4-6
888-2595-001
12/20/11
WARNING: Disconnect primary power prior to servicing.
Section 4 Theory of Operation
ZX Series
comparator U2C. In a non-faulted state, U2C will have a low output, based on a near
zero input at the (+) input at pin 10 and a 2.25V input at the (-) input at pin 11 due to
voltage divider R43-R51. U2C will pass to high (faulted) state whenever the (+) input
rises above the 2.25V level at the (-) pin (i.e. high reverse power from detector).
Positive feedback resistor R38 ensures that output remains high (approx. 3.5V) even
after the high reverse power sample condition is resolved. That is, it latches the fault
condition. The latched fault indication is cleared when a +5V pulse from the controller
on the ON_OFF_RESET line J7-3 is passed to the (-) input and is greater than the
+3.5V output signal being passed back to the (+) input via R38.
A high output at U2C is passed via current limiting resistor R39 to light LED DS1
indicating a latched fault condition. It is also passed through steering diode CR5 to
override the PA current sample from U2A with a fixed +3.5V level. The PA current
sample is normally 0 - 2.5V (0 - 25A). A +3.5V level on the PA current sample line is
interpreted by circuits on the PS interface board as a sign to shut down the PA module
because a fault condition is present.
4.5.2
PA Temperature Alarm
The operation of the PA temperature alarm is similar to the PA reverse power circuit
described in the previous paragraph, except that the sensor signal comes from a
thermistor on the PA module, rather than a directional coupler and RF detector. A 50k
thermistor on-board the PA module controls the gain of amplifier U1C (U1D) via pin
J5-N (J6-N). The temperature variable gain is applied to a 0.1V reference developed by
voltage divider R49-R37. The result is a temperature sample based on a natural
logarithmic scale and scaled such that the desired trip temperature produces an output
of 2.25V. The higher of the two samples (each half of module) is passed on to fault
comparator U2B. The operation of U2B is virtually identical to that of U2C, as
described in the previous paragraph. The highest reading is also passed via steering
diodes CR4A and CR4C to the MAX_PA_TEMP line to the controller via J7-5.
4.5.3
PA Current Alarm
The operation of the PA current alarm is similar to the PA reverse power circuit
described in a previous paragraph, except that the sensor signal comes from current
shunt R47-R48 and high-side current monitor U3, rather than a directional coupler and
RF detector. The current sample is buffered by U2A and sent to fault comparator U2D
via CR4G. It is also passed via RFI filter R33-C17-R34 to PA position selector
dipswitch S1. Switch S1 applies the PA current reading to one of eight possible PA
current sample lines J7-13…J7-20, thereby determining which slot in the chassis the
card occupies and which PS module it controls.
Summary of Contents for ZX10
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