56 cHAPTER 4: Operation
Model 335 Temperature Controller
ting will then provide multiple discrete current limit values that correspond to com-
mon heater power ratings. The available current limits keep the output operating
within the voltage compliance limit to ensure the best possible resolution. These
parameters work with the Heater Range parameter (
section 4.5.1.7.8) to provide
safety and flexibility.
If you are not using a standard heater resistance, set the Heater Resistance setting to
25 ) for any resistance less than 50 ), or to 50 ) for any higher heater resistance. The
user max current setting is useful when using a non-standard heater resistance value.
Refer to
section 4.5.1.3.1 for more information on User Max Current. TABLE 4-11 pro-
vides examples of different heater resistances and max current settings, and the
resulting maximum heater power. The maximum heater powers in bold represent the
discrete current limits available under the Max Current setting for 25 ) and 50 )
heaters.
The 1.732 A (75 W) Max Current setting is only available on Output 1 when Output 2 is
configured for Voltage Mode. When Max Current is configured to 1.732 A on Output 1, the
Current Mode setting will not be available on Output 2. In order to set Output 2 to Current
Mode, the Max Current setting on Output 1 must be configured to 1.41 A or less.
Menu Navigation:
Output SetupQOutput (1, or 2)Q QHeater Resistance (25 ) or 50 ))
Output SetupQOutput (1, or 2)Q QMax Current (User, 0.707 A, 1 A, or 1.732 A)
Default:
Heater ResistanceQ25 )
Output 1 QMax Current (1.414 A)
Output 2QMax Current (1 A)
Interface Command: HTRSET
4.5.1.3.1 User Max Current
The provided discrete Max Current settings of 0.707 A, 1 A, 1.414 A, and 1.732 A are
available for the common heater resistor values of 25 ) and 50 ), and for various
common heater power ratings. If you are using a heater that deviates from these
common values, the User Max Current setting is provided.
The optimal maximum current value should be calculated based on the heater’s
power rating or the maximum desired heater output power, whichever is lower. The
heater output compliance voltage should also be taken into account in order to maxi
-
mize heater setting resolution. This calculated current limit can then be entered
using the User Max Current setting.
To calculate the Max Current setting based on a heater or load power limit, calculate
current, I, using both of the following equations: I = Sqrt(P/R) and I = V/ R where P is
the maximum allowable power, V is the output compliance voltage (50 V for Output 1
and 35.5 V for Output 2), and R is the heater resistance. The load power limit and volt
-
age compliance limit of the heater output are in place at the same time, so the lower
calculated current is the correct User Max Current setting.
Example 1: A 50 ), 30 W heater is connected to Output 1.
Power lLimit Voltage compliance limit
I = Squrt(P/R) I = V/R
I = Squrt(30 W/50 )) I = 50 V/50 )
I = 0.77 A I = 1 A
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