Lakeshore 336, User Manual

Page: 52 / 192

40 c
HAPTER
3: Installation
Model 336 Temperature Controller
3.8.5.1 Choosing a Power Supply
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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 3.8.5.4.
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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.
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Output Type: most available voltage programmable power supplies are configured
for voltage output. This is different than Outputs 1 and 2 on the 336 which are
configured for current output. The differences between the two are not signifi-
cant when used in warm up mode.
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Output Voltage: Lake Shore recommends supplies with a working output voltage
between 10 V and 50 V. Voltage higher than 50 V poses a shock hazard and should
only be used if operator safety can be assured by the installer. Voltage lower than
10 V becomes impractical because the current necessary provide any meaningful
power is too high for most cryogenic wiring.
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Output Power: there is no limit to the maximum power of the supply. Typical warm
up applications normally range between 25 W and 200 W.
3.8.5.2 Power Supply Setup
Follow all operation and safety instruction in the power supply manual during setup.
Consider the following suggestions for protecting the power supply and heater load.
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Short circuits are common in cryogenic lead wiring. If the power supply does not
specify that it is short circuit protected the power output should be wired with a
fuse in series to prevent damage.
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Unipolar power supplies are designed to use a positive programming voltage and
some can be damaged if the programming voltage is negative. Be careful when
wiring the system to maintain the correct polarity. Also, never set the control out-
put of the Model336 to bipolar mode.
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Some power supplies can be damaged if there is a programming voltage present
at their input when they are turned off. This can happen if the Model 336 and
power supply use a different source of line power or are turned on and off individ-
ually. It can be avoided if the two instruments share a switched power strip.
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The heater and wiring in the system must be rated for both the maximum current
and maximum voltage provided by the power supply. The Model 336 can be set
to warm up using less than full power if the load will not tolerate the full power of
the supply.