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Direct Current (DC) : Part 1



In this blog post I would like to explain the overall basics of what Direct Current (DC) is, and how it is made and used, as I currently understand it to be.

Direct Current, abbreviated as DC, is an electric current that flows in one direction only. This means that whenever a conductor is connected between the two terminals of a DC power source (such as a battery, for example, since battery cells are DC power sources), the electrons flow from one terminal (negative) of the DC power source to the other terminal (positive) of the DC power source through the conductor continuously without reversing the direction of electron flow.

To understand the effects that direct current has on an electrical circuit, the relationship that exists between the different properties of electricity (current, voltage, resistance, etc..) must be understood.

Some examples of Direct Current (DC) power sources include:


Ohm's Law

Ohm's Law is used to describe a precise relationship that exists between current, voltage, and resistance. Ohm's Law states that the current in a circuit is directly proportional to the applied voltage and inversely proportional to the circuit resistance.

Ohm's Law may be expressed as an equation:

I = E / R    Which means current equals voltage divided by resistance.
where I ( as in upper case i ) = current in amperes, E = voltage in volts, and R = resistance in ohms.

Current is inversely proportional to resistance. This means, as the resistance in a circuit increases, the current decreases proportionately. Therefore, in the equation I = E / R, if any two quantities are known then the third quantity can be determined:

I = E / R  Which means current equals voltage divided by resistance.
R = E / I  Which means resistance equals voltage divided by current.
E = I * R  Which means voltage equals current times resistance.

This is especially helpful because by using Ohm's law we are able to find the resistance value of a circuit, knowing only the voltage and the current in the circuit.


Power

Power pertains to the rate at which work is being done. Work is done whenever a force causes motion. When a mechanical force is used to lift or move a weight, work is done. However, force exerted without causing motion, such as the force of a compressed spring acting between two fixed objects that does not actually move either of the two fixed objects, does not constitute work.

Voltage is an electrical force that forces electron current to flow in a closed circuit. However, when voltage exists but current does not flow because the circuit is open (such as when a switch is turned off), no work is done. When the circuit is closed and voltage causes electrons to move (such as when a switch is turned on), then work is done. The instantaneous rate at which this work is done is called the electric power rate, and is measured in Watts.

P = E * I  Which means power in watts is equal to voltage across a circuit multiplied by current through the circuit.

In practice, the only factors that can be changed in a circuit over time is voltage and resistance. If current is ever changed, it is because either voltage or resistance has been changed.

If the resistance in a circuit is held constant, the power varies directly with the square of the voltage.

Another way of proving that power varies as the square of the voltage when resistance is held constant is:

    Since:    I = E / R
    By substitution in:    P = E * I
    You get:    P = E * (E / R)
    Or:    P = (E * E) / R
    Therefore:    P = E2 / R

Power also varies was the square of current just as it does with voltage. Thus, another formula for power, with current and resistance, is P = I2 * R, as with:

    Since:    E = I * R
    By substitution in:    P = E * I
    You get:    P = I * R * I
    Or:    P = (I * I) * R
    Therefore:    P = I2 * R

Up to this point, four of the most important electrical quantities have been discussed. These are voltage (E), current (I), resistance (R), and power (P). It is necessary to understand the relationships that exist among these quantities since they are used in practically every electronic device.

There are 12 basic formulas that you should learn and know, which the four quantities (E, I, R and P) are at the center. The 12 basic formulas are shown in the formula wheel diagram below. Adjacent to each quantity are three segments. Note that in each segment, the basic quantity is expressed in terms of two other basic quantities, and no two segments are alike:

power wheel
Power Formula Wheel

Power Rating

Electrical components are often given a power rating. The power rating, in watts, indicates the rate at which the device converts electrical energy into another form of energy, such as light, heat, or motion. An example of such a rating is noted when comparing a 150-watt light bulb to a 100-watt light bulb. The higher wattage rating of the 150-watt light bulb indicates that it is capable of converting more electrical energy into light energy than the light bulb of the lower rating.

In some electrical devices the wattage rating indicates the maximum power the device is designed to use rather than the normal operating power. A 150-watt light bulb, for example, uses 150 watts when operated at the specified voltage printed on the light bulb. In contrast, a device such as a resistor is not normally given a voltage or a current rating. A resistor is given a power rating in watts and can be operated at any combination of voltage and current as long as the power rating is not exceeded. In most circuits, the actual power used by a resistor is considerably less than the power rating of the resistor because it is common practice to incorporate and use a 50% safety factor. For example, if a resistor was to normally use 2 watts of power, then a resistor that has a power rating of 3 watts would actually be used.

Resistors of the same resistance value are available in different wattage values. Carbon resistors, for example, are commonly made in wattage ratings of 1/8, 1/4, 1/2, 1 and 2 watts. The larger the physical size of a carbon resistor the higher the wattage rating. This is true because a larger surface area of material radiates a greater amount of heat more easily.

When resistors with wattage ratings greater than 5 watts are needed, wirewound resistors are used. Wirewound resistors are made in values between 5 and 200 watts. Special types of wirewound resistors are used for power in excess of 200 watts.

As with other electrical quantities, prefixes may be attached to the word watt when expressing very large or very small amounts of power. Some of the more common of these are the kilowatt (1,000 watts), the megawatt (1,000,000 watts), and the milliwatt (1/1000 of a watt).


Power Conversion and Efficiency

The term power consumption is common in the electrical field. It is applied to the use of power in the same sense that gasoline consumption is applied to the use of fuel in an automobile.

Another common term is power conversion. Power is used by electrical devices and is converted from one form of energy to another. An electrical motor converts electrical energy to mechanical energy. An electric light bulb converts electrical energy into light energy, and an electric stove cooktop converts electrical energy into heat energy. Power used by electrical devices is measured in watt-hours. This practical unit of electrical energy is equal to 1 watt of power used continuously for 1 hour. The term kilowatt hour (kWh) is used more extensively on a daily basis and is equal to 1000 watt-hours.

The efficiency of an electrical device is the ratio of power converted to useful energy divided by the power consumed by the device. This number will always be less than one (1.00) because of the losses in any electrical device. If a device has an efficiency rating of 0.95 then it effectively transforms 95 watts into useful energy for every 100 watts of input power. The other 5 watts are usually lost to heat, or to other losses which cannot be used.

Calculating the amount of power converted by an electrical device is a simple matter. You need to know the length of time the device is operated and the input power or horsepower rating.

Horsepower, a unit of work, is often found as a rating on electrical motors. One horsepower is always equal to 746 watts.


Continue with Direct Current - Part 2



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