# Resistors in parallel

### - The values produced by placing resistors in parallel including a reference table

When designing or building an electronic circuit, it may be necessary to use two or more resistors instead of one. Either the correct value of resistor may not be available, or it may be necessary to dissipate a higher power than that which a single resistor can dissipate on its own.

### Resistance of resistor combinations

Resistors can be placed in series or parallel. When placed in series the total resistance is equal to the sum of the individual resistors:

Resistors in series

It is also worth noting that the same current flows through each resistor, but the voltage across each resistor is proportional to the resistance of that particular resistor.

For resistors placed in parallel, the arithmetic is a little more complicated because the reciprocal of the total resistance is equal to the sum of the reciprocals of the constituent resistors:

Resistors in parallel

When there are only two resistors R1 and R2 in parallel this can be simplified to:

If both these resistors have the same value it can be seen that the overall value of the resistance is half the value for the individual resistor.

When resistors are placed in parallel the voltage across the resistors is the same, but the current through each one is inversely proportional to its resistance.

### Parallel resistor value table

While the values for resistors in parallel can be calculated relatively easily in most cases a table giving suitable values that are in the most case preferred is given below. Column A gives the required resistor value, and column B gives the first choice combination for the parallel resistors. A second choice is shown in column C. The values should be a close tolerance as possible, typically 0.5%.

Required resistor value "A" First selection of resistors "B" Alternative selection of resistors "C"
10.0 10   11 110
11.0 11   12  130
12.0 12   13 150
13.0 13   15 100
14.0 16 110 15 220
15.0 15   16 240
16.0 16   27 39
17.0 22 75 18 300
18.0 18   20 180
19.0 24 91 20 390
20.0 20   22 220
22.0 22   24 270
23.0 39 56 24 560
24.0 24   27 220
25.0 30 150 27 330
26.0 27 680 30 200
27.0 27   30 270
28.0 51 62 30 430
29.0 33 240 30 910
30.0 30   33 330
31.0 33 510 39 150
32.0 47 100 33 1100
33.0 33   36 390
34.0 68 68 162 75
35.0 36 1200 39 330
36.0 36   39 470
37.0 39 750 43 270
38.0 39 1500 43 330
39.0 39   68 91
40.0 47 270 43 560
41.0 82 82 43 910
42.0 43 1800 62 130
43.0 43   47 510
44.0 47 680 62 150
45.0 82 100 47 1100
46.0 47 2200 51 470
47.0 47   82 110
48.0 51 820 56 330
49.0 56 390 51 1300
50.0 75 150 56 470
51.0 51   56 560
52.0 68 220 56 750
53.0 68 240 56 1000
54.0 56 1500 62 430
55.0 110 110 56 3000
56.0 56   75 220
57.0 75 240 62 680
58.0 91 160 62 910
59.0 62 1200 62 1300
60.0 68 510 75 300
62.0 62   75 360
64.0 68 1100 75 430
66.0 91 240 68 2200
68.0 68   91 270
70.0 82 470 75 1000
72.0 75 1800 120 180
74.0 75 5600 82 750
76.0 82 1000 91 470
78.0 62 1600 110 270
80.0 120 240 82 3300
82.0 82   91 620
84.0 91 1100 110 360
86.0 100 620 91 1500
88.0 120 330 91 2700
90.0 120 360 91 8200
92.0 110 560 120 390
94.0 100 1600 120 430
96.0 100 2400 110 750
98.0 100 5100 110 910