Lake Shore Model 340 Temperature Controller User’s Manual
Introduction
1-3
Cooling Source (Continued)
The load must also accommodate a temperature sensor for feedback and a resistive heater for heating. The
sensor and heater are in thermal contact with the cooled load while the Model 340 stays at room temperature.
There are a large number of combinations of loads, sensors, and heaters to meet different sample and
experimental requirements. For this reason, sensors and heaters are not included with the instrument but are
either included with the cooling system or sold separately. Lake Shore has the most comprehensive line of
temperature sensors in the industry, and the Model 340 will work with most of them. Heaters and other
accessories are also available.
1.2.2 Feedback
Temperature sensing is important to the feedback parts of a control system as well as an important
experimental parameter. The sensitivity of a sensor in the temperature range plays a critical role in the
accuracy and resolution of the measurement as well as the control stability. The Model 340 provides current
to (excites) the sensors and measures the resulting signal. The signal is converted to temperature by
comparing it to a temperature response curve that is stored in the instrument. Every effort is made within the
instrument to keep the measurement as accurate and quiet as possible. Temperature feedback begins the
role of the Model 340 in a control system.
1.2.3 Control
Equation
The Model 340 can implement a control equation with an algorithm that calculates the amount of heating
needed to keep the load at the setpoint temperature. For most systems the cryostat or other cooling source is
running all of the time. It is necessary to tune the control equation because cooling sources and loads are
different. The Model 340 provides several different tuning options for choosing the best control settings. An
improperly tuned system will either not stabilize or will stabilize at a temperature below the setpoint.
1.2.4 Heating
Once the control equation computes the adjustment needed in heating, the Model 340 implements it by
changing the amount of current being sent to the resistive heater. There are significant differences between
the Model 340 and a home thermostat. First, the Model 340 can source the necessary power with an internal
direct current (DC) power supply while a thermostat depends on the furnace to provide heating power.
Second the Model 340 can make small adjustments in power to maintain a stable continuous temperature.
A thermostat can only turn the furnace on and off creating a abrupt change in temperature that includes
warming above the setpoint then cooling below. The heater output of a Model 340 can be optimized for
individual systems with heater ranges and limits.
1.3 MODEL
340
FEATURES
The following chapters of this manual are intended to help integrate the Model 340 into a temperature control
system with the fundamental building blocks described in Paragraph 1.2. This section is a brief overview of
some individual features of the instrument that may not be obvious in the setup and operation sections. These
features illustrate the Lake Shore goal of making the Model 340 a valuable part of the temperature control
environment. See Figure 1-3.
1.3.1 Configurable
Inputs
Each of the many different types of temperature sensor has qualities that make it the best choice for some
application. The Model 340 has the ability to operate with many different types of sensors so that it is not a
limiting factor when deciding which sensor to use. The two sensor inputs included in the Model 340 can be
configured to measure and control with nearly any diode or resistive sensor. Appropriate excitation and input
gain can be selected from the front panel. There are no modules or cards to swap or internal switches to
change. Resistive sensors that change many orders of magnitude are measured in an autorange mode that
keeps the power in the sensor low to reduce self heating. The Model 340 does require the installation of an
option card to read sensors other than diodes and resistors or to read more than two inputs continuously.