Field Unit Electronics - Ametek 5000 Instrument Manual

Field Unit Electronics

Power distribution
The AC supply is applied through circuit breaker CB1 and thermal cutout S2 to the primary of trans-
former T1. The T1 secondary (35 V ac) is directly connected to rectifier and regulator circuits on the
electronic control board. Diodes CR4 and CR5 supply unregulated +24 volts and feed the series-
connected voltage regulators. VR1 supplies +12 volts and VR2 supplies +5 volts. Diodes CR2 and
CR3 supply unregulated +24 volts for the cell oven; CR15 and CR16 supply unregulated +24 volts
for the moisture generator oven.
Power Distribution Failure Symptoms:
• Valves do not switch, but output reading could be on display
• One or more LEDs do not light, depending on faulty circuit
• No moisture generator heat
• No oven heat; CR1 and CR14 on steady
Electronics Control Board Voltage Checks:
• +5 volts — TP5 J7
• +12 volts — TP4 J6
• +24 volts — TP2 J4 (0 volt — TP1 J3)
Heat Control
There are two, identical, +24-volt heat control circuits: one for the cell oven and one for the moisture
generator. Each has a heater, an inverting amplifier (U1 or U2), a thermistor (RT1 or RT2), a triac (Q1
or Q2), an LED (CR1 or CR14), a temperature adjustment pot (R17 or R26), and miscellaneous resis-
tors and capacitors.
The thermistor has a negative coefficient; heat decreases, resistance increases. 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 resistor 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 increases and decreases gradually. The
LED lights when heater voltage is on.
The outputs of U1 and U2 also are fed to dual pulse detector U3. If an output is steady-state [(not oscil-
lating) (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; 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
(Figure 17)
Troubleshooting and Maintenance 6-9