5.4 Internal Resistance

This is the fourth week of Module 5 in the Navy Basic Electricity and Electronics series. This module is on the relationships between current, voltage, and resistance. The first three sections were:

5.1 Voltage, Resistance, and Current
5.2 Ohm's Law Formula
5.3 Power

1. This week is on the internal resistance of a source such as a battery. Let's say that you were to put a voltmeter across a battery in an open circuit and it were to read 12 volts, the "no-load" voltage. Then lets say you close the circuit and the voltmeter drops to 11 volts. This suggests that your battery has an internal resistance that is zapping some of the force.

2. To measure the internal resistance (R_i) of a source:

• Measure the no-load voltage.
• Energize the circuit and then measure the voltage across the source again.
• Subtract the difference (internal resistance decreases the voltage in the circuit).
• Measure the current in the circuit.
• Use R=E/I to determine the internal resistance of the source.

3. Hopefully, the internal resistance of your source is small enough to be negligible.

5.2 The Ohm's Law Formula

This is the second week of Module 5 in the Navy Basic Electricity and Electronics series. This module is on the relationships between current, voltage, and resistance. The first section was:

5.1 Voltage, Resistance, and Current

1. You can calculate the current in a system without shutting it down and measuring it. You can use "Ohm's Law" if you know the voltage and the resistance. The formula is:

E=IR

The voltage equals the current times the resistance. We can rearrange the formula as well. Current equals the voltage divided by the resistance, and the resistance equals the voltage divided by the current.

2. You can use this formula for an entire circuit. You can also use it for a distinct part of a circuit.

3. There can be more complex situations where the value of one resistor is known but not the value of another. In such cases, knowing algebra will come in handy so that you can set up an equation and then solve for the unknown.

5.1 Voltage, Resistance, and Current

Module 5 of the Navy Basic Electricity and Electronics series starts today. This module is called Relationships of Current, Voltage, and Resistance. The first four modules were:

1. Electrical Current
2. Voltage
3. Resistance
4. Measuring Current and Voltage in Series Circuits

The first section of the new module is titled: "Voltage, Resistance, and Current." "The relationship of voltage, resistance, and current is probably the most important concept you will learn in your study of electricity" (6).

• You cannot really address current directly. You can only change it by increasing the force moving it (relates to voltage) or the resistance opposing it.
• When voltage goes up, current goes up. When voltage goes down, current goes down (assuming that resistance is held constant).
• So the amount of current is "directly proportional" to the amount of voltage.
• When resistance goes up, current goes down. When resistance goes down, current goes up (assuming voltage is constant). 
• EMF (electromotive force, which relates to voltage) and resistance are inherent quantities that depend on the physical components you are using in a circuit. Current is a secondary characteristic--it changes on the basis of the physical components.
• The relationships between voltage, resistance, and current were unfolded by George Simon Ohm (1789-1854). Ohm's Law states, "Current is directly proportional to voltage and inversely proportional to resistance."

4.2 Voltage in a Series Circuit

This is the second week of Module 4 in the Navy Basic Electricity and Electronics series. The first week was:

• 4.1 Current in a Series Circuit

1. While amperage measures anywhere in a circuit, as long as it is connected in series. Voltage can only be measured where there is a difference in potential, and it is measured in parallel.

2. A difference in potential exists either where there is a "voltage rise" across a battery or source of electromotive force... or it can be measured across a "voltage drop," something that offers resistance to that force (e.g., a resistor, a lamp, etc...).

3. The voltage source can be symbolized with an E, or E_s (source voltage), or E_a (applied voltage) or E_T (total voltage).

4. Kirchoff's Voltage Law is that the total voltage drop will always equal the total applied voltage. This is more or less the same as the conservation of energy. The voltage used up by each resistor or load will add up to the total voltage across the battery or voltage source.

5. A second important rule is that in a series circuit, the largest voltage drop will take place over the largest resistance.

3.4 The Ohmmeter

This is the fourth and final week of Module 3 of the Navy Basic Electricity and Electronics series, which is on resistance. The three posts so far are:

• 3.1 What is resistance?
• 3.2 Resistors
• 3.3 Resistor identification

1. So an ammeter measures current (amps). A voltmeter in a sense measures the force of the current (volts). An ohmmeter measures the resistance of some circuit component (in ohms).

Of course usually these measurement functions are combined on a "multimeter," a device that can measure all of these. There are often "ranges" to these devices. So one might measure resistance in units of 1. Another setting might measure in units of 100 ohms. Another might have units of 10,000 ohms.

"It is best to use a meter range which causes the pointer to rest over the center two-thirds of the scale" (86-87). I imagine they're all digital today. :-)

2. Some miscellaneous elements of this section:

• Don't connect an ohmmeter to an energized circuit.
• The ohmmeter needs to be "zeroed" each time you change the range.
• You touch the leads together and turn on the "zero ohms" control to zero it.
• Turn the range switch to the 1000 volt scale when you are done to save the battery.
• An ohmmeter can help you find the point where a circuit is disconnected (an open circuit). At that point it will read infinity.

3.3 Resistor Identification

This is the third week of Module 3 of the Navy Basic Electricity and Electronics series. Last week was:

• 3.1 What is resistance?
• 3.2 Resistors

1. The unit of measure for resistance is the ohm, symbolized by the Greek letter omega, Ω

Resistors are commonly color coded to indicate their resistance value. To read the code, start at the end with the least body color showing. The first band gives the first number of the value. Then the second the second. The third band then tells the number of zeros behind the second digit.

The fourth band tells you the "tolerance," which means how precise the resistance is, how much its value is likely to vary.

Opensourse NearWiki

2. NBEE gives a mnemonic to remember the values of the numbers--"Bad boys race our young girls behind victory garden walls." Ooookkkk? Its black (0), brown (1), red (2), orange (3), yellow (4), green (5), blue (6), violet (7), grey (8), white (9).

For the tolerance, the mnemonic is "Get started now," which stands for gold (±5%), silver (±10%), no color (±20%).

3. Another system of giving the resistance value is the "part numbering system." So RB31P102G means RB31 (tells you the style), P (tells you how temperature affects it), 102 (resistance value), and G (gold, tells you the tolerance). With regard to the resistance number, the final number tells you the number of zeros (so it's two zeros here). If there is an R and then a number here, then that is where a decimal point goes.

3.2 Resistors

This is the second week of Module 3 of the Navy Basic Electricity and Electronics series. Last week was:

• 3.1 What is resistance?

This week is on resistors. I chuckled because so much is now done on integrated circuits that have billions of items on a small chip. I probably haven't seen a resistor in decades. But poking around I see that visible resistors still find their way onto circuit boards.

• Basically, a resistor is a circuit component that limits or controls the amount of current flow. Otherwise, other components might be damaged or might not function optimally. Think of slowing down the flow of a raging river, except the river is electrical current.  
• Resistors are rated in ohms (see the previous post). They are also rated in watts, which tells you how much power they can take without getting damaged.
• One type used to be made out of a mixture of carbon and clay (composition resistor). These could change in value with age and were not good for carrying large currents.
• Another type were e-wound resistors, with varying length of wire wrapped around a ceramic tube and painted with an insulating glaze. They could carry more current and could be more accurate. But they also used to be more expensive.



Taken from Wikipedia

• The symbol for a resistor with a fixed value is in the diagram to the right.
• "Tapped" resistors and sliding-contact resistors provide the possibility of varying the amount of resistance as desired. Tapped resistors have multiple connections available. The amount of resistance depends on which "tap" you are connected to.
• The sliding contact resistor can be slid to change the resistance (so it's a "variable" resistor). Once set, however, it usually isn't changed.
• There are also other kinds of variable resistors. The one is the potentiometer, which has three connections with three different resistance options (see picture below).
• The other is the rheostat, which used to have a movable contact in between two connections.

From Wikipedia

3.1 What is resistance?

We now move on to Module 3 of the Navy Basic Electricity and Electronics series. The first two modules were:

1. Electrical Current
2. Voltage

Today starts the booklet on resistance.

• If current (electron movement) and electromotive force (resulting in voltage, a difference in potential throughout a circuit) are two key elements of a circuit, resistance is a third factor always present.
• "Resistance is the property that opposes current flow" (6).
• Resistance is symbolized by R and the unit is the ohm, symbolized by Ω.
• Conductors have low resistance, insulators or non-conductors have high resistance.
• Some factors that determine resistance include the nature of the material (atomic structure), the area of the cross-section (e.g., of wire), and the length (e.g., of wire). The larger the cross-section, the less the resistance. The longer the wire, the more the resistance. Atomic structure determines how many free electrons are available to move around.

That's the big picture. Here are some additional details.

• One ohm is the amount of resistance that allows 1 amp to flow through a circuit when one volt is applied. 1 amp per volt.
• Resistance is sometimes needed to limit the current flow to a safe value through a circuit. Completely unopposed current can lead to a "short circuit," which is damaging. 
• The resistance of a toaster or iron or bulb is what provides the heat or light.