Heater Location; Heater Types - Lakeshore 335 User Manual

20 c 2: Cooling System Design and Temperature Control
HAPTER

2.5.2 Heater Location

2.5.3 Heater Types

Model 335 Temperature Controller
Example 1: A 20 ) heater is connected to output 1, and the heater resistance setting
is set to 25 ), which can provide up to 1.41 A of current, and up to 50 V.
Current limit
P = I R
2
P = (1.41 A) 2 × (20 ))
P = 40 W
The power limit is the smaller of the two, or 40 W, limited by current.
Example 2: A 60 ) heater is connected to Output 2 (in current mode), and the heater
resistance setting is set to 50 ), which can provide up to 0.71 A of current, and up to
35.4 V.
Current limit
P = I R
2
P = (0.71 A) 2 × (60 ))
P = 42.6 W
The power limit is the smaller of the two, or 20.9 W, limited by voltage.
Example 3: A 50 ) heater is connected to Output 2 (in voltage mode). The maximum
voltage for Output 2 in voltage mode is 10 V, and the maximum current is 100 mA.
Current limit
P = I R
2
P = (0.1 A) 2 × (50 ))
P = 0.5 W
The power limit is the smaller of the two, or 0.5 W, limited by current.
It is possible to choose a heater value that results in a maximum power greater than the
power rating for either current source output, but doing so can cause the Model 335 to
work improperly. In this situation the max user current setting should be used to limit the
power. Refer to section 4.5.1.3.1 for details on using the max user current setting.
The resistor chosen as a heater must be able to withstand the power being dissipated
in it. Pre-packaged resistors have a power specification that is usually given for the
resistor in free air. This power may need to be derated if used in a vacuum where con-
vection cooling cannot take place and it is not adequately anchored to a cooled sur-
face. The Model 335 has a current limit feature which allows you to specify the
maximum output current for each heater output (section 4.5.1), which when set
appropriately will help protect the heater from being overheated.
For best temperature measurement accuracy, position the heater so that tempera-
ture gradient across the sample is minimized. For best control, the heater should be in
close thermal contact with the cooling power. Geometry of the load can make one or
both of these difficult to achieve. That is why there are several heater shapes and
sizes.
Resistive wire like nichrome is the most flexible type of heater available. The wire can
be purchased with electrical insulation and has a predictable resistance per given
length. This type of heater wire can be wrapped around a load to give balanced, even
heating of the area. Similar to sensor lead wire, the entire length of the heater wire
should be in good thermal contact with the load to allow for thermal transfer. Ther-
mal anchoring also protects the wire from overheating and burning out.
Voltage limit
P = V /R
2
P = (50 V) 2 /(20 ))
P = 125 W
Voltage limit
P = V /R
2
P = (35.4 V) 2 /(60 ))
P = 20.9 W
Voltage limit
P = V /R
2
P = (10 V) 2 /(50 ))
P = 2 W