Lake Shore Model 370 AC Resistance Bridge User’s Manual
3.7.2
Heater Output Connection
BNC receptacle provides connections for the temperature control Heater Output. Signal is carried on the center
conductor and isolated heater output measurement common is on the outer conductor. The heater output has a maximum
voltage of ±10 V and can source 100 mA. Mating connector not included.
3.7.3
Heater Output Cabling
Coaxial cable is recommended for the heater output, outside of the test Dewar. The center conductor should be large
enough to carry the maximum 100 mA current to prevent accidental burn out of the conductor. The outer conductor,
heater output common, should be terminated at the bulk head connection or other conductive point on the Dewar to
provide the single connection between isolated heater output circuitry and the Earth grounded Dewar.
3.7.4
Calculating Control Heater Power
The control heater output current source on the Model 370 has eight 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
Ω
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 current 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 3-1. Maximum 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 voltage of 10 V. Heater output resolution is lost any time the compliance voltage limits 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 useful output ranges and maintain good control resolution.
Table 3-1. Heater Resistance
Heater
Resistance
10
Ω
100
Ω
1000
Ω
5000
Ω
Output
Range
100 mA
31.6 mA
10 mA
3.16 mA
1 mA
316
μ
A
100
μ
A
31.6
μ
A
100 mW
10 mW
1 mW
100
μ
W
10
μ
W
1
μ
W
100 nW
10 nW
1 W*
100 mW
10 mW
1 mW
100
μ
W
10
μ
W
1
μ
W
100 nW
100 mW
†
100 mW
†
100 mW
10 mW
1 mW
100
μ
W
10
μ
W
1
μ
W
20 mW
†
20 mW
†
20 mW
†
20 mW
†
5 mW
500
μ
W
50
μ
W
5
μ
W
* Indicates maximum power range and resistance
†
Indicates power limited by compliance voltage
3.7.5
Heater Output Protection
The heater output hardware is internally protected against open circuit and short circuit conditions. The output should
never be connected to an external power source. There are additional firmware features in the Model 370 to help prevent
damage to the load. The heater will automatically turn itself off if an open circuit condition is detected. The instrument
will not allow the control setpoint to be larger than the setpoint temperature limit stored with the temperature response
curve. The heater range can be limited so it is not accidentally set too high from the front panel.
Installation
3-13
