HomeElectrical SuppliesPower Supplies & AC/DC These durable, reliable power supplies and AC/DC converters from BK Precision are ideal for telecommunications, general electronic field service, school electronics, laboratory testing, powering up hobbyist's projects, and many other applications. For the DC Switching supply, choose from three different models to meet your DC voltage needs. What's the difference between AC, DC power? AC Power vs DC Power [Insert joke about the rock band here] You've probably heard the term “AC/DC” and figured it had something to do with electricity, but maybe you didn't know much else. If you ask any random person you meet about electricity, they may give an answer like “It's that magical lightning stuff that makes my lights turn on and my computer do things,” and if they want to sound smart they may include an anecdote about Benjamin Franklin flying a kite with a key tied to it during a lightning storm. Which, of course, is why instead of power lines, we all have kites with keys tied to them attached to our houses, just waiting for that stormy day to give us our much needed power.

If you ask any random person you meet about AC/DC in particular, there's a 99% chance they'll mention the Australian rock band that gave us such thought-provoking hits as “Highway to Hell”, “Hell’s Bells” and probably some other songs with Hell in the title. Fun fact: the band's name comes from the sister of band members Malcolm and Angus Young, who saw the initials on a sewing machine. To the band, it symbolizes the “raw power” of their music. There, now you have something to talk about at your next social gathering. But for realsies, what's the actual meaning behind AC/DC? What's the difference between them? Well, you're here already, so we may as well tell you. What is AC Power? AC stands for alternating current, which means the electrical current frequently reverses direction. AC electricity is measured according to its cycles, with one complete cycle being counted each time a given current travels in one direction and then doubles back on itself. Like it's doing laps in the most dangerous swimming pool ever.

An electrical current is able to complete many cycles per second, and is then given its frequency rating based on that number. The unit of measurement for an electric cycle is “Hertz” (Hz), not to be confused with the Hertz Donut (Hz DnT), which is not nearly as delicious as it sounds. The typical frequency in North America is 60 hertz (Hz), which indicates that the current is performing 60 cycles per second. In Europe and many other countries, they usually stick to 50 hertz. Shut up, that's why! No, I dunno, it's probably the same reason they spell some words differently in the United Kingdom. AC power is the type of electricity most commonly used in homes and offices, and is extremely versatile because its voltage can be changed through a transformer to suit a variety of transmission needs. What is DC Power? DC is the abbreviation for District of Columbia, and it's a type of electricity that gets paid a lot of money by lobbyists to sit around and not do anything important.

Just kidding, DC is actually short for direct current, which is a type of electrical current that travels through a circuit in only one direction. window ac unit cheapSo instead of doing laps like our athletic friend AC, DC gets from one end of the pool to the other and says “Ehh, screw it.”cleaning air conditioner wall unit This is the type of electrical power that is produced by fuel cells, batteries, and generators equipped with commutators. portable ac power unitWhile DC power was the first type of electricity to be commercially transmitted, it has been widely replaced by alternating current (AC) electricity, and is now used primarily in electrochemical and metal-plating applications. So when you see “AC/DC” it refers to a device that will run on either type of current.

Using DC power to save energy -- and end the war on currents Using DC power to save energy -- and end the war on currentsThursday, November 15, 2012 - 5:00am Here I'm talking about a device that takes 100-240V AC in, and outputs 10-20V DC out. In terms of grounding, I'm referring to the grounding offered by a standard outlet (e.g., a NEMA-5-15R receptacle). In this space, many devices have a grounded (three prong) cable and many don't. I'm particularly interested in say laptop power warts (AC-to-DC converters, which are generally always external to the device). The older laptops I had used two-prong ungrounded plugs. The two most recent laptops, however (one Mac, one Dell) have both used grounded plugs. This is really annoying when traveling as many adapters and even outlets don't support the ground plug. Adding to the confusion, it seems that the same transformer, for the same model laptop, comes with a two prong plug in other countries: my Dell has a detachable power cord, and I've an otherwise identical brick for the same laptop sold in another country with a different cord that omits the grounding plug.

In all cases, the AD-DC bricks are fully plastic with no exposed metal parts. When designing such a device, what are the considerations that would cause a fully enclosed plastic line lump for something like a laptop have a grounding prong? For what reason would a fully enclosed plastic line lump for something like a laptop have a grounding prong? Switched mode power supplies use what is known as a "flyback converter" to provide voltage conversion and galvanic isolation. A core component of this converter is a high frequency transformer. Practical transformers have some stray capacitance between primary and secondary windings. This capacitance interacts with the switching operation of the converter. If there is no other connection between input and output this will result in a high frequency voltage between the output and input. This is really bad from an EMC perspective. The cables from the power brick are now essentially acting as an antenna transmitting the high frequency generated by the switching process.

To suppress the high frequency common mode is is nessacery to put capacitors between the input and output side of the power supply with a capacitance substantially higher than the capacitance in the flyback transformer. This effectively shorts out the high frequency and prevents it escaping from the device. When desinging a class 2 (unearthed) PSU we have no choice but to connect these capacitors to circuitry that is referenced to the input "live" and/or "neutral". Since most of the world doesn't enforce polarity on unearthed sockets we have to assume that either or both of the "live" and "neutral" terminals may be at a sinificant voltage relative to earth and we usually end up with a symmetrical design as a "least bad option". That is why if you measure the output of a class 2 PSU relative to mains earth with a high impedance meter you will usually see around half the mains voltage. That means on a class 2 PSU we have a difficult tradeoff between safety and EMC. Making the capacitors bigger improves EMC but also results in higher "touch current" (the current that will flow through someone or something who touches the output of the PSU and mains earth).

This tradeoff becomes more problematic as the PSU gets bigger (and hence the stray capacitance in the transformer gets bigger). On a class 1 (earthed) PSU we can use the mains earth as a barrier between input and output either by connecting the output to mains earth (as is common in desktop PSUs) or by using two capacitors, one from the output to mains earth and one from mains earth to the input (this is what most laptop power bricks do). This avoids the touch current problem while still providing a high frequency path to control EMC. So why are laptop PSUs from major repuatable vendors class 1 nowadays when they didn't used to be? (and when cheap crap often still isn't) I don't know for sure but I expect it's a combination of. Even touch currents below the legal limits can be problematic. Some people are unusually sensitive to electricity and can feel currents below the legal limit. Some electronics can also be damaged by currents below the legal touch current limit during hotplugging.

EMC regulations have got tighter over the years. So how dangerous is it to use a laptop power supply that has a ground pin without actually connecting it? Unfortunately that is impossible to answer without knowing details of the internal construction. It may just result in a slight increase in touch current, or EMC emmisions, or it may leave you a "single fault" away from getting a shock off the mains. Not sure why, but Dell has its laptops grounded. This presents more problems than just those related to plugs, since the PC internal ground is also ground for external connectors such as USB, video, etc. When you connect external usb devices, especially data acquisition and audio equipment, you often find that the response can be different when the laptop is running from batteries vs. plugged into the wall, and these problems go away when you use an isolation plug or transformer. I always carry an isolation plug (along with my international converter plug kit) whenever I travel.