How DC/AC Power Inverters Work


Car Gadgets Image Gallery An inverter like this 200-watt unit is easy to use and install. It's very portable, but its best suited for powering small electronic devices. See more pictures of car gadgets.
DC AC Power Inverters

­Everyone uses some kind of electronic gadget while in their motor-home, SUV or car. You might listen to your MP3 player, check for directions on your global positioning system (GPS) or play a portable video game. These types of electronic devices can be recharged or powered by plugging them into the cigarette lighter (or power port) in your vehicle.

But what if you want to use something a little more elaborate while you're on the open road? Maybe you want to make toast, watch an LCD TV, or perhaps even write an article on your laptop computer. These devices plug into regular wall outlets, not cigarette lighters. Making sure your electronic gear gets the juice it needs while on the road isn't a simple matter of finding the right adapter. You need a power inverter.

­Power inverters convert direct current (DC), the power that comes from a car battery, into alternating current (AC), the kind of power supplied to your home and the power larger electronics need to function. What kind of power inverter is the right one for the job? How do you install one? And how exactly does an inverter change the current from one form to another? In this article, we'll explore all the positives and negatives of DC to AC power inverters.

Why Do I Need To Convert from DC to AC?

Most cars and motor homes derive their power from a 12-volt battery. In some cases, a heavy-duty 24-volt battery might be used. It's important to know your vehicle's voltage because the voltage rating of the inverter you select should match the voltage of the battery. In either case, the battery provides direct current. This means that the current flows continuously from the negative terminal of the battery, through the completed circuit and back to the positive terminal of the battery. The flow is in one direction only, hence the name direct current. The ability to provide direct current power is inherent to the nature of batteries.

Direct current is very useful, but batteries can generally only provide relatively low-voltage DC power. Many devices need more power to function properly than DC can provide. They're designed to run on the 120-volt AC power supplied to homes in the U.S. Alternating current or AC, constantly changes polarity, sending current one way through the circuit, then reversing and sending it the other way. It does this very quickly -- 60 times per second in most U.S. electrical systems. AC power works well at high voltages, and can be "stepped up" in voltage by a transformer more easily than direct current can.

An inverter increases the DC voltage, and then changes it to alternating current before sending it out to power a device. These devices were initially designed to do the opposite -- to convert alternating current into direct current. Since these converters could basically be run in reverse to accomplish the opposite effect, they were called inverters.

Up next, how do inverters invert?

Making Direct Current Alternate

This unit provides 1,500 watts of pure sine wave power. The wave is probably better than that provided by your home electrical system, but the manufacturer's price for one of these is almost $1,000.
This unit provides 1,500 watts of pure sine wave power. The wave is probably better than that provided by your home electrical system, but the manufacturer's price for one of these is almost $1,000.
DC AC Power Inverters

The earliest AC power inverters were electro-mechanical devices. Direct current would flow down one end of a circuit with an electromagnet. As soon as the current hit the magnet, the magnet would activate. This would pull a wire attached to a spring arm, forcing the wire to contact the circuit. This would change the flow of the current to the other side of the circuit, cutting power from the electromagnet. As soon as the magnet released, the spring would snap the wire back, allowing the current to flow on the other side of the circuit, once again activating the magnet. These old inverters were known for making a buzzing sound.

Modern inverters use oscillator circuits to accomplish the same process. They're made with transistors or semiconductors, so there's no longer the need for a spring arm flipping back and forth to alternate the current.

It's not quite as simple as that, however. Alternating current forms a sine wave. The output of an inverter is a very square wave, not like the smooth, round wave of a perfect sine. Some devices are inherently sensitive to the signal produced by an AC wave. Typically, these are devices that receive or broadcast some kind of signal, such as audio or video equipment, navigation devices or sensitive scientific equipment. You can see or hear the square waveform on a television as lines on the screen or a steady buzz or hum.

Cleaning up the sine wave requires a series of filters, inductors and capacitors. Inexpensive inverters have little or no filtering. The alternating current they produce has a very square wave, which is fine if you just want to make coffee or run something with a simple electric motor. If you need a smoother sine wave, you'll need an inverter with better filtering. Of course, better filtering also costs a little more. Inverters can get extremely expensive, even costing thousands of dollars, that is, if you're looking for an inverter with a smooth sine. The good news: Given a large enough budget, you can purchase an AC power inverter that produces virtually perfect AC sines. In fact, some high-end DC to AC inverters can make sine waves that are even smoother than the AC power supplied to your house.

In the next section, we'll learn how to pick the right inverter for the job.

Watts, Peaks and Surges

The first step in selecting an inverter is to match the inverter to the voltage of the battery you'll be using for power. In the majority of cases, you'll be using a 12-volt battery, so you would want to select a 12-volt inverter.

The next step is to determine which devices you plan to power with the inverter. Look for a label somewhere on each device that tells you the wattage it requires to operate. The wattage rating of your inverter must exceed the total wattage of all the devices you plan to run simultaneously. For instance, if you wanted to run a 600-watt blender and a 600-watt coffee maker at the same time, you'd need an inverter capable of a 1,200-watt output. However, if you knew you would never be making coffee and fruit smoothies at the exact same time, you'd only need a 600-watt inverter.

Unfortunately, things aren't quite that simple. Devices that have electric motors, as well as some televisions, draw a higher wattage than their normal operating wattage rating when they first start up. This is known as peak or surge, and this information should also be listed on the device's label. Most inverters also have a peak rating, so make sure the inverter's peak rating is higher than the peak wattage of the device you intend to power. Microwaves are a special case. As an example, you may know that your microwave is a 500-watt microwave. This is actually the cooking wattage. The power wattage might be twice that amount. Again, check the label on the device to make sure.

If you plan to run your inverter through the cigarette lighter in your car, it's a safe bet that you won't be using any high-wattage devices. In fact, if you try to pass more than about 400 watts through a cigarette lighter connection, it will fail -- and it might even start a fire in your vehicle.

The final specification to look for is the wave output of the inverter. If you'll be powering any of the equipment that is sensitive to square waves, look for an inverter with a "perfect sine" wave output. Be prepared for sticker shock -- a perfect sine inverter can cost almost 10 times as much as the same wattage inverter with a modified sine output. Modified sine means that the current is run through some filtering, so it isn't a square wave, but it isn't totally smooth either.

On the next page, we'll explain how to install an inverter.

Inverter Installation

If you have serious power requirements, you might need an inverter like this. It requires a heavy-duty 24-volt battery and has an output of 3,500 watts. You could run almost anything with this, including air-conditioners and other large appliances.
If you have serious power requirements, you might need an inverter like this. It requires a heavy-duty 24-volt battery and has an output of 3,500 watts. You could run almost anything with this, including air-conditioners and other large appliances.
DC AC Power Inverters

Inverters are very easy to install. Most of them are "plug and play" devices, especially smaller, low-wattage inverters. These inverters have a cable with a plug that fits into the cigarette lighter on your car or truck. They're meant to be portable, so there's no other mounting to be done.

­If you purchase an inverter that allows higher wattages, proper installation becomes a bit more critical. Below 400 watts, the cigarette lighter connection is still a possibility, but wattages above that require direct connection to the battery. The inverter's input cables have clips that can be attached to the terminals of the battery, similar to a set of jumper cables. If the installation is to be permanent, the cables can be bolted to the terminals. The inverter itself can be mounted anywhere, although it should be in a place with good air flow. Inverters generate a fair amount of heat, and they use cooling fans and heat dissipation fins to prevent overheating. Larger, heavier inverters have mounting holes in their chassis so they can be bolted to any surface. Obviously, with a permanent installation, you'll probably want to bolt your converter in place, but this isn't absolutely necessary. It's possible to simply place the inverter in a secure, stable position, clip the leads to the battery and plug in.

Just what does an inverter look like, anyway? Well, the smallest inverters can fit in your pocket, while higher-wattage models are roughly the size and weight of a large dictionary. As a general rule: The higher the wattage, the larger and heavier the inverter. At the top of the inverter wattage scale, some inverters can be more than two feet long and weigh over 30 pounds.

Modern inverters have some built in safety features that make them even easier to use. Some models sound an alarm when the battery's voltage gets too low. This is more of a convenience, but depending on what sort of equipment you're powering, it could also be a valuable safety feature. Inverters typically have automatic shut-off capabilities, too. If the unit detects a current overload or an overheating situation, it will shut down to lessen or prevent the chance of a fire. Inverters can also shut off in the event of a short circuit, such as a piece of metal falling into the chassis or the inverter getting wet. Short circuit shut-off is an effective way to prevent electrocution.

So how much is all of this going to cost, you ask? You can buy a modified sine inverter rated for continuous power of 200 watts for about $25 and the price of a 6,000-watt modified sine inverter can approach $1,000. Pure sine inverters cost much more -- these can be more than $200 for an inverter rated at just 300 watts.

If you'd like to read more about automotive electronics and other related topics, follow the links on the next page.

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Sources

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  • Public Broadcasting Service. "The American Experience: AC/DC: What's the Difference?" (Jan.11, 2009) http://pbs.gen.in/wgbh/amex/edison/sfeature/acdc.html
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  • Whitney, Chad. "How to Choose a Power Inverter for your RV or RoadTrip Vehicle." Roadtrip America. Aug. 14, 2005. (Jan. 9, 2009) http://www.roadtripamerica.com/dashboarding/Power-Inverters.htm