Because RFL™ and Hubbell® have a policy of continuous product improvement, we reserve the right to change designs and specifications without notice.
9.2.3.7 +5-VOLT OUTPUT CIRCUIT
The +5V output circuitry operates essentially the same as the other outputs. In order to increase the
output current capability, a series pass device was employed. The regulator U54 controls the base
current to the pass transistor Q5, which varies the collector to emitter voltage and provides a fixed
output. To compensate for tolerances of the voltage programming, an adjustment is provided by R76.
This eliminates all the fixed tolerances leaving only deviations for temperature, load and line regulation.
The 5 volt output is protected from overloads and short circuits via a Hick-Up Mode overcurrent circuit.
The circuit consists of a current shunt R60, differential amp U57A, comparitor U57B and a one shot
U58 and associated components. During overload conditions the output of U58 is triggered high and will
disable the 5 volt regulator U54 for 100ms. After the timeout U58 will enable U54. Within 1 ms the
circuit will detect if the overload is still active and trigger the one shot for another 100ms shutsown
period. This will continue until the fault is removed. This mode of operation has a 1% duty cycle thereby
eliminating any excessive power dissipation of Q5 or U54.
9.2.3.8 -48-VOLT OUTPUT CIRCUIT
Another output of -48V is provided as a reference for local talk battery. The winding of T1 is rectified
by CR53, and filtered by C54. The output is protected against short circuits by PTC thermistor RT2.
The circuit will open if the current exceeds 100mA at 55 DEG C or 230mA at 0 DEG C.
9.2.3.9 REDUNDANT OPERATION
When two power supplies are installed in the same multiplexer shelf, they operate in a redundant fashion
that is transparent to the user. Redundancy occurs automatically, as described below. (Since redundancy
works the same for the +15-volt, -15-volt, and +5-volt outputs, only the +15-volt output will be
described.)
The two power supply modules do not produce exactly the same +15-volt output; one will be slightly
higher than the other. The higher-voltage output will become the dominant (or primary) output, and will
bear the entire +15 volt load. This occurs because the feedback voltage present at U3-1 of the redundant
output is higher than its internal reference voltage; this will tend to turn the redundant output off.
(Resistor R51 is placed in the feedback divider network to prevent the output from turning off
completely.)
At this point, Schottky diode CR11 on the redundant output is reverse-biased, preventing current from
flowing from the primary output into the redundant output. If the primary output fails, CR11 will
become forward-biased, and the redundant output will then bear the entire +15-volt load. Switchover
occurs quickly and smoothly, preventing system misoperation.
M-DACS-T1 RFL Electronics Inc.
August 7, 2012 9-5 (973) 334-3100
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