3.7 Heater Output Setup
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3.7 Heater Output
Setup
The following section covers the heater wiring from the vacuum shroud to the instru-
ment for both heater outputs. Specifications are detailed in section 1.3.
3.7.1 Heater Output
Description
This section describes the sample heater and the warm-up heater.
3.7.1.1 Sample Heater
The sample heater output is a controlled DC current source capable of generating a
maximum of 1 W of power into a resistive heater that is attached to the cooled load.
During control, the heater output is adjusted, either manually or through a closed
loop PID control algorithm, to balance the continuous cooling power of the refrigera-
tion system and maintain steady load temperature.
The sample heater on the Model 372 has eight current ranges scaled in approximate
one-third steps. Each range will source one-tenth the power of the next largest range
with the highest range being 1 W. The current source voltage compliance is 10 V and
limits the heater resistance to 100
)
when operating at full power. The restriction on
heater resistance is not as severe on lower power ranges and higher heater resistance
is often preferred. Heater output can be displayed either as percent of full scale cur-
rent for a range or as power in watts. Power calculation requires an accurate heater
resistance be entered into the instrument.
Some examples of heater resistance and output range are given in TABLE 3-7. Maxi-
mum power for a range is the lower of the two; P = I
2
R and P = V
2
/R where P is heater
power in watts, I is the maximum current for the range and V is the compliance volt-
age of 10 V. Heater output resolution is lost any time the compliance voltage limits
the power.
If a permanently installed heater has a very high resistance value, a resistor may have
to be put in parallel with it on the outside of the Dewar to increase the number of use-
ful output ranges and maintain good control resolution.
3.7.1.2 Warm-Up Heater
The warm-up heater is a traditional control output for a cryogenic temperature con-
troller. It is a DC current source with software settable ranges and limits. It is designed
for higher power than the sample heater (up to 10 W), providing control at higher
temperatures, and more specifically, fast warm-up capability. The warm-up heater is
configurable for optimization using either a 25
)
or a 50
)
heater resistance. At the
Heater Resistance
10
)
100
)
1000
)
5000
)
Output
range
100 mA
100 mW
1 W*
100 mW
†
20 mW
†
31.6 mA
10 mW
100 mW
100 mW
†
20 mW
†
10 mA
1 mW
10 mW
100 mW
20 mW
†
3.16 mA
100 µW
1 mW
10 mW
20 mW
†
1 mA
10 µW
100 µW
1 mW
5 mW
316 A
1 µW
10 µW
100 µW
500 µW
100 µA
100 nW
1 µW
10 µW
50 µW
31.6 µA
10 nW
100 nW
1 µW
5 µW
*Indicates maximum power range and resistance
†
Indicates power limited by compliance voltage
TABLE 3-7
Sample heater resistance