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You might have had warts on your skin. They’re formed by viruses making a new home. If you’ve ever had one, you probably didn’t like it and found it hard to get rid of. Walls often have warts, too, although you probably didn’t identify them as such. “Wall Wart” is engineering talk for the black plastic protrusions you often find attached to the exterior of a wall outlet in modern homes. If you call them anything at all, it’s most likely “AC power adapters.” A typical wall wart is shown in Figure 1.
Figure 1 – A Typical Wall Wart Wall warts provide a handy, portable and easy to use conversionary power source for small electronic devices, including lamps, small appliances, and various modern day electronics. If you’re like me, you have lots of them scattered on the walls of your home and office. Most people come to use them when a need arises, say you bought a scanner for your computer. Beyond that they’re usually not given much thought, but today we’re going to explore them a bit. Suppose you’re an engineer and you’ve been asked to design an electronic product for household use. The product only requires 12 volts of direct current (DC) to operate, but you know that the typical home is wired to supply 120 volts of alternating current (AC). What can be done to rectify the discrepancy? Well, there are two distinct choices. One of the choices is to design electronic circuitry capable of converting 120 volts AC into 12 volts DC, then place it inside the product. But is this the best choice? Not really. It takes time to design custom circuitry, and doing so will add substantially to the design time and final cost of the product. This is especially true if the circuitry is produced in small quantities. Besides, if the electronic product is small, there may not be enough room inside to accommodate this type of circuitry. The smarter choice would be to buy a wall wart from another company that specializes in manufacturing them. They’re produced in huge quantities, so the cost is low. They also come in standard voltages, like 12 volts DC. And because the wall wart is external to the product housing, space inside is no longer a concern. It couldn’t be any easier or cost effective. Just plug the wall wart into your home electrical outlet, then plug in the product’s 12 volt DC cord. Done! Next time we’ll take a look at what’s going on inside your basic wall wart to see how it works. ____________________________________________
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Posts Tagged ‘power cord’
Ever Had a Wall Wart?
Sunday, August 28th, 2011
Wire Size and Electric Current – Joule Heating
Sunday, March 20th, 2011| Ever take a peek inside the toaster while you’re waiting for the toast to pop up? If so, you would have noticed a bright orange glow. That glow is produced when the toasting wires heat up, which in turn creates a nice crusty surface on your bread or waffle. It’s the same phenomenon as when the filament inside an incandescent bulb glows. The light and heat produced in both these cases are the result of the Joule, pronounced “jewel,” effect at work.
To understand Joule heating, let’s first refresh our memories as to electrical current resistance. We learned previously that wire is not a perfect conductor, and as such resistance to flow is encountered. This resistance causes power to be lost along the length of wire, in accordance with this equation: Power Loss = I2 × R Where I is the electric current flowing through a wire, and R is the total electrical resistance of the wire. The power loss is measured in units of Joules per second, otherwise known as watts, “watt” denoting a metric unit of power. It is named after the famed Scottish mechanical engineer, James Watt, who is responsible for inventing the modern steam engine. A Joule is a metric unit of heat energy, named after the English scientist James Prescott Joule. He was a pioneer in the field of thermodynamics, a branch of physics concerned with the relationships between different forms of energy. Anyway, to see how the equation works, let’s look at an example. Suppose we have 12 feet of 12 AWG copper wire. We are using it to feed power to an appliance that draws 10 amperes of electric current. Going to our handy engineering reference book, we find that the 12 AWG wire has an electrical resistance of 0.001588 ohms per foot, “ohm” being a unit of electrical resistance. Plugging in the numbers, our equation for total electrical resistance becomes: R = (0.001588 ohms per foot) × 12 feet = 0.01905 ohms And we can now calculate power loss as follows: Power = I2 × R = (10 amperes)2 × (0.01905 ohms) = 1.905 watts Instead of using a 12 AWG wire, let’s use a smaller diameter wire, say, 26 AWG. Our engineering reference book says that 26 AWG wire has an electrical resistance of 0.0418 ohms per foot. So let’s see how this changes the power loss: R = (0.0418 ohms per foot) × 12 feet = 0.5016 ohms Power = I2 × R = (10 amperes)2 × (0.5016 ohms) = 50.16 watts This explains why appliances like space heaters and window unit air conditioners have short, thick power cords. They draw a lot of current when they operate, and a short power cord, precisely because it is short, poses less electrical resistance than a long cord. A thicker cord also helps reduce resistance to power flow. The result is a large amount of current flowing through a superhighway of wire, the wide berth reducing both the amount of power loss and the probability of dangerous Joule heating effect from taking place. Our example shows that the electric current flowing through the 12 AWG wire loses 1.905 watts of power due to the inconsistencies within the wire, and this in turn causes the wire to heat up. This is Joule heating at work. Joule heating of 50.16 watts in the thinner 26 AWG wire can lead to serious trouble. When using a power cord, heat moves from the copper wire within it, whose job it is to conduct electricity, and beyond, on to the electrical insulation that surrounds it. There the heat is not trapped, but escapes into the environment surrounding the cord. If the wire has low internal resistance and the amount of current flowing through it is within limits which are deemed to be acceptable, then Joule heating can be safely dissipated and the wire remains cool. But if the current goes beyond the safe limit, as specified in the American Wire Gauge (AWG) table for that type of wire, then overheating can be the result. The electrical insulation may start to melt and burn, and the local fire department may then become involved. That’s it for wire sizing and electric current. Next time we’ll slip back into the mechanical world and explore a new topic: the principles of ventilation. _____________________________________________
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