MKS HPS 919 Series, Operation And Maintenance Manual

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Circuit Description
This section is intended to give the reader an overview of the internal work-
ings of the Series 919 Controller. This manual does not provide sufficient
detail to allow component level troubleshooting of the instrument. Refer to the
figure below, the system block diagram, while reading this section.
The Series 919 Controller is an analog instrument with a digital display, which
has circuits to control the hot cathode sensor and uses signals from the
sensor to display pressure, operate the process control relays, and supply
analog voltage vs. pressure output.
Filament Supply
The filament or cathode is heated by an AC transformer winding that is pulse-
width-modulated by a triac to regulate the filament temperature, which in turn
determines the electron emission current. The pulse-width modulation is
achieved by a proportional integral (PI) servo loop that compares the mea-
sured electron emission current to a reference voltage determined by the
sensitivity setting as set by the user. The servo signal is then compared to a
saw tooth oscillator that is synchronized with the AC line frequency. The triac
is fired at the appropriate phase angle to control the filament power.
The emission current is 1 mA when the pressure is below 1 x 10
-4
Torr and
100 mA when the pressure is above 8 x 10
-5
Torr. In the pressure range of 8 x
10
-5
to 1 x 10
-4
Torr, the emission is 100 mA or 1 mA, dependent upon
whether the pressure is increasing or decreasing, respectively. This prevents
unstable pressure readings in this transition region.
The filament or cathode bias is developed across a zener diode by the
emission current. It is a n30 V referenced to ground and -150 V
referenced to the grid.
Degas Supply
The degas supply is a transformer that is energized by a relay when the
degas function is enabled by the front panel switch or through the rear panel
Accessory connector. The filament switch must be on or the Remote Fila-
ment Enable pin on the Accessory connector must be in a TTL low state, and
the system must be at a suitable pressure, before the degas will engage.
Grid Supply
The grid supply is a linear DC supply that consists of a transformer winding,
bridge rectifier, a filter capacitor, and voltage regulator. The voltage is regu-
lated by a zener diode and pass transistor. A current limiting circuit keeps the
output of the grid supply from exceeding 3 mA, in case there is a short
circuit in the sensor or cable. The grid supply is disabled when the filament is
off, so that no high voltage is present on the sensor, except when the sensor
is in operation. The grid voltage is a n180 V referenced to ground.
The Series 919 Hot Cathode
System Design Overview
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