1 DC ELECTRICAL CIRCUITS RESISTANCE

2 DC ELECTRICAL CIRCUITS In short resistance limits the flow of current. Resistance is measured in ohms, the symbol used to refer to resistance is the omega symbol Ω. The letter R is used to represent resistance when working with formulas. The resistivity of material determines the overall resistance, however there are three other factors that determine resistance. In short resistance limits the flow of current. Resistance is measured in ohms, the symbol used to refer to resistance is the omega symbol Ω. The letter R is used to represent resistance when working with formulas. The resistivity of material determines the overall resistance, however there are three other factors that determine resistance.

3 DC ELECTRICAL CIRCUITS There are three factors that contribute to a material’s resistance, typically cable. LENGTHCROSS SECTIONAL AREATEMPERATURE

4 DC ELECTRICAL CIRCUITS Resistors are commonly used to perform two functions in a circuit. The first use is to limit the flow of current in a circuit. Resistors are commonly used to perform two functions in a circuit. The first use is to limit the flow of current in a circuit. I = E / R I = 15 V / 30 Ω I = 0.5 A

5 DC ELECTRICAL CIRCUITS The second use is to produce a voltage divider. A to B = 1.5 V A to C = 7.5 V A to D = 17.5 V B to C = 6 V B to D = 16 V C to D = 10 V

6 DC ELECTRICAL CIRCUITS Fixed resistors have only one ohmic value, which cannot be changed or adjusted. One type of fixed resistor is the composition carbon resistor.

7 DC ELECTRICAL CIRCUITS Carbon resistors are very popular for most applications because they are inexpensive and readily available in standard sizes and wattages. ½ Watt1 Watt2 Watt

8 DC ELECTRICAL CIRCUITS Metal Film resistors are another type of fixed resistor. These resistors are superior to carbon resistors because their ohmic value does not change with age and they have improved tolerance.

9 DC ELECTRICAL CIRCUITS Wire wound resistors are fixed resistors that are made by winding a piece of resistive wire around a ceramic core. These are used when a high power rating is required.

10 DC ELECTRICAL CIRCUITS The resistor color code can be used to determine the resistor’s ohmic value and tolerance.

11 DC ELECTRICAL CIRCUITS Exceeding the power rating causes damage to a resistor.

12 DC ELECTRICAL CIRCUITS Variable resistors can change their value over a specific range. A potentiometer is a variable resistor with three terminals. A rheostat has only two terminals.

13 DC ELECTRICAL CIRCUITS Schematic symbols are used to represent various types of fixed resistors.

14 DC ELECTRICAL CIRCUITS

15 DC ELECTRICAL CIRCUITS This resistor would equal 1-2 and 2 (0s) or 1200 Ω with a tolerance of + 20%.

16 DC ELECTRICAL CIRCUITS 1 2 X % 12 X 100 = 1200Ω + 20%

17 DC ELECTRICAL CIRCUITS What is the color code of a 280 ohm resistor ? RED GRAY BLACK What is the color code of a 280 ohm resistor ? RED GRAY BLACK

18 DC ELECTRICAL CIRCUITS A resistor’s tolerance is the amount a resistor may vary and still be acceptable. The larger the tolerance of a resistor the cheaper the resistor is. Resistors of + 10% are widely accepted for use in electronic circuits. A resistor’s tolerance is the amount a resistor may vary and still be acceptable. The larger the tolerance of a resistor the cheaper the resistor is. Resistors of + 10% are widely accepted for use in electronic circuits.

19 DC ELECTRICAL CIRCUITS 1000 ohm resistor has a color code of? 2.2 k ohm resistor color code? 45 ohm color code? 12.5 k ohm color code? 1 ohm color code? 1000 ohm resistor has a color code of? 2.2 k ohm resistor color code? 45 ohm color code? 12.5 k ohm color code? 1 ohm color code?

20 DC ELECTRICAL CIRCUITS The formula for finding total resistance in a series circuit is Rt = R 1 + R 2 + R 3 + R n, (Rn is any number or resistances in series).

21 DC ELECTRICAL CIRCUITS To find total resistance in a parallel circuit the formula is: This is called the reciprocal formula and used for 3 or more resistors in parallel, it can be used for 2 resistors in parallel however there is another simpler formula for finding 2 resistor in parallel. To find total resistance in a parallel circuit the formula is: This is called the reciprocal formula and used for 3 or more resistors in parallel, it can be used for 2 resistors in parallel however there is another simpler formula for finding 2 resistor in parallel.

22 DC ELECTRICAL CIRCUITS The total resistance of a parallel circuit will always be less than the lowest branch.

23 DC ELECTRICAL CIRCUITS To find Rt in a series parallel circuit we have to add the resistor to the branch circuit its in then use the reciprocal formula again. WE ALREADY DETERMINED THE VALUE OF THE PARALLEL RESISTANCES FOR THIS CIRCUIT, IT IS OHMS, IN THIS CASE WE WOULD SIMPLY ADD THE 37 OHM RESISTOR TO THE TOTAL SO IT WOULD EQUAL OHMS

24 DC ELECTRICAL CIRCUITS If the series resistor is at the end it gets added to the R3 resistor than we would apply the reciprocal formula again, in this case Rt will be different from the previous totals.

25 DC ELECTRICAL CIRCUITS To find total resistance in a parallel circuit with resistors that all have the same value we can use the simplified formula: R (resistance) = value of one like resistor (R) divide by the number of like resistors (N). To find total resistance in a parallel circuit with resistors that all have the same value we can use the simplified formula: R (resistance) = value of one like resistor (R) divide by the number of like resistors (N).

26 DC ELECTRICAL CIRCUITS What would 4, 100 ohm resistors in parallel equal? What would 6, 120 ohm resistor in parallel equal? What would 4, 100 ohm resistors in parallel equal? What would 6, 120 ohm resistor in parallel equal?

27 DC ELECTRICAL CIRCUITS To find two resistor in parallel you can use the product over sum method.