Lake Shore Model 340 Temperature Controller User’s Manual
1-2
Introduction
1.1 TEMPERATURE CONTROL SYSTEM
The goal of a temperature control system is to balance the effect of heating and cooling to provide a stable
temperature in an area of interest. In order to control temperature, you need the following elements:
1. A heating source.
2. A
cooling
source.
3. A means to measure temperature.
4. A means to compare the measured temperature to a desired temperature (called the setpoint).
5. A means to make any necessary changes to either the heating, the cooling, or both.
One familiar example is the temperature control system in a typical home. When the outside temperature
(cooling source) lowers the temperature inside (area of interest) below the temperature setting (desired
temperature) the thermostat measures the temperature and detects the difference (comparison) and turns on
the furnace (changes heat source). Substituting some control jargon would result in the description of a
temperature
control loop
: When the
cooling source
reduces the temperature of the
load
below the
setpoint
, a temperature
sensor
provides
feedback
to the
control equation
which detects the
error
and
actuates
the
heat source
.
In our house example, there is only one actuator: the heat of the furnace. If the house is equipped with air
conditioning, the cooling source is a second actuator allowing control over a greater range of temperature. All
of the parts are called a
closed loop
control system
and the operation is referred to as a
control loop
because there is a cycle of cause and effect: From the control equation to the actuator to the load to the
sensor to the control loop and so on. The loop operation allows the heat source to turn on and off repeatedly,
keeping the temperature constant over a long period of time.
1.2 TEMPERATURE CONTROL SYSTEM WITH THE MODEL 340
The Model 340 is designed to work
as an integral part of a cryogenic
temperature control system (as
described in Paragraph 1.1). The
Model 340 has many specialized
features that uniquely qualify it for
this difficult job. These features are
built on a fundamentally sound
design that can be used in many
applications requiring stable and
accurate control from cryogenic
temperatures, to room temperature
and above. Much of the information
in this manual refers to cryogenic
systems, but the ideas generally
apply at any temperature.
1.2.1 Cooling
Source
The Model 340 integrates many of the elements of a complete control system. There are a few parts that the
Model 340 does not provide. The largest and most notable part missing is a cooling source. The Model 340
provides power to a resistive heater acting as the heating source, but it does not provide cooling. When
operating at cryogenic temperatures a vacuum insulated cryostat or dewar typically provides cooling power to
a relatively small load that may be a block of copper or other thermally conductive material. The load has a
sample holder or mounting surface for the material that is being studied. Sample holders are often custom
made for individual experiments.
Setpoint
Model 340
(Control
Equation)
Cooling Source
(Cryogen)
Heater
Sensor
(Actual Temp.)
Feedback
Load
(Cryostat)
Temperature
You Want
Warm Side
Cold Side
Figure 1-2. Typical Temperature Control System With Model 340