5.4 Fi lament On-Off (Refer to Fig. 5-4)
Transistor Ql4 and filament relay K3 control the power to the filament. During manual
operation the base of Ql4 is turned on by the +I0V signal applied from C7 when the filament
switch is momentarily toggled to the on/off position. This energizes K3 establishing a holding
path through R2 6 and R24. The voltage at test point V then drops to -12V de. Momentarily
toggling SI again to the on/off position applies the negative signal to the base of Ql4 turning
it off causing relay K3 to de-energize. In the auto on position of switch SI the output of TC
lf.2 process control voltage comparator IC 13-A whose output is either in negative or positive
saturation is used, instead, to control the base of Ql4.
Remote operation con be performed at any time by use of the IG accessory connector.
A momentary short between J3-7, Hand J3-9, K will turn on the filament and between J3-8,
J _and J3-9, K will turn off the filament.
Once energized there are two automatic sources which will result in a filament
shutdown. If the output of the emission control amplifier exceeds -6V de the base of Q 14 wil I
be turned off causing the filament relay to de-energize. In addition when not in the auto on
mode of filament operation, should the ion current rise to a level corresponding to 3 x 10-3
Torr, transistor Q9 turns on removing the + 12V to relay K3, pin 6, removing the source of
base current for Q 14.
5.5 Emission Control (Refer to Fig. 5-4)
The emission control circuit consists of: An em1ss1on reference supply, an emission
sensing resistor RA9-4, a high-gain amplifier ICI0-A, a synchronized pulse generator
composed of QII , Ql7 and Tl, and a triac SCR-1 in series with the filament. The output of
ICIO-A is shorted to its input through contacts 14 and 15 of K3 until the filament circuit is
turned on. When the filament circuit is energized, the positive input from the emission
reference supply (0.3 to 1.2 volts depending upon the setting of the emission adjust
potentiometer R28) to pin 2 of IC
I
0-A results in a negative going output from IC
I
0-A. This
increases the output of the current source Q 11, which in turn fires uni junction Q 17 at earlier
and earlier times in the power line cycle. Pulses from Q 17 are coupled to the triac, SCR-1,
through transformer Tl. These pulses cause SCR-1 to conduct for the remainder of the half
cycle producing current through the filament. As ICI0-A output becomes more negative, the
filament is turned on for a larger fraction of the half cycle. The resulting heating of the
filament generates the emission current through emission sensing resistor RA9-4. The
resulting emission signal voltage serves as a feedback signal to ICI0-A, closing the control
loop. The signals which synchronize the pulses from Q6 with the power line are obtained from
the sync signal generator.
5.6 Emission Sensing Amplifier (Refer to Fig. 5-4)
IC I 0-B and its associated components form a conventional inverting amplifier with a
gain of two and are used to drive the ion gauge meter when the emission switch is held in the
adjust position.
5. 7 Electrometer Circuit (Refer to Fig. 5-5)
A high gain negative feedback type ampfi fier consisting of Q20 and IC9-A is used so
that the collector electrode is normally maintained within millivolts of ground potential. Due
to the high input impedance of Q20, essentially al
I
of the ions from the collector electrode
must flow through the feedback resistor, Rf' of Fig. 5-2. Since the amp Ii fier input is
maintained at ground potential by the high loop gain, the output voltage is the product of the
ion current times the feedback resistance. This output is used to supply information to the
recorder output, converter, and autoranging circuit. The collector lead is attached to the
gate of one side of the dual JFET Q20 through a protection network consisting of a surge
voltage protector, CI, R4, CR4 and CR36. The other gate is grounded. The JFET is used as a
source follower and is coupled to operational amplifier IC9-A's differential input. The loop is
5-3
I..
