4.6.1 Heater Outputs
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4.6.1.1 Sample Heater Resistance
The Model 372 does not need to know heater resistance to perform its control func-
tions. Heater resistance is only required to accurately display the heater output and
manual output in power. The instrument does not measure heater resistance so one
must be entered for these display values to be meaningful. A 100
)
heater is required
for maximum power output, but other values can be used. If a heater resistance is not
known, the heater output display should be configured for the current.
Menu navigation:
Output Setup
Q
Sample Heater
Q
Heater Resistance
Q
(1 to 2000)
Default: 100
)
Interface command:
HTRSET
4.6.1.2 Warm-up Heater Max Current and Heater Resistance
The Model 372 warm-up heater output is designed to work optimally into a 25
)
or
50
)
heater. The Heater Resistance and Max Current parameters work together to
limit the maximum available power into the heater. This is useful for preventing
heater damage or limiting the maximum heater power into the system. When using a
25
)
or 50
)
heater, set the Heater Resistance parameter accordingly. The Max Cur-
rent setting will then provide a discrete current limit value that corresponds to the
10 W warm-up heater power rating. These 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 4.6.1.5.9) 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 4.6.1.2.1 for more information on Max User Current. TABLE 4-10 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.
Menu navigation:
Output Setup
Q
Warm-up Heater
Q
Heater Resistance
Q
(25
)
or 50
)
)
Output Setup
Q
Warm-up Heater
Q
Max Current
Q
(User, 0.45 A, 0.63 A)
Default:
Heater Resistance-> 25
)
Output 1
Q
Max Current
Q
0.63 A
Interface command:
HTRSET
4.6.1.2.1 Max User Current
When using a heater that is not 25
)
/10 W or 50
)
/10 W, the provided discrete cur-
rent limits may not be appropriate. The User Max Current setting is available for this
case. 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
23 V heater output compliance voltage should also be taken into account in order to
maximize 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 = 23 V/ R where P is
the maximum allowable power, R is the heater resistance. The load power limit and
voltage compliance limit of the heater output (23 V) are in place at the same time, so
the lower calculated current is the correct Max Current setting.