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It can sometimes be useful to have a way to use a device that is normally powered by the house current while out in the car. Perhaps the device in question does not have a 12-volt car power adapter available, or perhaps the owner does not have that adapter because they normally use the apparatus in the house. Regardless of the reason, there are times when it would be nice to be able to power those devices using the car’s battery or alternator.
Power inverters were created for this very purpose: they are designed to convert a car’s 12-volt DC power to 120-volt AC, allowing an electronic device to be plugged in and operated just as if it were being used in the house.
The reason these devices are referred to as "inverters" is a bit of a play on the word "converters." A power converter is a device that changes one type of voltage to another; all inverters are also converters. However, most of the time, converters change a higher voltage to a lower one, such as would be done by a cell phone charger.
An inverter cconverts a lower voltage to a higher one; specifically, it converts the 12 volts a car operates on to the normal 120-volt house current. Although a car power inverter is not the only type of inverter that exists, it is the most common type of inverter and the focus of this guide.
Understanding Volts, Amps, and Watts
To understand how inverters work and to be able to select the right inverter for a particular application, it is helpful to have some understanding about the electronic terms used to describe them.
Electrical current is the movement of electrons from one atom to another in a material. Looking in a high school chemistry textbook shows that atoms are made up of three types of materials: protons, neutrons, and electrons. Protons and neutrons form the nucleus of the atom, while the electrons run around the outside, like planets around a sun. Protons are positively charged; electrons are negatively charged; and neutrons have no charge at all.
It is these negatively charged electrons, moving from atom-to-atom, that provide the electrical power that all electronics use. There are various terms used to describe this movement, telling us how much work those electrons can do in an electronic apparatus.
"Volts" is the term used to describe the amount of potential an electrical charge has. To visualize this term, the metaphor of a garden hose can be used. Volts are like the amount of pressure applied to water passing through the hose. The higher the pressure, the higher the voltage. Normally, cars operate at 12 volts; small electronics, such as cell phones and laptop computers, operate at 5 volts; and home appliances operate at 120 volts.
"Amps" refers to the amount of voltage that exists. Going back to the example of the garden hose, amps refers to the amount of the water that is passing through the hose. This can be increased in two ways: by increasing the speed at which the water moves through the hose, or by making the hose wider to allow more water to pass through it.
"Watts" refers to the amount of voltage consumed by the device that is connected to the source of electrical power. In the example of a garden hose, this would be how much of the water is actually being used and how much is allowed to continue flowing through the hose.
Putting the Three Together
The relation between watts and amps is an important one. Products can be rated by watts or by amps, depending on the type of product and the manufacturer; this can be rather confusing for consumers. Typically, devices with motors have an "amp rating" to show off how powerful the motor is. However, if the voltage inverter is rated in watts, most people cannot tell if the inverter can power the device. Fortunately, two simple formulas can be used to convert amps to watts and vice versa:
Volts x Amps = Watts
Watts ÷ Volts = Amps
With these two formulas, anyone can convert from watts to amps and from amps to watts in order to determine how much power is actually required for a particular device and how much power the inverter produces. It is important to note that manufacturers are required to provide this information. Items like power supplies will have this information written on them, and for other devices, it will be listed in the specifications section of the owner’s manual.
It is also important to note that the conversion should be based on the output of the power inverter, not the input, as the output is the part that will be powering the device. Some manufacturers will rate their power inverters for both a maximum output and wattage. The unit cannot be used at the maximum power for more than a few minutes.
AC Voltage vs. DC Voltage
AC and DC form another pair of confusing terms that are important for understanding the function of inverters and converters. AC stands for "alternating current" and DC stands for "direct current." Direct current means that the voltage (electrons) only travels in one direction. Going back to the hose example, the water would only be flowing in one direction, i.e. from the spigot to the end of the hose. Alternating current means that the voltage constantly changes direction, spending as much time traveling from the spigot to the end of the hose as it does traveling from the end of the hose back to the spigot.
When AC voltage is measured over time, as shown on an oscilloscope, the voltage swings from positive to negative and back to positive again in a very consistent pattern. By comparison, when DC voltage is measured over time, it always stays positive by the same amount, not changing.
This change from positive to negative is accomplished smoothly in what is known as a sine wave. A sine wave can be visualized by thinking of trying to graph the movement of a pendulum. As the pendulum starts moving from the extreme end of its swing, it moves very slowly. The farther it moves, the faster it goes, until it reaches the exact middle of its range, when it moves the fastest. As it moves from there to the end of its arc of travel, it slows down again, until it comes to a stop at the end of its swing.
To graph this movement, comparing speed to time, the line would start at the top of the graph (the end of the swing), with almost no vertical movement, and a lot of horizontal movement. A time progressed, the speed would increase, even though time would march on at the same rate. So, in the exact middle of the pendulum’s arc of travel, the line on the graph would be vertical, with lots of speed and almost no time. Then it would swing the other way, reaching a point once again of lots of horizontal movement for time, with almost no vertical movement for speed.
This would create a smooth, curving line on the graph, oscillating from the top of the graph to the bottom and curving back up again, crossing over zero right in the middle of every swing.
Devices are designed to operate either on AC or DC voltage. Cars and small electronic devices all operate on DC current. Devices
that are designed to connect to the house current, such as appliances, all operate on AC current. If the right type of current is not supplied to the device, it will not work.
How Inverters Work
An inverter connects to the car’s power system, drawing 12-volt DC electricity from it. This can be accomplished through the cigarette lighter, through a direct connection to the battery, or by being permanently hard-wired into the car’s electrical system.
The electrical voltage that is drawn from the battery is then multiplied, usually by the use of a transformer. Since the inverter is starting with 12 volts and needs to boost it to 120 volts, the voltage must be multiplied ten times. To provide this much power, the amperage has to be divided by ten. Therefore, an inverter that is producing 1 amp of power at 120 volts AC needs to draw 10 amps of power from the car’s battery.
However, the transformer does not change the voltage from DC to AC. That requires additional circuitry. Older inverters did this with a device called a vibrator. This is an automatic switch that is actuated by heat. It is essentially a larger version of the "flasher," which is used to make a car’s turn signals work. Newer ones do this with oscillator circuitry. The major advantages of oscillator circuits are that they are not noisy, and they produce a cleaner sine wave.
A vibrator works by creating heat from the electrical current passing through it. This is easily accomplished with carefully sized electrical contacts inside the vibrator. Since the contacts are barely big enough for the necessary amount of amps of current to pass through, they heat up. These contacts are each made of two different types of metal, one on one side and another on the other side (called a "bi-metal contact"). Since each type of metal expands at its own particular rate, the contact bends as it heats. This causes the contact to break and connect to another contact. Through this action, the power switches from positive to negative and back to positive in a very consistent rhythm.
However, this is still not AC current. Instead of the nice sine wave the house current has, this current is a square wave. Square waves look like a line jumping from negative to positive, staying there a moment, and jumping back to negative once again.
The square wave needs to be filtered to turn it into something that resembles a sine wave. This is done by a big capacitor. The easiest way to think of a capacitor is like a battery. The major difference is that batteries release their power at a slow, constant rate. Capacitors, on the other hand, absorb and release power almost immediately. Therefore, the capacitor is charged by the high positive or negative voltage, and then when the voltage switches, the capacitor releases the charge.
The resulting wave is not a square wave and it is not quite a sine wave. It is more like a square wave with rounded corners. Due to this difference, power produced by a voltage inverter may not work well with some electronic devices, although most are able to work fine off of the power produced by an inverter.
Once again, those inverters that use oscillator circuits to create a sine wave produce a cleaner wave, which works better with all electronic equipment.
A Warning About Power Consumption
Since a voltage inverter uses ten times as many amps of power as it produces (actually slightly more due to inefficiencies) it can drain a car’s battery quickly. To determine how fast an inverter will drain a particular battery, it is necessary to know the inverter’s current draw in amps and the battery’s reserve capacity in amps. Dividing the reserve current by the current draw will determine the maximum amount of time that the inverter will run off of that car’s battery.
However, battery reserve current ratings are based on a new car battery. As a battery ages, the reserve current capacity drops. The rate depends on the design and quality of the battery in question.
It is also important to consider the amount of power that will be needed to start the car’s engine. As a car battery loses its charge, its voltage level drops as well. So, the lower the battery’s charge, the more current that is needed to start the car. If a battery drops below a 50-percent charge, it may not start the car.
The other consideration is the damage that drawing the car battery’s charge down can cause to the battery itself. Car batteries are not designed for deep cycling, i.e. they are not designed for having a large amount of the charge used up before recharging. The more times a battery deep cycles, the shorter the battery’s life expectancy.
Sizing a Power Inverter
When selecting a power inverter, it is important to select one that provides enough current in amps or watts for the device it will be used for. If too small an inverter is used, the device connected to it may not work properly. Another possibility when using too small an inverter is incurring damage to the inverter itself, to the point where it will no longer function.
Buying Power Inverters on eBay
Although power inverters are normally used with automobiles, they are located in the consumer electronics category on eBay, rather than in the automotive section. To find them, first select Electronics from the main navigation menu. From the main electronics page, allow your mouse pointer to hover over Car Audio, Video & GPS in the navigation menu. This will open a flyout where you can select "All Categories." In the pop-up menu, find the Vehicle Electronics & GPS heading. There will be a category listed under it for Car Electronics Accessories. From there, a filter allows selection of the Power Inverters category.
Alternatively, you can also do a general keyword search to find power inverters on eBay by entering a key phrase, such as "car power inverter," into the search bar at the top of the eBay home page.
Power inverters are extremely useful devices when one wants to use a device away from home power. They amplify a car’s 12-volt DC power to be essentially equal to a home’s 120-volt AC power, allowing connection and operation of electronic devices.
The most common mistake that people make when purchasing and using voltage inverters is to buy one that is too small for its intended purpose. This can either cause the device connected to the inverter to not work properly, or cause permanent damage to the inverter itself. It is important to select an inverter that properly matches the application it will be used for.
It is also important to take into consideration the car’s power system when buying and using inverters. If the inverter is going to be used extensively, it will be necessary to install a second battery in the car or to run the engine periodically in order to recharge the car’s battery and prevent deep cycling of the battery.
Marine and RV batteries are designed to permit deep cycling without damage. If a voltage inverter is going to be used in a situation where a lot of deep cycling is expected, installing one of these batteries may be a good option to prevent damage to the battery. However, these types of batteries have a lower "cold cranking amps" rating, meaning they are not as effective for starting a car’s engine.