by H. Ward Silver
An ohm is an ohm, right? Not so
fast — there are many different
types of resistors. To insure that
your circuit works and stays
working, use the right type of
resistor. In this article, you’ll
learn about the common types
of resistors and their special
Every conductor exhibits some
resistance to the flow of electrical
charge (except for superconductors).
Georg Ohm discovered the exact relationship between voltage (V), current
(I), and resistance (R), formulating
the law that bears his name and is
learned by every electronics student:
atoms, which vibrate in response.
These vibrations result in an increased
temperature of the material. The
energy that heats the material is the
power dissipated, calculated as:
As electrons flow through a material in response to an electric field,
they collide with the atoms that make
up the material. The collision transfers
some of the electron’s energy to the
A perfect resistor does not care
whether the current flowing through it
is AC or DC. The electrons collide with
atoms going in either direction.
Nevertheless, practical construction
details, as shown in Figure 1, create
some parasitic effects, making an
actual resistor act like the circuit model
shown in Figure 2. The model describes
how the actual resistor’s characteristics
make its electrical behavior depend on
frequency and on how voltage and
current are applied to it.
The series inductance, LS, is primarily created by the leads attached
to the resistor. Notice that for the
surface-mount resistor, there are no
leads, which greatly reduces LS. The
electrodes also form a very small
capacitor, CP, that affects the resistor’s behavior at very high frequencies. Although the resistor’s coating
is a very good insulator, current can
still flow in very small amounts
across the surface of the resistor as a
leakage current, represented by RP.
This becomes important when the
resistor has a very high value or is
To learn more about resistors,
definitions are available from the
Resistor Term Glossary at www.
s.pdf Download the article on
choosing resistors “Ask the
Applications Engineer #24” from
Analog Devices’ website at
neer.html. Serious designers will
want to find a copy of The Resistor
Handbook by Kaiser (CJ Publishing,
2851 W. 127th St., Olathe, KS
66061) which is a good reference.
FIGURE 1. Resistors can be
constructed in a number of ways
to optimize power handling,
stability, or size.