Lakeshore 335, Руководство пользователя, Страница: 49 / 172

3.7.4 Heater Output Noise 37
|
www.lakeshore.com
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.
3.7.4 Heater Output
Noise The heater output circuitry in the Model 335 is capable of sourcing 75 W of power.
This type of circuitry can generate some electrical noise. The Model 335 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 nearby. If the Model 335 heater leads are too noisy and the above wiring tech-
niques do not help, Lake Shore offers the Model 3003 heater output conditioner to
help reduce noise (section 7.7).
3.7.5 Powering
Output 2 Using an
External Power Supply
Output 2 in voltage mode can be used to program an external power supply, which in
turn powers the heater. This can be useful if more than 75 W is needed for either
closed loop PID control, warm up control, or open loop operation. This section
describes choosing and installing an external supply. Section 5.5 describes operation
of the warm up supply mode.
3.7.5.1 Choosing a Power Supply
D
Voltage programmable: the power supply must be voltage programmable so that
Output 2 in voltage mode can control it. Ideally the supply’s programming input
should have a range of 0 V to 10 V that corresponds to 0 V 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 3.7.5.4.
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.
D
Output type: most available voltage programmable power supplies are configured
for voltage output. This is different than Outputs 1 and 2 (current mode) on the
Model 335, which are configured for current output. The differences between the
two are not significant when used in warm up mode.
D
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 to provide
any meaningful power is too high for most cryogenic wiring.
D
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.