Components are most easily
procured in standard values. They are
less expensive by many orders of magnitude.
Quite often the precise
value of a component is of no consequence.
Examples are a bleeder resistor in a power supply, a series current
limiting resistor or a pull-up resistor.
A good designer also attempts to make use of resistor ratios, as in an
op amp stage, where gain is the ratio, while avoiding precise values.
However, there may be times
when you just need that 14K resistor.
Active filters come to mind.
When try to implement a non
standard value, there is a natural tendency to immediately think or series
resistors, with good reason. R1 + R2 +
R3 is a whole lot easier to calculate in your head than are expressions for
parallel resistors.
However, consider the
following table. Standard values appear
in the first row and two decades of standard values appear in the first
column. The parallel equivalent appears
at the intersection.
Note that a 14K resistor
might be implemented with a 22K resistor in parallel with a 39K resistor. Another possibility is a 220K in parallel
with a 15K resistor.
In developing this table, I
used only ten percent standard values.
This might be expanded to also include five percent standard values; 11,
13, 16, 20, 24, 30, 36, 43, 51, 62, 75 and 91.
My reluctance in doing so was simply the size of the table.
In addition, one might
extend this to include three parallel resistors. Rather than a three dimensional spreadsheet, I might consider
using a spreadsheet to guess at which three standard values in parallel come
close to the desired value.
|
|
10.00 |
12.00 |
15.00 |
18.00 |
22.00 |
27.00 |
33.00 |
39.00 |
47.00 |
56.00 |
68.00 |
82.00 |
100.00 |
|
10.00 |
5.00 |
5.45 |
6.00 |
6.43 |
6.88 |
7.30 |
7.67 |
7.96 |
8.25 |
8.48 |
8.72 |
8.91 |
9.09 |
|
12.00 |
5.45 |
6.00 |
6.67 |
7.20 |
7.76 |
8.31 |
8.80 |
9.18 |
9.56 |
9.88 |
10.20 |
10.47 |
10.71 |
|
15.00 |
6.00 |
6.67 |
7.50 |
8.18 |
8.92 |
9.64 |
10.31 |
10.83 |
11.37 |
11.83 |
12.29 |
12.68 |
13.04 |
|
18.00 |
6.43 |
7.20 |
8.18 |
9.00 |
9.90 |
10.80 |
11.65 |
12.32 |
13.02 |
13.62 |
14.23 |
14.76 |
15.25 |
|
22.00 |
6.88 |
7.76 |
8.92 |
9.90 |
11.00 |
12.12 |
13.20 |
14.07 |
14.99 |
15.79 |
16.62 |
17.35 |
18.03 |
|
27.00 |
7.30 |
8.31 |
9.64 |
10.80 |
12.12 |
13.50 |
14.85 |
15.95 |
17.15 |
18.22 |
19.33 |
20.31 |
21.26 |
|
33.00 |
7.67 |
8.80 |
10.31 |
11.65 |
13.20 |
14.85 |
16.50 |
17.88 |
19.39 |
20.76 |
22.22 |
23.53 |
24.81 |
|
39.00 |
7.96 |
9.18 |
10.83 |
12.32 |
14.07 |
15.95 |
17.88 |
19.50 |
21.31 |
22.99 |
24.79 |
26.43 |
28.06 |
|
47.00 |
8.25 |
9.56 |
11.37 |
13.02 |
14.99 |
17.15 |
19.39 |
21.31 |
23.50 |
25.55 |
27.79 |
29.88 |
31.97 |
|
56.00 |
8.48 |
9.88 |
11.83 |
13.62 |
15.79 |
18.22 |
20.76 |
22.99 |
25.55 |
28.00 |
30.71 |
33.28 |
35.90 |
|
68.00 |
8.72 |
10.20 |
12.29 |
14.23 |
16.62 |
19.33 |
22.22 |
24.79 |
27.79 |
30.71 |
34.00 |
37.17 |
40.48 |
|
82.00 |
8.91 |
10.47 |
12.68 |
14.76 |
17.35 |
20.31 |
23.53 |
26.43 |
29.88 |
33.28 |
37.17 |
41.00 |
45.05 |
|
100.00 |
9.09 |
10.71 |
13.04 |
15.25 |
18.03 |
21.26 |
24.81 |
28.06 |
31.97 |
35.90 |
40.48 |
45.05 |
50.00 |
|
120.00 |
9.23 |
10.91 |
13.33 |
15.65 |
18.59 |
22.04 |
25.88 |
29.43 |
33.77 |
38.18 |
43.40 |
48.71 |
54.55 |
|
150.00 |
9.38 |
11.11 |
13.64 |
16.07 |
19.19 |
22.88 |
27.05 |
30.95 |
35.79 |
40.78 |
46.79 |
53.02 |
60.00 |
|
180.00 |
9.47 |
11.25 |
13.85 |
16.36 |
19.60 |
23.48 |
27.89 |
32.05 |
37.27 |
42.71 |
49.35 |
56.34 |
64.29 |
|
220.00 |
9.57 |
11.38 |
14.04 |
16.64 |
20.00 |
24.05 |
28.70 |
33.13 |
38.73 |
44.64 |
51.94 |
59.74 |
68.75 |
|
270.00 |
9.64 |
11.49 |
14.21 |
16.88 |
20.34 |
24.55 |
29.41 |
34.08 |
40.03 |
46.38 |
54.32 |
62.90 |
72.97 |
|
330.00 |
9.71 |
11.58 |
14.35 |
17.07 |
20.63 |
24.96 |
30.00 |
34.88 |
41.14 |
47.88 |
56.38 |
65.68 |
76.74 |
|
390.00 |
9.75 |
11.64 |
14.44 |
17.21 |
20.83 |
25.25 |
30.43 |
35.45 |
41.95 |
48.97 |
57.90 |
67.75 |
79.59 |
|
470.00 |
9.79 |
11.70 |
14.54 |
17.34 |
21.02 |
25.53 |
30.83 |
36.01 |
42.73 |
50.04 |
59.41 |
69.82 |
82.46 |
|
560.00 |
9.82 |
11.75 |
14.61 |
17.44 |
21.17 |
25.76 |
31.16 |
36.46 |
43.36 |
50.91 |
60.64 |
71.53 |
84.85 |
|
680.00 |
9.86 |
11.79 |
14.68 |
17.54 |
21.31 |
25.97 |
31.47 |
36.88 |
43.96 |
51.74 |
61.82 |
73.18 |
87.18 |
|
820.00 |
9.88 |
11.83 |
14.73 |
17.61 |
21.43 |
26.14 |
31.72 |
37.23 |
44.45 |
52.42 |
62.79 |
74.55 |
89.13 |
|
1000.00 |
9.90 |
11.86 |
14.78 |
17.68 |
21.53 |
26.29 |
31.95 |
37.54 |
44.89 |
53.03 |
63.67 |
75.79 |
90.91 |