Updated February 21,
here to go to our main page on resistors
here to go to our page on thick-film resistors
here to go to our page on thin-film resistors
Even at microwave frequencies,
resistors obey Ohm's law:
However, in the real world, there
is often some non-linearity
associated with the term "R". This might be because you
are dealing with a semiconductor, which might have a saturation
effect, or perhaps because of the temperature effect on resistance
due to power dissipation. Watch out!
By the way, one definition of
"semiconductor" is a device that doesn't follow Ohm's
law. Thus the need for curve
For April 2010, sheet resistance
math has been moved
to its own page.
depth considerations: in a microstrip transmission line, the
part of the conductor and ground plane that carry the most current
is the closest to the substrate. The skin depth affect is what makes
RF sheet resistance
greater than DC sheet
All resistors exhibit some degree
of variation with temperature. Usually the variation is close to
linear. The temperature coefficient of resistance, ,
is usually expressed in ppm/degrees Celsius:
The temperature coefficient of
resistance can be negative or positive. In the former case the resistance
is decreased with temperature. In the latter case it increases with
temperature. Pure metals have a positive coefficient. Some alloys
have been formulated to have a near-zero temperature coefficient
(constantin and maganin for example). Carbon and its associated
binders usually has a negative temperature coefficient.
are resistors that are built specifically to exploit the temperature
coefficient, and are often used as temperature control elements.
A thermistor with negative temperature coefficient is called "NTC",
while a positive temperature coefficient thermistor is called "PTC".
Did we mention that the temperature
coefficient is always at least a slight function of temperature?
Something you need to consider, thermistors are not perfectly linear.
Power rating of a resistor specifies
the most power that a resistor can dissipate up to a maximum temperature,
which will not damage the resistor. Power rating usually implies
that a maximum hot-spot temperature must not be reached, surpassing
the limit may result in permanent damage
Power rating specifies two temperatures;
the first is the temperature up to which the maximum power rating
applies, the second temperature is where the rating must be derated
to zero dissipation, in between the rating derated linearly with
temperature. Two things can be inferred from these ratings: the
maximum storage temperature is equal to the derated temperature.
Also, the slope of the derating can be used to calculate an equivalent
thermal resistance in degrees C per watt. The difference between
the no-load and maximum full load temperature is less than or equal
to the temperature rise at full load.
Let's run through the math, for
the component who's derating curve is shown. The calculated thermal
(Thanks for the correction, Rod!)
Nothing could be easier!