Lake Shore 370, Руководство пользователя

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Lake Shore Model 370 AC Resistance Bridge User’s Manual
3.7.6 Boosting Heater Power Above 1W
An external power supply can be used to boost the power output of a Model 370 if a temperature control application
requires more than the 1 W of heater power. The control heater output can be used to program a voltage programmable
DC power supply of almost any size. A resistor must be placed across the heater output to convert the output current into
a programming voltage. An appropriate resistor value can be calculated as
R = V/I
where R is the resistor value in ohms,
V is the full scale programming input voltage of the power supply and I is the full scale current in amps of the control
heater output range. For example if the supply requires a 0 to 10 V programming input a 100
Ω
resistor is required for
the 100 mA (0.1 A) heater output range 100
Ω
= 10 V/0.1 A. The resistor must be rated for the power dissipated in it,
P = VI
, which would be 1 W. Lower heater output ranges can be used to lower the power dissipated in the programming
resistor. The maximum output current of 100 mA and the maximum compliance voltage of 10 V cannot be exceeded in
this configuration. The analog voltage outputs can be used to directly control a programmable DC power supply in open
loop control mode but the closed loop algorithm is not available for those outputs.
3.8 ANALOG OUTPUT AND STILL HEATER OUTPUT
The Model 370 has two analog voltage outputs. They have the same hardware configuration but Analog Output 2 has
additional firmware features that make it more useful as a still heater output for a dilution refrigerator. Both signal
outputs are referenced to their common that is internally tied to chassis ground.
Analog Outputs.bmp
Figure 3-15. Analog Output Connectors
3.8.1 Analog Output Connections
Two, BNC receptacles provides connections for Analog Voltage Output 1 and 2 with ±10 V range, 100 mA maximum
current into a 100
Ω resistor. Signal is carried on the center conductor and common is on the outer conductor. The
outputs are short circuit protected to prevent them from being damaged but operation into a resistance of less than 100
Ω
is not recommended. Mating connector not included.
3.8.2 Calculating Still Heater Power
The heating power sourced by an analog output can be calculated as
P = V
2
/R
where P is power in watts, V is the output
in volts and R is the heater resistance in ohms. Output voltage ranges from 0 to 10 V and is capable of driving a
resistance of 100
Ω
or greater. The maximum power output of 0 to 1 W can be achieved with a 100
Ω
heater. Most stills
do not require the maximum power. Using a larger resistor will reduce the range of power and improve setting
resolution. For example a 1000
Ω
resistor will produce a 0 to 100 mW power range with full setting resolution. If a small
resistive heater is permanently installed in the cooling system, lower power can be created by adding a series resistor in a
heater lead outside the Dewar.
3.8.3 Calculating Still Heater Current
The current sourced by an analog output can be calculated as
I = V/R
where I is current in amps, V is the output in volts
and R is the heater resistance in ohms. Output voltage ranges from 0 to 10 V and is capable of driving a resistance of
100
Ω
or greater. The maximum current output of 0 to 100 mA can be achieved with a 100
Ω
heater. Most stills do not
require the maximum current. Using a larger resistor will reduce the full scale current and maintain good setting
resolution. For example a 1000
Ω
resistor will produce a 0 to 10 mA current range with full setting resolution. If a small
resistive heater is permanently installed in the cooling system, lower current can be created by adding a series resistor in
a heater lead outside the Dewar.
3-14 Installation