Solenoid Control; Cell Output - Ametek 5000 Manual

The outputs of U1 and U2 also are fed to dual pulse detector U3. If an output is steady-
state [(not oscillating) (fault)], a Q output of U3 turns off either transistor Q7 or Q8,
which generates a fault number.
There also is a field unit environmental heater comprised of resistors R1 and R2 and a 40
°C thermostat switch S1.

Solenoid Control

There are two, similar, +24-volt solenoid circuits: one cycles wet/dry solenoids L1 and
L2 and the other operates the moisture generator/sample flow solenoids simultaneously.
Their circuits include transistor switches, diodes, LEDs, and current limiting resistors.
With no valve signal (0 volt) at TB1-7, transistor switch S6 is biased on, grounding
the control side of the solenoid L1 winding and lighting LED CR11. Sample gas flows
through the cell.
Every 30 seconds, +5 volts is applied to transistor Q5, turning it on, grounding the con-
trol side of solenoid winding L2 and LED CR10. The solenoid opens reference gas flow
through the cell, and the LED lights.
In normal operation, +5 volts is on TB1-8, biasing transistor switch Q4 on, turning Q3
off, de-energizing solenoids L3 and L4, allowing sample gas to flow. Pressing the MOIST
GEN key applies 0 volt to the base of Q4, turning it off. This turns Q3 on, grounding the
control sides of L3 and L4, opening moisture generator gas flow.

Cell Output

There are two crystal oscillator (Y
fier/inverters U1 and U2 and rc filters. Except for their frequencies, the two oscillators
function alike.
U2A-1/-2 and U2F-13/-12 provide positive feedback to sustain the Y
(Measure) nominal oscillations at 8.982500 MHz and 8.982000 MHz, respectively.
44 | 5000 Single-Point Moisture Analyzer
•
The thermistor has a negative coefficient: heat decreases, resistance in-
creases. This decreases the voltage to its amplifier, making the output
go high, gating the triac on, applying +24 volts to the heater.
•
As less heat is needed, the reverse occurs. Capacitor C6 (C9) and resis-
tor R20 (R28) provide positive feedback to start U1(U2) oscillating and
to establish a time constant for frequency control. These oscillations
gate the triac on and off at a control frequency rate so that heat in-
creases and decreases gradually. The LED lights when heater voltage
is on.
— reference and Y
1
— measure) circuits with ampli-
2
(Reference) and Y
1 2