Chapter Three: Overview Front Panel Controls
41
Flow Control Switch
This switch controls the circuit which applies the set point signal to the MFC as follows:
ON: The switch activates the output switch which allows the set point signal to flow
through it, through the unity gain buffer amplifier, and on to the MFC.
OFF: The switch places a small negative voltage on the set output line so that the
control valve will be closed positively.
REMOTE: A logic signal coming in through Interface connector P6 may be used to gate the
set point signal to the MFC. Flow may be turned ON/OFF by an external TTL
signal; a logic low turns the flow ON, a logic high or open turns the flow OFF.
Digital Panel Meter
The Digital Panel Meter (DPM) is a 3½ digit, 2 V full scale device. The DPM—set to read
1 VDC as 1000 counts full scale—displays the flow rate of the channel selected by the Channel
Selector. The DPM also displays the set point signal for an MFC when its READ/SET POINT
switch is held in the SET POINT position.
The Scaling Controls on the rear panel (refer to Figure 6, page 43) must be properly set for the
meter to display a
direct
flow or set point reading, in sccm or slm. Refer to
Scaling Controls
,
page 46, for more information.
Channel Selector
The Channel Selector switch controls input to the Digital Panel Meter.
Set Point Source Switch
This 3-position switch selects the source of the set point signal to be sent to the MFC. The
FLOW and RATIO positions are driven by the output of the SET POINT CONTROL; the EXT
position is driven by an externally applied voltage level. Refer to
Set Point Signal
, page 36, for
more information.
FLOW Position: Selects the set point signal from the front panel Set Point Control, which is
driven from the + 5 V internal reference.
The switch must be in this position
when using Manual Flow Control.
RATIO Position: Selects the set point signal from the front panel Set Point Control, which is
driven from the Channel 1 Transducer Output or the External Ratio Amplifier,
driven by an external controller (external set point input at Interface connector
P5).
The switch must be in this position when using External Ratio Flow
Control.
EXT Position: Selects the set point signal from a pin on Interface connector P6, allowing the
flow rate to be controlled directly from an external 0 to +5 V signal.
The
switch must be in this position when using TTL logic control.
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