Lake Shore Model 340 Temperature Controller User’s Manual
Installation
3-11
3.5.11 Boosting the Output Power
There are temperature control systems that require more power than the Model 340 can provide. An auxiliary
DC power supply can be used to boost the output of the Model 340. Programmable power supplies are
available that use a low current programming voltage as an input to control a high current voltage output.
Analog output 2, used for Loop 2 output, provides an ideal programming voltage for an auxiliary power supply.
The only drawback with using the analog output to program auxiliary supplies is it only has one voltage range.
The heater output for Loop 1 has several ranges that can improve resolution, but its output is in current not
voltage. A programming resistor can be placed across the heater output to produce a programming voltage.
The programming voltage is related to output current by:
V
R
I
program
program
output
=
×
The resistor must be chosen to convert a full scale current from the highest heater output range being used to
the full scale programming voltage of the auxiliary supply. For example, it the auxiliary supply has a full scale
programming voltage of 10 V and the maximum current for the highest heater output range being used is 0.25
A the programming resistor should be 10 V / 0.25 A = 40
Ω
. The programming resistor must be rated for the
power being dissipated in it which is
Power
I
R
output
program
=
×
2
or 2.5 W. Lower heater output current ranges
can be selected to reduce the power dissipated in the programming resistor.
3.6 ANALOG
OUTPUTS
Analog Output 1 and Analog Output 2 on the rear panel of the
Model 340 are voltage outputs that can be used for monitor and
control applications. See Figure 3-5. Their most basic function
is a temperature monitor where they put out a voltage that is
proportional to temperature. They can be configured to monitor
the output of a linear equation which allows the user to select
an offset and gain. ANALOG OUT 2 can be used as the control
output for Loop 2. The analog outputs can also be controlled
manually by the user for any application that requires a
programmable voltage source.
Both analog outputs are variable DC voltage sources that can
vary from -10 V to +10 V. The resolution of the analog output
is 1.25 mV or 0.0125% of full scale. The output can source up
to 100 mA of current providing a maximum of 1 W of power.
They can drive a resistive load of no less than 100
Ω
. The
output is short protected so the instrument is not harmed if the
heater resistance is too small. It is not recommended because
the additional load on instrument power supplies causes noise on internal circuits.
The connectors for the analog outputs are BNC sockets. The inner conductor is the output voltage terminal,
the outer conductor is the outputs ground and is attached to chassis ground inside the instrument. A coaxial
cable with attached BNC plug is recommended for this output. These connectors are not included with the
Model 340 but is available from local electronic suppliers.
It is not recommended to attach the analog output ground to a ground outside the instrument. The output
should be read by an instrument with an isolated or differential input wherever possible. Connecting to an
external ground can cause noise in the analog output voltage or the sensor input measurement. If this cannot
be avoided, try to keep the chassis of the two instruments at the same potential with a ground strap.
PIN
DESCRIPTION
1
2
Analog Output, Center Conductor
Ground, Connector Shell
Figure 3-5. ANALOG OUT 1 and 2
BNC Rear Panel Connector Details