Series and parallel resistors
- The values produced by placing resistors in series and 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 |
|
