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Note:
In the diagrams below we are considering the battery to be ideal, no
internal resistance and always at a constant 12 volts. Current flow will be 'conventional' flow (positive to negative).
Resistors as Voltage Dividers:
You already know that a resistor can be used to limit the current flow in a circuit. When multiple resistors are used in series, they will divide the voltage from the power supply (a battery in this example). In this first diagram you can see that the voltage across the resistor is the same as the voltage across the battery.
Equivalent Circuits:
The following 3 circuits are identical. Don't let the different configurations confuse you.
2 Resistors in Series:
The resistors in the following diagram are in series. Since they are the same value (1000 ohms), the voltage drop across each resistor is the same. Each resistor drops half the supply voltage (6 volts).
3 Resistors in Series:
If there were 3 equal value resistors, the voltage would be divided equally between them. They would each have a voltage drop of 4 volts (3*4=12). As you can see, the voltage drop across all of the resistors will add up to the power supply voltage.
Different Value Resistors:
If the resistor values are different, you can still calculate the voltage difference across the resistors. There are a few different ways to calculate the voltage. I'll show you the most versatile way. This is the circuit:
We know:
The total voltage
The individual resistor values
We can calculate the current flow and then the voltage drop across the individual resistors. From the Ohm's law page, we will use the formula:
I = V/R
I = 12/3000 ohms
I = 0.004 amps or 4 milliamps
The current flow through the resistors is 4 milliamps. Since the resistors are in series, we know that the current flow through each resistor is the same.
Then, to find the voltage drop across the 1000 ohm resistor, we can use the formula:
V = I*R
V = .004*1000
V = 4 volts across the 1000 ohm resistor
And to find the voltage drop across the 2000 ohm resistor, we can use the formula:
V = I*R
V = .004*2000
V = 8 volts across the 2000 ohm resistor
The previous method (using current flow to calculate voltage drop) will work for any number of series connected resistors. There is another method to find the voltage drop across a resistor when there are only 2 resistors. The formula is:
V = (R1/(R1+R2))*battery voltage
For the following diagram, these are the calculations:
V = (4700/4700+2200))*12
V = 8.17 volts across the 4700 ohm resistor.
Using Resistors to Increase an Amplifier's Power Output
As we found earlier on this page, a resistor can be used to dissipate power. Some people believe that they will have an increase in system SPL if they reduce the amplifier load's impedance with resistors. The fact is, the SPL will likely be reduced. Just because the amplifier is producing more power, it does NOT mean the SPL will increase. The reason? The extra power is dissipated in the form of heat and produces no audio. The reason that the SPL will likely drop is because the amplifier's internal power supply will lose some rail voltage with the lower impedance load (the loss may not be significant on amps with highly regulated power supplies). When the rail voltage drops, the output power to the speaker drops. Even if you have an amplifier with a regulated power supply and the power to the speakers doesn't fall, the amplifier will draw more current and run hotter.
In the following diagram, you can see a few different things:
- You can see that the amplifier is producing almost twice the power with the resistor in parallel with the load.
- The rail voltage is reduced with the heavier load.
- The power to the speaker is reduced because less rail voltage is available.
- The current draw has more than doubled. It is more than twice due to inefficiencies within the amplifier.
- The amplifier will run hotter because of more voltage drop across and more current through all of the semiconductors.
Resistor Construction:
There are several different ways to make resistive components. I'll try to cover a few of them here.
Film Resistors:
In the following diagram, you can see a ceramic substrate covered in a resistive film. The substrate is held, on each end, by metallic end caps. The wire leads are welded onto the end caps. Film resistors are (generally) made from etching a resistive element from a film of resistive material. The composition of the resistive film can vary from one type of resistor to another but the following description covers most type of film resistors. The ceramic (or glass) substrate is covered with a resistive material. The resulting component is effectively a relatively low ohm component. To change the component's value, the resistive film is lengthened by cutting a helical grove in it.
The resistance can be varied by varying the way the element is cut. In this diagram, you can see that leaving a wide and relatively short resistive element results in a low ohm resistor. A narrower longer helix results in a higher value resistor.
Types of Film Resistors
There are several different types of film resistors. The following are a few of their characterisitcs.
Carbon Film Resistors:
Carbon film resistors are some of the least expensive and therefore the most common resistors in use today. They are formed in one of 2 ways. The first is as described above. A carbon film is deposited deposited on the ceramic substrate when the substrate is exposed to hydrocarbon gasses in a vacuum (at high temperatures). The film is then cut to produce the desired resistor value. Another way the carbon film resistor can be formed is by painting a carbon filled polymer onto the former/substrate. The resistor value is determined by the amount of carbon in the polymer, the width and the length of the resistive element. Carbon film resistors are most commonly available in 5% tolerance.
Metal Film Resistors:
Metal film resistors are much like the carbon film resistors but instead of having a carbon material deposited on the former, a metal film such as nichrome is deposited.
Metal Oxide Resistors:
The resistive element in a metal oxide resistor is formed by the process of oxidation of a chemical like tin-chloride on the ceramic substrate. Metal oxide resistors can withstand higher temperatures than metal film or carbon film resistors. They can also better withstand short term surges.
Carbon Composition Resistors
Carbon Composition Resistors:
Carbon composition resistors are formed a little differently that the previously described film resistors. In all of the film resistors, the resistive element has very little thermal mass. If there is a short term surge through the resistor, the small thin element can quickly overheat and fail. In a carbon composition resistor, the resistive element is much thicker and therefore more able to handle short term surges without failing. The following diagram shows how the carbon composition resistor differs from film resistors. The value of the resistor can be controlled by the amount of the carbon in the 'slug'. Due to cost, carbon composition resistors are not used very often in car audio equipment.
Wire Wound Resistors:
There are many different styles of wire wound resistors. The 2 most common are the ceramic cased (cement) resistor and the type that look much like a large version of a film resistor. The ceramic type generally have a small element inside of a large casing. The large casing is needed to help dissipate heat and prevent the temperature from getting too high (which would cause the resistor to fail). The other type has the wire wound on top of the former. Many times, the wire is visible as a ridge under the insulating coating.
The following are 3 different wire wound resistors. The first is a 1 watt with its insulator scraped away to show the resistive element. The middle image is a 3 watt with its ceramic insulator broken away. The last is a 50 watt wirewound resistor in an aluminum case.
Flame-Proof Resistors:
Flame-proof resistors are available in several different materials (carbon film and metal film are most common). The main thing that distinguishes a flame-proof resistor from a common resistor is its coating. Most resistors will overheat and burn when too much current flows through them. The coating on a flame-proof resistor will not flame up (although it may turn dark or even black). This type of resistor is very commonly used in the audio section of home amplifiers.
Power Handling:
One of the factors that determines the power rating of a resistor is it's ability to dissipate heat. The maximum temperature that a resistor can withstand, without being damaged, is determined by the materials used in its construction. To prevent the resistor's temperature from getting too high, there has to be enough of a heat sink to soak up and/or dissipate the heat. If the resistor is not mounted onto a heat sink, it's physical size generally determines its power rating. Larger resistors have more surface area and can dissipate heat at a greater rate than smaller resistors. Even some large resistors (like the aluminum resistor above) need an additional heat sink to dissipate its rated power. Without a heat sink, its rated to dissipate only about 10-15 watts. To dissipate 50 watts, a significantly larger heat sink would be required.
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