Lake Shore Model 325 Temperature Controller User’s Manual
4-24 Operation
4.13 HEATER RANGE AND HEATER OFF
The heater output for Loop 1 is a well-regulated variable DC current source, while the heater output for Loop 2 is a
variable DC voltage source. Both heater outputs are optically isolated from the sensor input circuits to reduce
interference and ground loops. The heater output for the main control loop (Loop 1) can provide up to 25 W of
continuous power to a resistive heater load and includes a low range for systems with less cooling power, while the
Loop 2 heater output can provide up to 2 W of continuous power. Both Loop 1 and Loop 2 Heater outputs are
short-circuit protected to prevent instrument damage if the heater load is accidentally shorted.
NOTE: During normal operation, if the input type or input curve is changed for the control input, the heater
will automatically shut off.
Specifications of the heater outputs are provided in Section 1.2 – Specifications. Heater theory of operation is provided
in Section 2.4 – Heater Selection and Installation. Various Heater installation considerations are provided in Section 3.6
– Heater Output Setup.
Once control setup parameters are configured (Section 4.7), and the active control loop is selected (Section 4.6.1), the
desired heater range is selected by pressing the Heater Range key.
Use the s or t key to cycle through Loop 1 Heater settings: Off, Low, and High. Once the desired heater setting is
displayed, press the Enter key. You will return to the normal display.
Use the s or t key to toggle between Loop 2 Heater settings: Off and On. Once the desired heater setting is displayed,
press the Enter key. You will return to the normal display.
To immediately turn the heater off, press the Heater Off key. If the Heater Range is not being displayed on the front
panel, the user should immediately press the Heater Range key to verify that the proper loop is displayed and the heater
shows Off.
4.14 HEATER RESISTANCE SETTING
The Model 325 Loop 1 and Loop 2 heater outputs are designed to accommodate two common heater resistance
values: 25 and 50 . In order to achieve full output power, and stable temperature control over the full output range
(0 – 100%) the heater resistance setting must be set properly for both control loops. For Loop 1, the heater resistance
setting controls the heater output compliance voltage (50 setting = 36 V nominal, 25 setting = 25 V nominal).
Loop 1 was designed to provide 0 – 1 A of output current for heaters up to 20% lower than the nominal heater resistance
when the proper heater resistance setting is used. For Loop 2, the heater resistance setting controls the heater output full-
scale voltage (50 setting = 10 V, 25 setting = 5 V). If the heater resistance setting is not set properly limiting could
occur, which could result in temperature control instability. An exception occurs when using a heater greater than the
heater resistance setting on Loop 2. In this situation the maximum heater power is lowered as the heater resistance
increases, but control over the full output range will not be limited.
To set the heater resistance for the currently displayed loop, press the Control Setup key, then press the Enter key until
you see the following display.
Use the s or t key to select 25 or 50 . Press the Enter key to save changes and return to the normal display.
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