Thursday, January 22, 2009

What's a Watt

Knowing more about voltage, current and power can help when you are putting together an IP camera system. This is especially true when you are selecting the power supply and running the wire. Computers, light bulbs and cameras have a power rating that is measured in Watts and sometimes in Volt-Amps (VA). Ever wonder what a Watt is, or why we sometimes use watts and other times Volt-Amps? It’s important to know about the power, because if you don’t have enough of it you’re in trouble. This article provides the information you need to stay out of trouble.

Current is the flow of electric charge. The unit of measure for current is the ampere, or abbreviated it’s “amp”. If you stick your finger in an outlet (please don’t do this), it’s the current that knocks you on your rear. The more current you have, the larger the size (or gauge) of the wire you need to carry the flow of the current. If the wire gauge is too small, it resists the flow of the current and it gets hot. (See the description of a resister below.)

Resistance is what generates the heat when voltage and current are applied to an electric circuit. An electric heater is an example of a resister. The more current that follows through a resister the hotter it will get. Resistance is measured in ohms.
C:\Documents and Settings\Bob Mesnik\My Documents\My Pictures\voltage current no resistance example.jpgVoltage and current are related in electrical equipment by Ohm’s law. Voltage (V) is related to current (I) by the resistance (R) of the electrical circuit. The formula is V=I X R. This means that if you run 2 amps of current through an IP camera that has a resistance of 6 ohms you will get 12 volts (2 X 6 = 12 volts).

Almost everything, including wire, has some resistance. The thicker the wire the less resistance it has to the flow of the current. It’s like having a larger size pipe for water. The thinner the wire gauge the more resistance the wire will have, and the lower the voltage will be at the end of the wire. Also, the longer the length of the wire the more resistance is added. When you have a long run of wire, you should use a larger gauge size wire (thicker) to reduce the resistance.

For those who need more details, here they are, otherwise just go to Voltage description below.
C:\Documents and Settings\Bob Mesnik\My Documents\My Pictures\voltage current example.jpgTo illustrate the problem of wire resistance, I will use a slightly more complicated drawing. In this example I added the 2 ohm resister to represent the resistance of the wire. Now there is a total of 8 ohms in the circuit (6 + 2 = 8). Since there is now more resistance in the circuit, the amount of current changes and so does the amount of voltage available at the camera. The equation to calculate the current is: I = V/R. So now the current equals 12/8 or 1.5 amps. The camera that was getting 12 Volts before we added the resistance of the wire is now getting only 9 volts (I used the formula V=I X R for this. 1.5 A X 6 R = 9 volts.) This lower voltage could be a problem for the camera. There are some charts on the Internet such as the one at It describes the American Wire Gauge (AWG) for various gauges vs. the resistance of 1000ft of wire.

Voltage is the difference of potential between two points of an electrical circuit. The higher the potential the more volts you have. For example you can generate a lot of voltage by rubbing your feet on a carpet, and make quite a large spark when you touch a metal surface (or someone else).

Another example can be seen at museums that have scientific displays. Sometimes they have displays that show the effect of voltage. They let kids touch the surface of a large metal ball. When they touch it, their hair sticks out in all directions. This is called a Van der Graf generator (but you probably don’t need to know this). It generates a very large voltage on the surface of the metal ball, and the static electricity makes your hair stand on end.

Now for Watts:
Here’s the definition of a watt: the watt (symbol: W) is equal to one joule of energy per second (and about 745 watts equals 1 horsepower). It measures a rate of energy conversion. It was named after our old friend James Watt. Remember his work with the steam engine? If not, you’re not smarter than a fifth grader.

Here are some examples of the Watt. A human climbing a flight of stairs is doing work at a rate of about 200 watts. An automobile engine can produce mechanical energy at a rate of 25,000 watts (which is equal to approximately 33.5 horsepower) while cruising. A household incandescent light bulb uses electrical energy at a rate of 25 to 100 watts, while compact fluorescent lights typically consume 5 to 30 watts. We also have Kilowatts (1,000 watts), Megawatts (1,000,000 watts) and if you have a power station available Gigawatts (too many zeros (9 actually)).

First off, you may know this, but there are two different types of electrical systems. There is Direct Current (DC) systems and Alternating Current (AC) systems. I’m going to start by explaining DC systems which are simpler than AC systems.

The Watt is related to the voltage and current. The formula for calculating the watt is: Watts (W) equal Current (I) X Voltage (V) or W = I x V. For example if I have a camera that requires 12 V DC and it takes 2 amps, it will require a power supply that can provide at least 24 watts. (2 x 12 = 24).

Volt-Amps or Power in an Alternating Current System
Power is fairly simple, when you are using a DC power supply. It gets a little more complicated when you use an AC power supply. In the alternating current world the voltage cycles up and down. This is called a sine wave. The voltage goes positive and then negative and back again. The current goes back and forth as well. These two cycles can be affected by the load, especially when the load includes other components such as a transformer or motor. Pan tilt cameras with motors are an example of a complex load.

When there is a complex load, the sine wave for the voltage can get displaced from the sine wave for the current and this changes the real power we will need. When we compute power in an AC network we use Volt-Amps instead of Watts.
We use the Power Factor to adjust for this variable power load. Now the Volt-amps equal the watts divided by power factor. The power factor is a number between 0 and 1 and its value is determined by the properties of the load. So if you have a system that needs 20 watt but there is a power factor of 0.5, the Volt-Amp required is 20 / 0.5 = 40 watts. Notice that the unit of measure is the same (further confusing things).

If you have a camera or other device that doesn’t have a complex load then the power factor equals “1”. This is true in most fixed IP cameras. In this case, Watts equals Volt-Amps.

In summary, current, resistance and power are important specifications to consider when installing IP cameras or any type of camera system. The gauge (thickness) and length of wire is important because it can affect the voltage. Power is measured in Watts, but it can also be the same as Volt-Amps, it’s just determined by the type of load (or camera) that’s used.

If you would like more information about how voltage, current and power affect your IP camera system, just contact us at 914-944-3425 or use our contact form.

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