92
c
HAPTER
4:
Operation
Model 372 AC Resistance Bridge and Temperature Controller
Example 1: a 25
)
/5 W heater is connected to the warm-up heater.
User Max Current should be set to the smaller of the two or 0.48 A. In this example,
the desired 5 W of power is available to the heater.
Example 2: a 75
)
/25 W heater is connected to the warm-up heater.
User Max Current should be set to the smaller of the two, or 0.31 A. In this example,
only 7 W of the desired 25 W of power is available to the heater.
To enter a User Max Current, first set the Heater Resistance setting to 25
)
for any
resistance less than 50
)
, or to 50
)
for any higher heater resistance. Set the Max Cur-
rent setting to User. The User Max Current setting now becomes available in the
warm-up heater Output Setup menu. Enter the calculated current limit value in the
User Max Current parameter.
4.6.1.3 Heater Output Display
The warm-up heater output can be displayed in units of percent of full scale current
or percent of full scale power.
The sample heater output can also be configured to display the output range as its full
scale current and the output as a percent of that current. However, unlike the warm-
up heater, the sample heater can also be configured to display the output range by its
full scale power and the output as power in watts. This configuration is better for pre-
dicting the power entering the load, but it relies on an accurate heater resistance
being entered into the instrument (section 4.6.1.1).
With both heaters, the heater output display on the front panel is displayed in these
units, and the Manual Output parameter is set in these units. Available full scale cur-
rent and power are determined by the heater resistance, max current setting, and
heater range.
The heater output display is a calculated value intended to aid in system setup and tun-
ing. It is not a measured value, and may not accurately represent actual power in the
heater.
Power Limit
Voltage Compliance Limit
I = Sqrt(P/R)
I = 23 V/R
I = Sqrt(5 W/25
)
)
I = (23 V/25
)
)
I = 0.48 A
I = 0.92 A
Power limit
Voltage compliance limit
I = Sqrt(P/R)
I = 23 V/R
I = Sqrt(25 W/75
)
)
I = (23 V/75
)
)
I = 0.58 A
I = 0.31 A
Max current
Heater resistance (
)
)
0.63
10
25
30
40
50
100
4
10
12
x
x
x
0.5 (user)
3
6
8
10
x
x
0.45 2
5
6
8
10
Shaded black: max current is too high for these resistances due to the voltage compliance limit.
Bold font: discrete options that are available for 25
)
and 50
)
heaters are under the max current setting.
TABLE 4-10
Warm-up heater user max currents