Lake Shore 350, User Manual

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54 c
HAPTER
3: Installation
Model 350 Temperature Controller
3.6.3 Heater Output
Wiring Heater output current is what determines the size (gauge) of wire needed to connect
the heater. The maximum current that can be sourced from heater Output 1 is
1.732 A. When less current is needed to power a cooling system, it can be limited with
range settings.
When setting up a temperature control system, the lead wire for the heater must be
capable of carrying a continuous current that is greater than the maximum current.
Wire manufacturers recommend 26 AWG or larger wire to carry 1.732 A of current,
but there is little advantage in using wire smaller than 20 AWG to 22 AWG outside the
cryostat. Inside the cryostat, smaller gauge wire is often desirable.
It is recommended to use twisted heater leads. Large changes in heater current can
induce noise in measurement leads and twisting reduces the effect. It is also recom-
mended to run heater leads in a separate cable from the measurement leads to fur-
ther reduce interaction.
There is a chassis ground point at the rear panel of the instrument for shielding the
heater cable if necessary. The cable shield can be tied to this point using a 3.18 mm
(#4) spade terminal, or ring connector. The shield should not be connected at the
opposite end of the cable and should never be tied to the heater output leads.
For best noise performance, do not connect the resistive heater or its leads to ground.
Also avoid connecting heater leads to sensor leads or any other instrument inputs or
outputs.
3.6.4 Heater Output
Noise The heater output circuitry in the Model 350 is capable of sourcing 75 W of power.
This type of circuitry can generate some electrical noise. The Model 350 was designed
to generate as little noise as possible, but even noise that is a small percentage of the
output voltage or current can be too much when sensitive measurements are being
made near by. Heater output noise can be most detrimental when controlling at very
low temperatures. Heater Output 2 uses a fully linear power supply to minimize
heater noise. The switching power supply used for Output 1 is turned completely off
when Output 1 is off. For best results use Output 2 for control, and ensure Output 1 is
off when controlling at very low temperatures. If the Model 350 heater leads are too
noisy and the above wiring techniques do not help, Lake Shore offers the Model 3003
Heater Output Conditioner that may help (section 17.).
3.6.5 Powering Outputs
3 and 4 Using an
External Power Supply Outputs 3 and 4 cannot power heaters directly when used in warm up control mode.
These unpowered outputs must be used to program an external power supply which
in turn powers the heater. This section describes choosing and installing an external
supply. Section 5.5 describes operation of warm up control mode.
3.6.5.1 Choosing a Power Supply
D
Voltage Programmable: the power supply must be voltage programmable so that
Outputs 3 or 4 (control output) can control it. Ideally the supply’s programming
input should have a range of 0 to 10 V that corresponds to 0 to 10 V range of the
control output. This guarantees that 0 to 100% of the control output scales to

0 to 100% power out of the supply. Supplies with different programming input
ranges can be used as described in section 3.6.5.4.
Be aware that if the input voltage is not within the range of the power supply, damage
may result.
D
DC Output Capable : the power supply must be capable of continuous DC output.
Most commercial audio amplifiers are not suitable because they are AC coupled
and cannot provide a DC output.