20-07-2012, 12:24 PM
Simplified Thermocouple Interfaces and PICmicro MCUs
[attachment=28164]
INTRODUCTION
Thermocouples are the simplest form of temperature
sensors. Thermocouples are normally:
• Very inexpensive
• Easily manufactured
• Effective over a wide range of temperatures
Thermocouples come in many different types to cover
nearly every possible temperature application.
In Application Note AN684, thermocouple basics are
covered along with some circuits to measure them.
This Application Note begins where AN684 leaves off
and describes methods of obtaining good accuracy
with minimal analog circuitry. Also covered in this Application
Note are:
• Different linearization techniques
• Cold junction compensation
• Diagnostics
Linearization
Linearization is the task of conversion that produces a
linear output, or result, corresponding to a linear
change in the input. Thermocouples are not inherently
linear devices, but there are two cases when linearity
can be assumed:
1. When the active range is very small.
2. When the required accuracy is low.
Pilot lights in water heaters for example, are typically
monitored by thermocouples. No special electronics is
required for this application, because the only accuracy
required is the ability to detect a 600 degree increase
in temperature when the fire is lit.
Absolute Temperature Scaling
Thermocouples are relative measuring devices. In
other words, they measure the temperature difference
between two thermal regions. Some applications are
only interested in this thermal difference, but most
applications require the absolute temperature of the
device under test. The absolute temperature can be
easily found by adding the thermocouple temperature
to the absolute temperature of one end of the thermocouple.
This can be done at any point in the thermocouple
circuit. Figure 1 shows the scaling occurring after
the linearization.
Results
The result of the thermocouple circuit is a usable indication
of the temperature. Some applications simply
display the temperature on a meter. Other applications
perform some control or warning function. When the
results are determined, the work of the thermocouple
circuit is finished.
Simplified Digital
Most analog problems can be converted to a digital
problem and thermocouples are no exception. If an
analog-to-digital converter (ADC) were placed at the
end of the analog solution shown in Figure 2, the result
would be a simple digital thermometer (at least the software
would be simple). However, the analog/linear circuitry
could be made less expensive to build and
calibrate by adding a microcontroller.
[attachment=28164]
INTRODUCTION
Thermocouples are the simplest form of temperature
sensors. Thermocouples are normally:
• Very inexpensive
• Easily manufactured
• Effective over a wide range of temperatures
Thermocouples come in many different types to cover
nearly every possible temperature application.
In Application Note AN684, thermocouple basics are
covered along with some circuits to measure them.
This Application Note begins where AN684 leaves off
and describes methods of obtaining good accuracy
with minimal analog circuitry. Also covered in this Application
Note are:
• Different linearization techniques
• Cold junction compensation
• Diagnostics
Linearization
Linearization is the task of conversion that produces a
linear output, or result, corresponding to a linear
change in the input. Thermocouples are not inherently
linear devices, but there are two cases when linearity
can be assumed:
1. When the active range is very small.
2. When the required accuracy is low.
Pilot lights in water heaters for example, are typically
monitored by thermocouples. No special electronics is
required for this application, because the only accuracy
required is the ability to detect a 600 degree increase
in temperature when the fire is lit.
Absolute Temperature Scaling
Thermocouples are relative measuring devices. In
other words, they measure the temperature difference
between two thermal regions. Some applications are
only interested in this thermal difference, but most
applications require the absolute temperature of the
device under test. The absolute temperature can be
easily found by adding the thermocouple temperature
to the absolute temperature of one end of the thermocouple.
This can be done at any point in the thermocouple
circuit. Figure 1 shows the scaling occurring after
the linearization.
Results
The result of the thermocouple circuit is a usable indication
of the temperature. Some applications simply
display the temperature on a meter. Other applications
perform some control or warning function. When the
results are determined, the work of the thermocouple
circuit is finished.
Simplified Digital
Most analog problems can be converted to a digital
problem and thermocouples are no exception. If an
analog-to-digital converter (ADC) were placed at the
end of the analog solution shown in Figure 2, the result
would be a simple digital thermometer (at least the software
would be simple). However, the analog/linear circuitry
could be made less expensive to build and
calibrate by adding a microcontroller.