most require an in-country representative of the company to handle filings and communication. Some have lead times in the months.
The first thing you’ll see on every power supply is the Input and Output. The input is almost always “100-240V~50-60Hz. The world runs power to outlets in this range. It means that as an input, the plug expects to be connected to that range of input voltage and frequency. The United States uses 120V/60Hz, Europe uses 230V/50Hz, so it’s nice that the input has a range within all of the countries.
The output line has three pieces of information: the output voltage (typically 5V, 9V, 12V), a solid line over a dashed line indicating DC or a ~ indicating AC, and a current rating, usually in hundreds of milli-amps for smaller blocks that plug in, and amps for supplies where the brick is separate from the plug. When replacing a power supply, you’ll want to match the output voltage, match the AC/DC output, and the output amperage must be at least as big as the previous supply and it can be bigger. That number is just the maximum the supply is rated for, not how much it will deliver.
The next piece is the polarity. This looks like a circle with a + in it, a circle with a – in it, and a C in the center. Almost always, the – will point to the C and the + will point to a dot inside the C. This means that the plug has – (ground) on the outside and positive voltage on the inside. Some older plugs don’t conform to this, so you should always check before you uses a supply.
The house symbol means it’s meant for indoor use only, and the square inside a square means that the mains electricity is double insulated. The X through the garbage can means it should not be disposed of normally but instead recycled with other electronics.
There a few big companies that do the testing that have their own icons. It lends validity to the rest of the symbols if you can call up these companies and verify from a single source if they really do have each certificate.
You’ll most often see the UL symbol. UL is Underwriters Laboratories, which is a safety organization. They have a barrage of standard tests that they will run against the device to make sure that it is safe. In most cases, a UL certificate isn’t required for sale, but if your house burns down and it’s because of a non-UL listed supply blowing up, then the insurance company is going to put up a fight because you weren’t using safe equipment in your home. Many large retailers will require that your device be listed as well, since they don’t want to deal with any potential recalls or lawsuits from bad products. Next to each UL symbol should be a license number.
This is a good point to mention that many of these marks may be fake — I’ve run into that when sourcing USB power supplies for a product. Customs agents are going to see the symbol and may not follow up to see if the appropriate certificate actually applies to that product, so it’s not uncommon to look up a UL listing number and see pictures of a similar product. There’s some sort of balance, then, when investigating a product’s certificates. You want to see relevant certs and make sure they are legitimate, but you can’t check everything you touch.
The rest of the symbols are going to be country specific, and there are a lot of countries with strange requirements for testing. Power supplies are one thing, but adding intentional radio emissions, like a WiFi or Bluetooth product, steps it up to a whole new level of testing and certifications that are beyond the scope of this article.
In general, the more certificates you see on a product, the less sketchy it is, and the bigger the company manufacturing the product. Small manufacturers aren’t going to have the money or interest to pursue a lot of certifications, and may be flying under the radar on a lot of their sales. It’s also an indicator that the product doesn’t change frequently, and that they’ve locked down their assembly line. You won’t see the manufacturer removing critical components to shave costs at the expense of safety.
Polarity symbols are a notation for electrical polarity, found on devices that use direct current (DC) power, when this is or may be provided from an alternating current (AC) source via an AC adapter. The adapter typically supplies power to the device through a thin electrical cord which terminates in a coaxial power connector often referred to as a "barrel plug" (so-named because of its cylindrical shape). The polarity of the adapter cord and plug must match the polarity of the device, meaning that the positive contact of the plug must mate with the positive contact in the receptacle, and the negative plug contact must mate with the negative receptacle contact. Since there is no standardization of these plugs, a polarity symbol is typically printed on the case indicating which type of plug is needed.
The commonly used symbol denoting the polarity of a device or adapter consists of a black dot with a line leading to the right and a broken circle (like the letter "C") surrounding the dot and with a line leading to the left. At the ends of the lines leading right and left are found a plus sign (+), meaning positive, also sometimes referred to as "hot", and a minus sign (−), meaning negative, also sometimes referred to as "neutral".
The symbol connected to the dot (usually the symbol found to the right) denotes the polarity of the center/tip, whereas the symbol connected to the broken circle denotes the polarity of the barrel/ring. When a device or adapter is described simply as having "positive polarity" or "negative polarity", this denotes the polarity of the center/tip.[citation needed]
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