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3.5.3
Heater Output Wiring for Loop 1
Heater output current is what determines the size (gauge) of wire needed to connect the heater. The
maximum current that can be sourced from the Loop 1 heater output is 2 A. When less current is needed to
power a cooling system it can be limited in software with range settings. The load can be protected against
catastrophic instrument failure with heater fuse described below.
When setting up a temperature control system it is recommended that the lead wire for the heater be capable
of carrying a continuous current that is greater than the maximum current or heater fuse rating. Wire
manufactures recommend 26 AWG or larger wire to carry 2 A of current, but there is little advantage in using
wire smaller than 20 to 22 AWG.
It is recommended to use twisted, shielded cable for heater leads. Large changes in heater current can
induce noise in measurement leads and twisting reduces the effect. It is also recommended to run heater
leads in a separate cable from the measurement leads to further reduce interaction. 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 with a single banana plug. The shield should not be connected at the opposite end of the cable.
The Loop 1 heater output is isolated from chassis ground to reduce noise. For best noise performance, do not
connect the resistive heater or its leads to ground. Also avoid connection heater leads to sensor leads or any
other instrument inputs or outputs.
3.5.4
Heater Protection and Fuse for Loop 1
The heater output of the Model 340 is a current source and has protection that prevents it from being
damaged even if shorted. The heater fuse on the rear panel of the instrument is there to provide one more
level of protection for delicate cooling systems. A fuse is a current limiting device, it burns out if too much
current is passed through it. Some systems require little heater power which requires only a small amount of
current. Even though the Model 340 has software current limits, the user may wish to put a smaller fuse into
the heater fuse holder to protect against inadvertent changes in instrument settings.
The heater fuse is shipped from the factory with a 2.5 A, ¼ × 1¼ inch fuse. This fuse can be replaced with a
value less than 2.5 A but should never be replaced with a higher value.
3.5.5
Loop 1 Heater Noise
The heater output circuitry in the Model 340 must be capable of sourcing 100 W of power. This type of
circuitry can generate some excess electrical noise. The Model 340 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. If the Model 340 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.
3.5.6
Model 3003 Heater Output Conditioner
The Lake Shore Model 3003 heater output conditioner is a passive filter which reduces the already low noise
present in the heater output of the Model 340. The Model 3003 connects between the heater output terminals
on the rear panel of a controller and a resistive heater. See Figure 3-4. Specifications are as follows:
Max Current: 2 A
Max Voltage: 60 V
Attenuation: 50 or 60 Hz. (line frequency): 20 dB
100 Hz. and above (line frequency harmonics): 40 dB
Size:
144 mm wide × 72 mm long × 165 mm deep (5.7 × 2.8 × 6.5 inches)
Weight:
1.6 kilograms (3.5 pounds)
Installation
Lake Shore Model 340 Temperature Controller User's Manual
3-9
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Troubleshooting
