Lakeshore 325, Руководство пользователя, Страница: 44 / 166

Lake Shore Model 325 Temperature Controller User’s Manual
3-8 Installation
Loop 1 Heater Output Wiring (continued)
There is a chassis ground point at the rear panel of the instrument for shielding the heater cable. The cable shield can be
tied to this point using a #4 spade, 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 Loop 1 Heater Output Noise
The heater output circuitry in the Model 325 must be capable of sourcing 25 W of power. This type of circuitry can
generate some electrical noise. The Model 325 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.
3.6.5 Loop 2 Output
The Model 325 has a second control loop called Loop 2. Loop 2 is an auxiliary control loop with the capability of
powering a small sample heater, or controlling a larger, programmable heater power supply. Loop 2 has a different
output from Loop 1, it uses analog voltage output as its actuator. It is a variable DC voltage source with an output range
from 0 V to +10 V. The output can source up to 200 mA of current providing a maximum of 2 W with a 50

heater at
the 50

setting. The output voltage range is 0 V to +5 V when set to the 25

setting, providing a maximum power of
1 W into a 25

heater.
3.6.6 Loop 2 Output Resistance
The power delivered by the Loop 2 output is calculated as:
P = V 2 / R
heater
. The output is rated for no more than 200 mA
of current and has a built in current limit. For the maximum of 2 W output power use a 50

resistive heater with a
power rating greater than 2 W. A 25

heater can be used to provide 1 W of power. The 25

setting for Loop 2 changes
the output voltage range to allow for control over the entire range of output. Using a 25

heater at the 50

setting
would still provide 1 W of power, but the maximum power will be reached at a setting of about 50%, at which point the
200 mA current limit will begin to limit output power and could cause temperature control instability.
3.6.7 Loop 2 Output Connector
The connector for the Loop 2 output is a 2 pin detachable terminal block. See Figure 8-5. Twisted pair of small gauge
wire is recommended.
3.6.8 Loop 2 Heater Protection
The output is short protected so the instrument is not harmed if the heater resistance is too small. It is not recommended
because control over the full output voltage range is lost when in power limit mode. The user must be careful to build a
robust system and account for the voltage range and power up state of the control output.
3.6.9 Boosting Output Power
There are temperature control systems that require more power than the Model 325 can provide. An auxiliary DC power
supply can be used to boost the output of the Model 325. Programmable power supplies are available that use a low
current programming voltage as an input to control a high current voltage output. Loop 2 provides an ideal programming
voltage for an auxiliary power supply.
The only drawback with using the Loop 2 output to program an auxiliary supply is that it has only one heater range.
Although the heater resistance setting for Loop 2 does provide two different voltage scaling options (25

setting: 0 to
+5 V, 50

setting: 0 to +10 V), the output resolution of each setting is the same. The heater output for Loop 1 has two
ranges. Using the Low range will improve resolution, but the Loop 1 output is in current not voltage. To use Loop 1 to
program a larger power supply, 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
program
= R
program
× I
output
.