Basic Electronics for Computer Engineering 1 Chapter 3 Ohm’s Law

Basic Electronics for Computer Engineering 2 Introduction  Ohm’s law is perhaps the single most important tool for the analysis of electric circuits.  Ohm’s law is one of the basic foundation elements upon which the rest of your study and work in electronics will be built.  Ohm’s law describes mathematically how voltage, current, and resistance are related.

Basic Electronics for Computer Engineering 3 Ohm’s Law The effect of changing voltage on current, if resistance is held constant.

Basic Electronics for Computer Engineering 4 Ohm’s Law The effect of changing resistance on current, if voltage is held constant.

Basic Electronics for Computer Engineering 5 Formula for Current If the values of Voltage and Resistance are known, Current can be calculated as: I = V/R

Basic Electronics for Computer Engineering 6 Formula for Voltage If the values of Current and Resistance are known, Voltage can be calculated as: V = IR

Basic Electronics for Computer Engineering 7 Formula for Resistance If the values of Voltage and Current are known, Resistance can be calculated as: R = V/I

Basic Electronics for Computer Engineering 8 Calculating Current Using the formula I = V/R, voltage must be in volts, and resistance must be in ohms in order to get current in amperes. V IR

Basic Electronics for Computer Engineering 9 Calculate Current I = V/R = 100 V/22  = 4.55 A

Basic Electronics for Computer Engineering 10 Calculate Current I = V/R = 25 V/4.7 M  = 5.32  A

Basic Electronics for Computer Engineering 11 Larger Units of Resistance In electronics, resistance values of thousands (k  ) of ohms or millions of ohms (M  ) are common. If volts are applied, the resultant currents will be in milliamperes (mA) or microamperes (  A) respectively.

Basic Electronics for Computer Engineering 12 Calculating Voltage Using the formula V= IR, current must be in amperes, and resistance must be in ohms in order to get voltage in volts.

Basic Electronics for Computer Engineering 13 Calculate Voltage (V)

Basic Electronics for Computer Engineering 14 Calculate Voltage (V) V = I R = (5 mA)(56  ) = 280 mV

Basic Electronics for Computer Engineering 15 Smaller Units of Current Current values of milliamperes (mA) or microamperes (  A) with resistance values of ohms, will result in voltage values of millivolts (mV) or microvolts (  V) respectively.

Basic Electronics for Computer Engineering 16 Calculating Resistance Using the formula R = V/I, current must be in amperes, and voltage must be in volts in order to get resistance in ohms.

Basic Electronics for Computer Engineering 17 Calculate Resistance (R)

Basic Electronics for Computer Engineering 18 Calculate Resistance (R) R = V/I = 12 V/3.08 A = 3.90 

Basic Electronics for Computer Engineering 19 Current and Voltage relationship Current and voltage are linearly proportional. –In resistive circuits, with a constant resistance, if voltage increases or decreases by a certain percentage, so will current.

Basic Electronics for Computer Engineering 20 Current and Voltage are linearly proportional

Basic Electronics for Computer Engineering 21 Current and Resistance relationship Current and resistance are inversely related. –With constant voltage, if resistance is reduced, current goes up; when resistance is increased, current goes down.

Basic Electronics for Computer Engineering 22 Current and Resistance are Inversely Related

Basic Electronics for Computer Engineering 23 Voltage Measurements To measure voltage, the voltmeter is placed in parallel across the component; that is, one lead is place on each side of the component.

Basic Electronics for Computer Engineering 24 Resistance Measurements To measure resistance, the ohmmeter is connected across a component; however, the voltage must be first disconnected, and usually the component itself must be removed from the circuit.

Basic Electronics for Computer Engineering 25 Current Measurements To measure current, the ammeter must be placed in series with the component; that is, it must be in line with the current path.

Basic Electronics for Computer Engineering 26 Energy and Power Energy is the ability to do work; and power is the rate at which energy is used. Power = energy/time P = W (J) / t (s) (watt)

Basic Electronics for Computer Engineering 27 Units of Energy and Power Energy is measured in joules (J) Power is measured in watts (W) By definition: One watt is the amount of power when one joule of energy is used in one second.

Basic Electronics for Computer Engineering 28 Amounts of Power Amounts of power less than one watt are expressed as milliwatts (mW), microwatts (  W), and even picowatts (pW). Electrical utilities and transmitting stations may use large amounts of power, expressed as kilowatts (kW), and megawatts (MW).

Basic Electronics for Computer Engineering 29 Kilowatt-hour The kilowatt-hour (kWh) is frequently used as a unit of energy. One kWh is used when one thousand watts is used for one hour. Power utilized over a period of time represents energy consumption. W = Pt Energy can also be expressed as watt- seconds (Ws), watt-hour (Wh).

Basic Electronics for Computer Engineering 30 Typical power rating for several household appliances

Basic Electronics for Computer Engineering 31 Heat produced by Current When there is current through resistance, the collisions of the electrons produce heat, as a result of the conversion of electrical energy.

Basic Electronics for Computer Engineering 32 Power in an Electric Circuit There is always a certain amount of power in an electric circuit, and it is dependent on the amount of resistance and the amount of current, expressed as: P = I 2 R

Basic Electronics for Computer Engineering 33 Equivalent Expressions for Power Using Ohm’s law, and substituting V for IR: P = VI Using Ohm’s law, and substituting I for V/R: P = V 2 /R

Basic Electronics for Computer Engineering 34 Watt’s Law The power relationships are known as Watt’s law. Current (I) must be in amperes, Voltage (V) must be in volts, Resistance (R) must be in ohms.

Basic Electronics for Computer Engineering 35 Resistor Power Rating Resistor power rating is not related to ohmic value (resistance) but rather is determined by the physical composition, size and shape of the resistor.

Basic Electronics for Computer Engineering 36 Resistor Power Rating Power rating of a resistor is the maximum amount of power that a resistor can dissipate without being damaged by excessive heat buildup. Power rating is directly related to surface area.

Basic Electronics for Computer Engineering 37 Metal-film Resistors Metal-film resistors have standard power ratings of 1/8 W, 1/4 W, 1/2 W, and 1 W.

Basic Electronics for Computer Engineering 38 Selecting the Proper Power Rating A resistor used in a circuit must have a power rating in excess of what it will have to handle. Ideally, a rating that is approximately twice the actual power should be used when possible.

Basic Electronics for Computer Engineering 39 Resistor Failures When excessive power is applied to a resistor, the resistor will overheat. The resistor will burn open, or its resistance value will be greatly altered. Overheated resistors may be charred, or the surface color may change. Resistors suspected of being damaged should be removed from the circuit and checked with an ohmmeter.

Basic Electronics for Computer Engineering 40 Energy Conversion and Voltage Drop in Resistance As electrons flow through resistors, some of their energy is given up as heat. The same number of electrons entering a resistor will exit it, only their energy will be less, so the voltage exiting a resistor is less than the voltage entering the resistor.

Basic Electronics for Computer Engineering 41 Energy Conversion and Voltage Drop in Resistance

Basic Electronics for Computer Engineering 42 Power Supplies A power supply is a device that provides power to a load.

Basic Electronics for Computer Engineering 43 Power Supplies A battery is a dc power supply that converts chemical energy into electrical energy. Electronic power supplies generally convert 220 VAC (alternating current) from a wall outlet into a regulated dc (direct current) at a level suitable for electronic components.

Basic Electronics for Computer Engineering 44 Ampere-hour Ratings of Batteries Because of their limited source of chemical energy, batteries have a certain capacity that limits the amount of time over which they can produce a given power level. The ampere-hour rating determines the length of time that a battery can deliver a certain amount of current to a load at the rated voltage.

Basic Electronics for Computer Engineering 45 Power Supply Efficiency An important characteristic of electronic power supplies is efficiency, which is the ratio of output power to input power. Efficiency = P out /P in

Basic Electronics for Computer Engineering 46 Power Loss The output power of an electronic power supply is always less that the input power, because some of the input power is used to operate the power supply circuitry. P OUT = P IN - P LOSS

Basic Electronics for Computer Engineering 47 Efficiency of Power Supplies High efficiency means that little power is lost and there is a higher proportion of output power for a given input power.

Basic Electronics for Computer Engineering 48 Troubleshooting

Basic Electronics for Computer Engineering 49 Summary Voltage and current are linearly proportional. Ohm’s law gives the relationship of voltage, current, and resistance. Current is directly proportional to voltage. Current is inversely proportional to resistance.

Basic Electronics for Computer Engineering 50 Summary A kilohm (k  ) is one thousand ohms. A Megohm (M  ) is one-million ohms. A microampere (  A) is one-millionth of an ampere. A milliampere (mA) is one-thousandth of an ampere.

Basic Electronics for Computer Engineering 51 Summary Use: V = IR, when calculating voltage. Use: I = V/R, when calculating current. Use: R = V/I, when calculating resistance.

Basic Electronics for Computer Engineering 52 Summary The power rating in watts of a resistor determines the maximum power that it can handle safely. A resistor should have a power rating higher than the maximum power that it is expected to handle in the circuit. Power rating is not related to resistance value. A resistor normally opens when it burns out. Energy is the ability to do work and is equal to power multiplied by time.

Basic Electronics for Computer Engineering 53 Summary The kilowatt-hour equals one thousand watts used for one hour or any other combination of watts and hours that has a product of one. A power supply is an energy source used to operate electrical and electronic devices. A battery is one type of power supply that converts chemical energy into electrical energy. An electronic power supply converts commercial energy into regulated dc at various voltages levels.

Basic Electronics for Computer Engineering 54 Summary The output power of a supply is the output voltage times the load current. A load is a device that draws current from the power supply. The capacity of a battery is measured in ampere-hours (Ah). One ampere-hour equals one ampere used for one hour, or any combinations of amperes and hours that has a product of one. A circuit with high efficiency wastes less power than one with a lower efficiency.