R-410A versus R-22 Refrigerants

r-410a versus r-22 refrigerant R-22 is a refrigerant that has been widely used in air conditioners and heat pumps for many years. It has been deemed hazardous to the environment and it will begin phase-out by law in the year 2010. The replacement refrigerant for R-22, R-410A, is being produced to meet the rising demand for new air conditioner and heat pumps systems. Why do consumers need to understand the impact this law is having on the HVAC market? Because it will determine what HVAC consumers pat for future repairs made to their equipment. Everything boils down to the new law, refrigerant and HVAC equipment manufacturers retrofitting factories for the new HVAC equipment, and supply and demand. The changeover has already begun and there is some apprehension about this changeover.

First of all with any new technology there is always some doubt. Does the new technology offer stability and reliability on par with the old time tested system? How many resources need to go into the changeover? These questions are valid and need to be addressed so there is a successful changeover from R-22 refrigerant to R-410A refrigerant. Technicians and have to be trained, consumers need to understand the new technology to have confidence in it, but more importantly, HVAC contractors need to have the confidence in these systems to recommend the technology to their customers. If the HVAC contractors do not have confidence in the new technology they will not recommend it and the new changeover, even though it is required by law or the changeover will be less than welcome.

This has happened with R-410A. New technology has bugs and it takes time for these bugs to be addressed by engineers. R-410A when operating at normal operating pressures and temperatures has higher pressures there fore the old R-22 refrigerant systems needed to be buffed up some to handle the higher pressures. This was done by compressor manufacturers but only after some problems occurred. Another issue that affects the confidence of the HVAC contractors is the oils used for R-410A are different than the oils used for R-22. If the two are haphazardly mixed then there will be a sure failure of the system. The oil in R-410A refrigeration systems also absorbs moisture more easily than the old R-22 systems. This makes it critical that the technicians perform proper evacuation when installing new HVAC systems. A less than proper evacuation will leave moisture in the HVAC refrigeration system and this will ultimately lead to premature failure. These initial engineering problems coupled with the new oil and its sensitivity to moisture along with the new technology apprehensiveness has led to a lack of confidence in the new R-410A systems available.

The root of this lack of confidence lies with HVAC contractors and not the equipment and refrigerant manufacturers or distributors of HVAC equipment and refrigerants. The refrigerant manufacturers and the HVAC equipment manufacturers are beginning to changeover production lines to accommodate the new R-410A systems. These manufacturers are also addressing issues that make HVAC contractors nervous or lack confidence in the new R-410A systems. The manufacturers are doing this through education to help the contractors gain confidence in the new systems. The have reengineered the new R-410A systems and worked all the initial bugs out of the system. Slowly but surely confidence is gaining ground and the new R-410A HVAC systems are being installed.

There really is no other choice as the law will begin the phase-out period in 2010 and the prices of R-22 will be much higher than R-410A. This will cause repair costs for the end consumer to rise. The choice is always with the end consumer and what they want. The HVAC contractor can influence the end consumers decision but if the HVAC contractor lacks confidence then no influence will be offered for the new R-410A systems.

Saving Money With Best Practices and an Energy Inspection Checklist

by Ron Auvil 

With the economic downturn, many facility managers are facing very tight operation and maintenance budgets. Luckily, many facilities are sitting on multiple energy conservation opportunities that can save thousands of dollars per year with relatively little capital investment.

If capital expenditures at your customers’ facilities for new HVAC mechanical equipment have been frozen or eliminated, one way to free up financial resources is to tune up existing HVAC equipment. Follow best maintenance practices and the increased operating efficiency will reduce energy consumption. Cumulative energy bill savings can then supplement the general budget.

While HVAC systems vary widely in terms of equipment and operational characteristics, the following checklist should generally apply to most main equipment types and energy waste areas:

Steam system checklist:
• Insulation checks: Loose or missing insulation costs thousands of dollars in extra fuel costs per heating season. Use a thermal imager to inspect system insulation and identify insufficient areas.

• Steam trap checks: Steam traps remove condensate from heating and process equipment. If the traps fail open, valuable steam is lost. To spot a failed trap, use a non-contact thermal imager, infrared thermometer, or a contact thermometer to compare the temperature of the trap to the pipe on either side of it. Thermography and infrared thermometers hold obvious advantages for traps mounted in awkward areas. Normal temperature differences across steam traps are approximately 20°F.

• Check air temperatures in boiler room: Proper combustion requires that the combustion air be between proper limits. Use a digital thermometer designed to measure ambient air temperatures and consider checking the carbon monoxide level in the boiler room, as well, to ensure safety.

• Other: In addition to the steps made above, also check the feedwater temperature, fuel oil heater temperature, and other items such as lubricant. These checks may not only save energy but also costs due to equipment repair or failure.

Cooling system checklist: 

Electrical checks: Check compressor voltage and amperage to ensure that the motor is operating at manufacturer-recommended levels.

Temperature checks: Check the temperatures of inlet and outlet air and water temperatures at heat exchangers to verify they are operating at their most efficient levels. Chilled water supply temperatures for most chilled water systems should be between 42° and 44°.

Pressure checks: Use a digital multimeter with a pressure attachment to take the inlet and outlet pressures of shell and tube heat exchangers, to ensure the tubes are not fouled. Fouling will result in lower heat transfer and higher energy consumption at the compressor. Pressure drop across the heat exchanger (Delta P) may be anywhere from 5 psig to 25 psig and varies by manufacturer. Check refrigerant and pump pressures, as well, to ensure equipment is operating at baseline levels.

Cooling tower checks: For cooling equipment to operate at its most efficient levels, the cooling tower must operate properly. Since cooling towers reject heat at a specific rate to the atmosphere, use a temperature/humidity meter to measure the outside air temperature and humidity and analyze cooling tower operation. Also measure the temperature of the condensing water supplied to the chiller. A common value is 85°.

Fan system checklist:
• Electrical checks: Measure the voltage and amperage of the fan motor and compare to nameplate conditions.

• Temperature checks: Check the following values at air handlers:

• Outside air temperature
• Mixed air temperature
• Return air temperature
• Discharge or supply air temperature
• Coil face discharge air temperatures
• Space temperature sensors
• Economizer and related dampers
• Wet bulb temperature or relative humidity (rh) sensors

Take readings with a handheld temperature meter and compare them against those indicated by the control system to ensure accuracy. Sensor calibration should be an integral part of all maintenance programs.

• Air pressure checks: Check pressure drops across filter banks to ensure adequate airflow. Excessive pressure drops will cause higher than normal energy consumption. Measure air pressure drops across heating and cooling coils to ensure that the coils are clean. Measure the inlet and outlet air temperatures to ensure proper temperature rise or drop.

Lighting system checklist:
• Electrical checks: Thermography applications for lighting include ballast and breaker contact temperatures. Look for hotspots and compare values with baselines or previous temperature measurements for evaluation.

• Illumination levels: Use a light meter to measure foot-candle or lumen levels. Where possible, reduce illumination levels to industry standards. Especially consider reducing illumination during seasons when more natural light is available.

• Operational checks: Set up a recording meter to sample illumination levels during a facility’s unoccupied hours. This will help identify areas where the lights are left on inadvertently or unnecessarily.

Building automation system checklist:
• Energy consumption checks: Use a power quality meter to evaluate your facility’s voltage, current, and kWh energy consumption. If possible, log energy consumption over time on specific circuits, and by specific equipment and areas. Compare your hourly usage rate to your utility fee structure and adjust usage patterns out of high-demand, high-cost times of day. Also check control system supply voltages to ensure proper controller operation. Controller communication bus values may be checked to ensure the integrity of the communication wiring.

• Control system: Check all sensors and indicating thermometers for properly calibrated gauges. Check valve and damper actuators as well. Actuator problems can cause higher than normal heating and cooling usage and high energy bills. Actuator output voltages and current signals can be checked using a regular digital multimeter.

• Temperature checks: Check the temperatures of inlet and outlet air and water temperatures at heat exchangers to make sure that they are operating at their most efficient levels. For example, chilled water supply temperatures for most chilled water systems are between 42° and 44°.

• Operational checks: Log air temperature over time, over occupied and unoccupied schedules, to identify areas left in the wrong temperature mode at night or on weekends, wasting energy.

• Setpoint checks: Check the accuracy of all temperature, pressure, and humidity control setpoints by logging those values over time. Control inaccuracy may waste 1 percent of energy per degree of error.

Explaining Static vs. Dynamic Sheave Balancing

by Don R. Sullivan

BALANCING

Before detailing the differences of static vs. dynamic, let’s first explore balance and its importance to sheaves and the overall drive. When an object like a sheave or even the wheel on a car rotates, centrifugal force causes the mass to naturally move away from the axis of rotation. If a sheave is not balanced, it will have an inherent uneven distribution of weight. This uneven weight distribution (even very minimal) magnifies vibration and noise when rotated and as speed of rotation increases. Unbalanced sheaves can also lead to premature life of other drive components due to the vibration and stresses placed on the components. In order to balance the sheave, we must remove small amounts of material (weight) so that the center of the sheave becomes optimally concentric with the axis of rotation.


STATIC BALANCE

The Mechanical Power Transmission Association (MPTA) is a body of manufacturers, including Browning, that defines criteria for the manufacture of power transmission products including sheaves. Static sheave balance is defined by MPTA as “one plane or static balance.” MPTA states, “A rotating body is said to be in static balance (sometimes called standing balance) when its center of gravity coincides with the axis upon which it revolves.” One plane balance is the recommended norm for most sheave products. As a result most sheaves, such as Browning’s, are statically balanced. The MPTA body corrects for static imbalance by removing weight (typically a drilled hole) from the heavy spot.

NOTE: MPTA one plane balancing guidelines are based on the allowable peripheral speed of cast iron (6,500 ft/min or 33 m/sec).


DYNAMIC BALANCE

With dynamic unbalance vibration also occurs when the part is rotated. MPTA states “dynamic unbalance is a condition where opposite radial centrifugal forces are acting on a sheave but not in the same radial plane. These centrifugal forces form an unbalanced ‘couple’ tending to cause vibration at each end of the sheave, but in opposite directions as shown below.”

Correcting this condition requires a machine that can balance each end independently of the other end. For critical applications or when the sheave is to be operated at a high rpm, a two plane dynamic balance is recommended.

Warning: MPTA and Browning stress — When belt speeds exceed 6,500 feet per minute, special materials must be used. Contact the manufacturer for assistance in selection.


SUMMARY

Static balance is the most common form of sheave balancing and most applications do not require dynamic balance. For those applications that do, manufacturers such as Browning can perform dynamic balancing for an added charge. However, it is best to speak with an application engineer to determine if the application is a candidate.

Explaining V-Belt Drive Misalignment

by Don R. Sullivan

V-belt drive misalignment exists when the driver and driven sheaves are not properly aligned. Misalignment can take either the form of angular or parallel (offset) misalignment, or a combination of both.


Figure 2. Parallel misalignment.
Angular misalignment occurs when the faces of the sheaves do not form a straight line. (See Figure 1.)

With parallel misalignment, the sheaves may be in angular alignment, but their position on the shaft creates a parallel offset. (See Figure 2.)

Ideally, you want to have both parallel and angular harmony. The objective is to have the shafts parallel and the center lines of the two sheaves in line with each other. (See Figure 3.)

IMPACTS OF V-BELT MISALIGNMENT

Figure 3. Desired alignment.
Excessive V-belt drive misalignment causes one or more of the following drive conditions:

1. Premature V-belt failure.

2. Uneven and premature sheave wear.

3. Higher V-belt operating temperature and energy consumption.

4. Uneven wear on belt sidewall.

5. V-belts rolling over in the sheave groove.

6. Transfer of load to one side of the V-belt, which can destroy the belt cords on that side.

7. Increased probability of vibration.

8. Drive instability.


BEST PRACTICES FOR V-BELT DRIVE ALIGNMENT

Acceptable V-belt misalignment for raw edge cogged V-belts is 1/2° or 1/10 inch per foot of center distance. Non-cogged V-belts can tolerate sheave misalignment of about 2° maximum.

The easiest way to quickly ensure sheave alignment is to use a straight edge or multi point contact string method.

Special Note: Sheave alignment should be checked before and after belt tensioning. Always adhere to strict safety and lock out and tag out procedures when performing maintenance on a V-belt drive.

V-Belt Drive Video 

An informative V-belt drive sheave installation and tensioning instructional video

Circuit wiring

So far, we've been analyzing single-battery, single-resistor circuits with no regard for the connecting wires between the components, so long as a complete circuit is formed. Does the wire length or circuit "shape" matter to our calculations? Let's look at a couple of circuit configurations and find out:

When we draw wires connecting points in a circuit, we usually assume those wires have negligible resistance. As such, they contribute no appreciable effect to the overall resistance of the circuit, and so the only resistance we have to contend with is the resistance in the components. In the above circuits, the only resistance comes from the 5 Ω resistors, so that is all we will consider in our calculations. In real life, metal wires actually do have resistance (and so do power sources!), but those resistances are generally so much smaller than the resistance present in the other circuit components that they can be safely ignored. Exceptions to this rule exist in power system wiring, where even very small amounts of conductor resistance can create significant voltage drops given normal (high) levels of current.

If connecting wire resistance is very little or none, we can regard the connected points in a circuit as being electrically common. That is, points 1 and 2 in the above circuits may be physically joined close together or far apart, and it doesn't matter for any voltage or resistance measurements relative to those points. The same goes for points 3 and 4. It is as if the ends of the resistor were attached directly across the terminals of the battery, so far as our Ohm's Law calculations and voltage measurements are concerned. This is useful to know, because it means you can re-draw a circuit diagram or re-wire a circuit, shortening or lengthening the wires as desired without appreciably impacting the circuit's function. All that matters is that the components attach to each other in the same sequence.

It also means that voltage measurements between sets of "electrically common" points will be the same. That is, the voltage between points 1 and 4 (directly across the battery) will be the same as the voltage between points 2 and 3 (directly across the resistor). Take a close look at the following circuit, and try to determine which points are common to each other:

Here, we only have 2 components excluding the wires: the battery and the resistor. Though the connecting wires take a convoluted path in forming a complete circuit, there are several electrically common points in the electrons' path. Points 1, 2, and 3 are all common to each other, because they're directly connected together by wire. The same goes for points 4, 5, and 6.

The voltage between points 1 and 6 is 10 volts, coming straight from the battery. However, since points 5 and 4 are common to 6, and points 2 and 3 common to 1, that same 10 volts also exists between these other pairs of points:

Between points 1 and 4 = 10 volts                             
Between points 2 and 4 = 10 volts                               
Between points 3 and 4 = 10 volts (directly across the resistor) 
Between points 1 and 5 = 10 volts                              
Between points 2 and 5 = 10 volts                              
Between points 3 and 5 = 10 volts                               
Between points 1 and 6 = 10 volts (directly across the battery)  
Between points 2 and 6 = 10 volts                             
Between points 3 and 6 = 10 volts                      

Since electrically common points are connected together by (zero resistance) wire, there is no significant voltage drop between them regardless of the amount of current conducted from one to the next through that connecting wire. Thus, if we were to read voltages between common points, we should show (practically) zero:

Between points 1 and 2 = 0 volts    Points 1, 2, and 3 are  
Between points 2 and 3 = 0 volts     electrically common    
Between points 1 and 3 = 0 volts                          
                                                             
Between points 4 and 5 = 0 volts    Points 4, 5, and 6 are 
Between points 5 and 6 = 0 volts     electrically common   
Between points 4 and 6 = 0 volts                         

This makes sense mathematically, too. With a 10 volt battery and a 5 Ω resistor, the circuit current will be 2 amps. With wire resistance being zero, the voltage drop across any continuous stretch of wire can be determined through Ohm's Law as such:

It should be obvious that the calculated voltage drop across any uninterrupted length of wire in a circuit where wire is assumed to have zero resistance will always be zero, no matter what the magnitude of current, since zero multiplied by anything equals zero.

Because common points in a circuit will exhibit the same relative voltage and resistance measurements, wires connecting common points are often labeled with the same designation. This is not to say that the terminal connection points are labeled the same, just the connecting wires. Take this circuit as an example:

Points 1, 2, and 3 are all common to each other, so the wire connecting point 1 to 2 is labeled the same (wire 2) as the wire connecting point 2 to 3 (wire 2). In a real circuit, the wire stretching from point 1 to 2 may not even be the same color or size as the wire connecting point 2 to 3, but they should bear the exact same label. The same goes for the wires connecting points 6, 5, and 4.

Knowing that electrically common points have zero voltage drop between them is a valuable troubleshooting principle. If I measure for voltage between points in a circuit that are supposed to be common to each other, I should read zero. If, however, I read substantial voltage between those two points, then I know with certainty that they cannot be directly connected together. If those points are supposed to be electrically common but they register otherwise, then I know that there is an "open failure" between those points.

One final note: for most practical purposes, wire conductors can be assumed to possess zero resistance from end to end. In reality, however, there will always be some small amount of resistance encountered along the length of a wire, unless its a superconducting wire. Knowing this, we need to bear in mind that the principles learned here about electrically common points are all valid to a large degree, but not to an absolute degree. That is, the rule that electrically common points are guaranteed to have zero voltage between them is more accurately stated as such: electrically common points will have very little voltage dropped between them. That small, virtually unavoidable trace of resistance found in any piece of connecting wire is bound to create a small voltage across the length of it as current is conducted through. So long as you understand that these rules are based upon ideal conditions, you won't be perplexed when you come across some condition appearing to be an exception to the rule. 

REVIEW:

• Connecting wires in a circuit are assumed to have zero resistance unless otherwise stated.
• Wires in a circuit can be shortened or lengthened without impacting the circuit's function -- all that matters is that the components are attached to one another in the same sequence.
• Points directly connected together in a circuit by zero resistance (wire) are considered to be electrically common.
• Electrically common points, with zero resistance between them, will have zero voltage dropped between them, regardless of the magnitude of current (ideally).
• The voltage or resistance readings referenced between sets of electrically common points will be the same.
• These rules apply to ideal conditions, where connecting wires are assumed to possess absolutely zero resistance. In real life this will probably not be the case, but wire resistances should be low enough so that the general principles stated here still hold.

Nonlinear conduction

Bernhard Haisch, Astrophysicist

Ohm's Law is a simple and powerful mathematical tool for helping us analyze electric circuits, but it has limitations, and we must understand these limitations in order to properly apply it to real circuits. For most conductors, resistance is a rather stable property, largely unaffected by voltage or current. For this reason we can regard the resistance of many circuit components as a constant, with voltage and current being directly related to each other.

For instance, our previous circuit example with the 3 Ω lamp, we calculated current through the circuit by dividing voltage by resistance (I=E/R). With an 18 volt battery, our circuit current was 6 amps. Doubling the battery voltage to 36 volts resulted in a doubled current of 12 amps. All of this makes sense, of course, so long as the lamp continues to provide exactly the same amount of friction (resistance) to the flow of electrons through it: 3 Ω.

However, reality is not always this simple. One of the phenomena explored in a later chapter is that of conductor resistance changing with temperature. In an incandescent lamp (the kind employing the principle of electric current heating a thin filament of wire to the point that it glows white-hot), the resistance of the filament wire will increase dramatically as it warms from room temperature to operating temperature. If we were to increase the supply voltage in a real lamp circuit, the resulting increase in current would cause the filament to increase temperature, which would in turn increase its resistance, thus preventing further increases in current without further increases in battery voltage. Consequently, voltage and current do not follow the simple equation "I=E/R" (with R assumed to be equal to 3 Ω) because an incandescent lamp's filament resistance does not remain stable for different currents.

The phenomenon of resistance changing with variations in temperature is one shared by almost all metals, of which most wires are made. For most applications, these changes in resistance are small enough to be ignored. In the application of metal lamp filaments, the change happens to be quite large.

This is just one example of "nonlinearity" in electric circuits. It is by no means the only example. A "linear" function in mathematics is one that tracks a straight line when plotted on a graph. The simplified version of the lamp circuit with a constant filament resistance of 3 Ω generates a plot like this:

The straight-line plot of current over voltage indicates that resistance is a stable, unchanging value for a wide range of circuit voltages and currents. In an "ideal" situation, this is the case. Resistors, which are manufactured to provide a definite, stable value of resistance, behave very much like the plot of values seen above. A mathematician would call their behavior "linear."

A more realistic analysis of a lamp circuit, however, over several different values of battery voltage would generate a plot of this shape:

The plot is no longer a straight line. It rises sharply on the left, as voltage increases from zero to a low level. As it progresses to the right we see the line flattening out, the circuit requiring greater and greater increases in voltage to achieve equal increases in current.

If we try to apply Ohm's Law to find the resistance of this lamp circuit with the voltage and current values plotted above, we arrive at several different values. We could say that the resistance here is nonlinear, increasing with increasing current and voltage. The nonlinearity is caused by the effects of high temperature on the metal wire of the lamp filament.

Another example of nonlinear current conduction is through gases such as air. At standard temperatures and pressures, air is an effective insulator. However, if the voltage between two conductors separated by an air gap is increased greatly enough, the air molecules between the gap will become "ionized," having their electrons stripped off by the force of the high voltage between the wires. Once ionized, air (and other gases) become good conductors of electricity, allowing electron flow where none could exist prior to ionization. If we were to plot current over voltage on a graph as we did with the lamp circuit, the effect of ionization would be clearly seen as nonlinear:

The graph shown is approximate for a small air gap (less than one inch). A larger air gap would yield a higher ionization potential, but the shape of the I/E curve would be very similar: practically no current until the ionization potential was reached, then substantial conduction after that.

Incidentally, this is the reason lightning bolts exist as momentary surges rather than continuous flows of electrons. The voltage built up between the earth and clouds (or between different sets of clouds) must increase to the point where it overcomes the ionization potential of the air gap before the air ionizes enough to support a substantial flow of electrons. Once it does, the current will continue to conduct through the ionized air until the static charge between the two points depletes. Once the charge depletes enough so that the voltage falls below another threshold point, the air de-ionizes and returns to its normal state of extremely high resistance.

Many solid insulating materials exhibit similar resistance properties: extremely high resistance to electron flow below some critical threshold voltage, then a much lower resistance at voltages beyond that threshold. Once a solid insulating material has been compromised by high-voltage breakdown, as it is called, it often does not return to its former insulating state, unlike most gases. It may insulate once again at low voltages, but its breakdown threshold voltage will have been decreased to some lower level, which may allow breakdown to occur more easily in the future. This is a common mode of failure in high-voltage wiring: insulation damage due to breakdown. Such failures may be detected through the use of special resistance meters employing high voltage (1000 volts or more).

There are circuit components specifically engineered to provide nonlinear resistance curves, one of them being the varistor. Commonly manufactured from compounds such as zinc oxide or silicon carbide, these devices maintain high resistance across their terminals until a certain "firing" or "breakdown" voltage (equivalent to the "ionization potential" of an air gap) is reached, at which point their resistance decreases dramatically. Unlike the breakdown of an insulator, varistor breakdown is repeatable: that is, it is designed to withstand repeated breakdowns without failure. A picture of a varistor is shown here:

There are also special gas-filled tubes designed to do much the same thing, exploiting the very same principle at work in the ionization of air by a lightning bolt.

Other electrical components exhibit even stranger current/voltage curves than this. Some devices actually experience a decrease in current as the applied voltage increases. Because the slope of the current/voltage for this phenomenon is negative (angling down instead of up as it progresses from left to right), it is known as negative resistance.

Most notably, high-vacuum electron tubes known as tetrodes and semiconductor diodes known as Esaki or tunnel diodes exhibit negative resistance for certain ranges of applied voltage.

Ohm's Law is not very useful for analyzing the behavior of components like these where resistance varies with voltage and current. Some have even suggested that "Ohm's Law" should be demoted from the status of a "Law" because it is not universal. It might be more accurate to call the equation (R=E/I) a definition of resistance, befitting of a certain class of materials under a narrow range of conditions.

For the benefit of the student, however, we will assume that resistances specified in example circuits are stable over a wide range of conditions unless otherwise specified. I just wanted to expose you to a little bit of the complexity of the real world, lest I give you the false impression that the whole of electrical phenomena could be summarized in a few simple equations.

REVIEW:

• The resistance of most conductive materials is stable over a wide range of conditions, but this is not true of all materials.
• Any function that can be plotted on a graph as a straight line is called a linear function. For circuits with stable resistances, the plot of current over voltage is linear (I=E/R).
• In circuits where resistance varies with changes in either voltage or current, the plot of current over voltage will be nonlinear (not a straight line).
• A varistor is a component that changes resistance with the amount of voltage impressed across it. With little voltage across it, its resistance is high. Then, at a certain "breakdown" or "firing" voltage, its resistance decreases dramatically.
• Negative resistance is where the current through a component actually decreases as the applied voltage across it is increased. Some electron tubes and semiconductor diodes (most notably, the tetrode tube and the Esaki, or tunnel diode, respectively) exhibit negative resistance over a certain range of voltages.

Resistors

Because the relationship between voltage, current, and resistance in any circuit is so regular, we can reliably control any variable in a circuit simply by controlling the other two. Perhaps the easiest variable in any circuit to control is its resistance. This can be done by changing the material, size, and shape of its conductive components (remember how the thin metal filament of a lamp created more electrical resistance than a thick wire?).
Special components called resistors are made for the express purpose of creating a precise quantity of resistance for insertion into a circuit. They are typically constructed of metal wire or carbon, and engineered to maintain a stable resistance value over a wide range of environmental conditions. Unlike lamps, they do not produce light, but they do produce heat as electric power is dissipated by them in a working circuit. Typically, though, the purpose of a resistor is not to produce usable heat, but simply to provide a precise quantity of electrical resistance.
The most common schematic symbol for a resistor is a zig-zag line:

Resistor values in ohms are usually shown as an adjacent number, and if several resistors are present in a circuit, they will be labeled with a unique identifier number such as R₁, R₂, R₃, etc. As you can see, resistor symbols can be shown either horizontally or vertically:

Real resistors look nothing like the zig-zag symbol. Instead, they look like small tubes or cylinders with two wires protruding for connection to a circuit. Here is a sampling of different kinds and sizes of resistors:

In keeping more with their physical appearance, an alternative schematic symbol for a resistor looks like a small, rectangular box:

Resistors can also be shown to have varying rather than fixed resistances. This might be for the purpose of describing an actual physical device designed for the purpose of providing an adjustable resistance, or it could be to show some component that just happens to have an unstable resistance:

In fact, any time you see a component symbol drawn with a diagonal arrow through it, that component has a variable rather than a fixed value. This symbol "modifier" (the diagonal arrow) is standard electronic symbol convention.
Variable resistors must have some physical means of adjustment, either a rotating shaft or lever that can be moved to vary the amount of electrical resistance. Here is a photograph showing some devices called potentiometers, which can be used as variable resistors:

Because resistors dissipate heat energy as the electric currents through them overcome the "friction" of their resistance, resistors are also rated in terms of how much heat energy they can dissipate without overheating and sustaining damage. Naturally, this power rating is specified in the physical unit of "watts." Most resistors found in small electronic devices such as portable radios are rated at 1/4 (0.25) watt or less. The power rating of any resistor is roughly proportional to its physical size. Note in the first resistor photograph how the power ratings relate with size: the bigger the resistor, the higher its power dissipation rating. Also note how resistances (in ohms) have nothing to do with size!
Although it may seem pointless now to have a device doing nothing but resisting electric current, resistors are extremely useful devices in circuits. Because they are simple and so commonly used throughout the world of electricity and electronics, we'll spend a considerable amount of time analyzing circuits composed of nothing but resistors and batteries.
For a practical illustration of resistors' usefulness, examine the photograph below. It is a picture of a printed circuit board, or PCB: an assembly made of sandwiched layers of insulating phenolic fiber-board and conductive copper strips, into which components may be inserted and secured by a low-temperature welding process called "soldering." The various components on this circuit board are identified by printed labels. Resistors are denoted by any label beginning with the letter "R".

This particular circuit board is a computer accessory called a "modem," which allows digital information transfer over telephone lines. There are at least a dozen resistors (all rated at 1/4 watt power dissipation) that can be seen on this modem's board. Every one of the black rectangles (called "integrated circuits" or "chips") contain their own array of resistors for their internal functions, as well.
Another circuit board example shows resistors packaged in even smaller units, called "surface mount devices." This particular circuit board is the underside of a personal computer hard disk drive, and once again the resistors soldered onto it are designated with labels beginning with the letter "R":

There are over one hundred surface-mount resistors on this circuit board, and this count of course does not include the number of resistors internal to the black "chips." These two photographs should convince anyone that resistors -- devices that "merely" oppose the flow of electrons -- are very important components in the realm of electronics!
In schematic diagrams, resistor symbols are sometimes used to illustrate any general type of device in a circuit doing something useful with electrical energy. Any non-specific electrical device is generally called a load, so if you see a schematic diagram showing a resistor symbol labeled "load," especially in a tutorial circuit diagram explaining some concept unrelated to the actual use of electrical power, that symbol may just be a kind of shorthand representation of something else more practical than a resistor.
To summarize what we've learned in this lesson, let's analyze the following circuit, determining all that we can from the information given:

All we've been given here to start with is the battery voltage (10 volts) and the circuit current (2 amps). We don't know the resistor's resistance in ohms or the power dissipated by it in watts. Surveying our array of Ohm's Law equations, we find two equations that give us answers from known quantities of voltage and current:

Inserting the known quantities of voltage (E) and current (I) into these two equations, we can determine circuit resistance (R) and power dissipation (P):

For the circuit conditions of 10 volts and 2 amps, the resistor's resistance must be 5 Ω. If we were designing a circuit to operate at these values, we would have to specify a resistor with a minimum power rating of 20 watts, or else it would overheat and fail.

• REVIEW:
• Devices called resistors are built to provide precise amounts of resistance in electric circuits. Resistors are rated both in terms of their resistance (ohms) and their ability to dissipate heat energy (watts).
• Resistor resistance ratings cannot be determined from the physical size of the resistor(s) in question, although approximate power ratings can. The larger the resistor is, the more power it can safely dissipate without suffering damage.
• Any device that performs some useful task with electric power is generally known as a load. Sometimes resistor symbols are used in schematic diagrams to designate a non-specific load, rather than an actual resistor.