The Laws of Circuit - you can learn and practice by just
reading
Copyright. Charles Kim 2006
- Resistor
Specification
-
- Value, Tolerance, and Power Rating
- There are three important parameters that
specify a resistor: value, tolerance, and power rating.
- Have you ever wondered why the part store
(or part cabinet) in your school does not have 115 ohm
resistor you need in your implementation of a circuit on
your breadboard? If yes, probably you also wondered
why these weird values of resistors in the store:
3.9 kohm, 4.7 kohm, 22 ohm, 33 ohm, but not 25 ohm. Even
though 25 ohm value is more reasonable than 33 or 22 ohm
resistors? To discuss this resistor value, we
need to talk about the second parameter: tolerance.
Tolerance is simply error from the value specified.
In other words, if tolerance is 10%, then the expected
value from a resistor of 10 ohm is between 9 (i.e., 10 - 1) ohm and
11 (i.e., 10 + 1) ohm. Two most common tolerance values are
5% and 10%. There is 1% tolerance also but you can
rarely see in the cabinet.
-
- Standard Resistor Values
- The following are a part of standard
resistor values in ohm for 5% and 10 % tolerance:
-
- 5% resistors: 1.0, 1.1, 1.2, 1.3, 1.5,
1.6, 1.8, 2.0, 2.2, 2.4, 2.7, 3.0, 3.3, 3.6, 3.9, 4.3,
4.7, 5.1, 5.6, 6.2, 6.8, 7.5, 8.2, 9.1, 10, 11, 12, 13,
15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51,
56, 62, 68, 75, 82, 91, 100, 110, 120, 130, 150, 160,
180, 200, 220, 240, 270, 300, 330, 360, 390, 430, 470,
510, 560, 620, 680, 750, 820, 910, 1.0k, 1.1k, 1.2k,
1.3k, 1.5k, 1.6k, 1.8k, 2.0k, 2.2k, 2.4k, 2.7k, 3.0k,
3.3k, 3.6k, 3.9k, 4.3k, 4.7k, 5.1k, 5.6k, 6.2k, 6.8k,
7.5k, 8.2k, 9.1k, 10k, 11k, 12k, 13k, 15k, 16k, 18k, 20k,
22k, 24k, 27k, 30k, 33k, 36k, 39k, 43k, 47k, 51k, 56k,
62k, 68k, 75k, 82k, 91k.
-
- 10% resistors: 1.2,1.5, 1.8, 2.2, 2.7,
3.3, 3.9, 4.7, 5.6, 6.8, 12, 15, 18, 22, 27, 33, 39, 47,
56, 68, 82, 120, 150, 180, 220, 270, 330, 390, 470, 560,
680, 820, 1.2k, 1.5k, 1.8k, 2.2k, 2.7k, 3.3k, 3.9k, 4.7k,
5.6k, 6.8k, 8.2k, 12k, 15k, 18k, 22k, 27k, 33k, 39k, 47k,
56k, 68k, 82k
-
- Now we wonder why we have such, weird,
so-called "standard" resistor
values. Who made these? The EIA
(Electronic Industries Association: http://www.eia.org/) did. EIA specifies "standard
values" for resistors, sometimes referred to as the
"preferred value" system. The preferred
value system has its origins in the early years of the
last century at a time when most resistors were
carbon-graphite with relatively poor manufacturing
tolerances. The rationale is simple - select values
for components based on the tolerances with which they
could be manufactured. Using 10% tolerance devices
as an example, suppose that the first preferred value is
100 ohms. It makes little sense to produce a 105
ohm resistor since 105 ohms falls within the 10%
tolerance range of the 100 ohm resistor. The next
reasonable value is 120 ohms because the 100 ohm resistor
with a 10% tolerance is expected to have a value
somewhere between 900 and 110 ohms. The 120 ohm resistor
has a value ranging between 110 and 130 ohms. Following
this logic, the preferred values for 10% tolerance
resistors between 100 and 1,000 ohms would be 100, 120,
150, 180, 220, 270, 330 and so on.
-
- E Series Resistors
- EIA has 6 series of resistor values with
different tolerances. The E6 series (6 values for each
multiple of ten, for resistors with 20% tolerance) has
the following values: 10, 15, 22, 33, 47, 68, ... then it
continues 100, 150, 220, 330, 470, 680, 1000 etc.
Notice how the step size increases as the value
increases. For this series the step (to the next value)
is roughly half the value. On the other hand, the E12
series (12 values for each multiple of ten, for resistors
with 10% tolerance) has the following resistor values:
10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82, ... then
it continues 100, 120, 150 etc.
-
- Power Rating
- The power rating specifies the maximum power that can be
dissipated by a resistor.
Electrical energy is converted to heat when current flows
through a resistor. Usually the effect is negligible, but
if the resistance is low (or the voltage across the
resistor is high) a large current may pass and make the
resistor becomes noticeably warm. Remember the
resistive heating is proportional to I2R.
The resistor must be able to withstand the heating effect
and resistors have power ratings up to which they
can stand. Some typical ratings are 1/8 W(Watts), ¼ W, ½
W, 1W, and 2W. Power ratings of resistors are
rarely quoted in part list because for most circuits
the standard power ratings of ¼ W or ½ W are suitable.
For rare cases where a higher power is required, it
should be clearly specified in the part list, specifically for circuits using low value resistors (less than about
300ohm) or serving high voltages (more than
15V). Anyway, in selecting a resistor for some
particular application, remember that one important criterion is the
expected power dissipation.
-
- Color Coding and Size
- Resistor values and tolerances are color
coded, and power ratings are usually recognizable by the
physical size. This information is readily available online or in
print, so I do not dispaly it here.
-
- WWW.MWFTR.COM