Electronic components are the vocabulary of electronic design. In the same way that English vocabulary constrains the ways that we can express ourselves, the the available components determine how designs are expressed. Our language also sets limits on what thoughts we can express at all. Similarly, the available components constrain what things we can design. I think that electronic components provide an interesting case study in the interaction between physical possibility, physical laws (or models), technology and society.

The relationship between fixed voltage across two terminals and the corresponding current flow is described by Ohm's law:

E = I * RR is the component characteristic known as

All real components deviate from the ideal in every possible way. Passive components are designed to be the best practical approximation of an ideal resistance, capacitance or inductance. For example, a resistor is designed to provide a fixed value of resistance with very little capacitance or inductance.

There are three main classes of non-ideality:

*parasitic impedances*- All components have frequency-dependent impedances, so all resistors have
some capacitance and inductance. These undesired impedances are called
*parasitic*impedances. In many cases, the components are good enough that designers can get away with using the ideal model and ignoring the effect of parasitics. Even when the parasitics are important, you can still use a linear model by pretending that the resistor is a combination of an ideal resistor, ideal capacitor and ideal inductor. *limited linearity*- Real components are somewhat nonlinear. Their impedance varies depending on how much voltage you apply. (Some are designed to be highly nonlinear, but that's another story.) If you apply way too much voltage, the impedance will be irreversibly changed, with much production of smoke. Usually nonlineary can be ignored as long as you stay well away from the smoke-producing operating region.
*inaccuracy and instability*- The value of all real components is not precisely what it is supposed to be when the component is manufactured, and this inaccurate value keeps changing as the component ages and the temperature changes. Humidity and mechanical stress or vibration will also affect the value. Often ordinary components are good enough, but sometimes the accuracy and stability of a precision component is needed.

Manufacturers don't really guarantee that all of their components will meet all
of their specifications all of the time, but if a particular component doesn't
meet specifications, then it is said to have *failed* -- it's busted.

A key thing to realize here is that the specification is just a
*more general model*, and not a complete description. Of course,
at some microscopic level, each individual component is distinct, so
the manufacturer must resort to statistical approximations. The only
exact model of a thing is the thing itself, and this is a useless
model because it has no generality. But what is more interesting from
the designer's perspective is what the manufacturer chooses to
specify.

But what if the resistor will have to absorb 100 watts, but only for one millionth of a second? Our physical understanding says that the resistor can only heat up so fast, and the main problem is the final temperature. If the resistor has enough mass, the temperature will hardly change at all with a short pulse. If this pulse happens once a second, the average power is only 100 millionths of a watt, or 0.0001 watts. This is 1/5000 of the continuous power rating, so linear thinking would say this is extremely safe.

But linearity is limited in the real world... In fact, figure 2 in our datasheet tells us that one millionth of a second is o.k., but four millionths is a no-no. The shape of the curve in the datasheet has to do with specific details of the resistor materials and construction. The graph was probably constructed by experiment rather than theoretical analysis: zap it and see if it smokes.

For this discussion, the main point is that this table is useful, and
that *most resistor data sheets don't have this table*. It is up
to the manufacturer what they want to specify. Without a pulse power
rating, you would have use a huge expensive 100 watt resistor or you
would have to trust your physical intuition and experience that a much
smaller resistor is o.k.

One of the oldest resistor designs is the carbon composition resistor. Although it is noisy, has poor accuracy, stability, linearity, and is bulky for a given continuous power rating, engineers found that it was much better at withstanding high-power zaps than many newer designs. Engineers came to trust the performance of carbon composition resistors, and have resisted manufacturer's attempts to phase them out. The interesting point is that these resistors were never rated for pulse power, and still are not.

This is a nice new model. It's probably just as much of an oversimplification as the constant power model, but people will buy it because it's easy to work with. They don't care if the component can actually stand higher pulse energies at particular pulse lengths.

Why are there two different specs? The two main reasons are yield and
testing costs. After manufacture, electronic compoponents are tested
to see if they meet their specs. The fraction that passes the test is
the *yield* Although 90% of the components made may meet the
typical spec, there are many different specs, and the actual component
values vary independently. It is likely any given component will fail
at least one of the typical specs.

I've seen typical specs ranging from 0.001 millonths of an amp to 0.000001 millonths of an amp, from 100x to 10,000x better than the guaranteed spec. Furthermore, when I've tested these parts, the actual performance is significantly better even than these typical specs, in some cases 1000x better than the "typical" spec.

What is going on here? Basically we have a component that is uselessly close to ideal. The leakage current is so close to zero that detecting it requires specialized lab instruments, careful technique and a fair amount of time. So the manufacturer only tests that it is close enough to zero for typical use.

Sometimes engineers do need the true component performance, or at least a level of performance much higher than the tested worst-case performance. Then they can either pay a lot more for testing up to the needed spec, test it themselves, or just cross their fingers and use the component without any special testing.

*Last update 25 June 2004*

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