1-4 Subject to Export Control, see Cover Page for details.
A. Power Supply PCB Assy (62A1A1) (Figure 1-2) (cont)
ON/OFF Control
The ON/OFF control circuit provides debounce for the POWER Key and turn off delay, as well as
monitoring the protection circuitry.
Pressing the POWER Key shorts that point to ground providing a low to the ON/OFF control circuit
input, which turns the power supply circuits ON or OFF. To turn the unit ON, the POWER Key must
be pressed for a minimum of 128 ms. To turn the unit OFF, the PWR button must be pressed for
approximately 250 ms. After the 250 ms, an interrupt signal is sent by the ON/OFF circuit to the
software, indicating that the power supply circuits are shutting down and allowing the software to
perform an orderly shutdown. Approximately 500 ms after the generation of the interrupt the power
supply circuits shuts down.
The ON/OFF circuit also monitors the protection circuitry for a +3.3 V or +5 V over current condition
or a low battery condition, and shuts down the power supply circuits if any of these conditions are
present.
Source Select Circuitry
The source select circuitry is designed as a low-loss switch that selects the appropriate source when
the output of the ON/OFF circuitry is high. The external DC (+Vdc) is selected if present. If external
DC is not present, the battery (VBAT+) connects to +VS. If the Test Set is running the power source,
either external DC or battery may be switched without interrupting system operation.
The second function of the switching circuitry is to prevent battery current from flowing into the input
converter section when the Test Set is running on battery power. A low voltage cutoff is also
incorporated into the ON/OFF circuitry.
Protection Circuitry
The protection circuitry has four basic functions related to the DC input. The battery has internal
protection circuitry. The external DC input has a fuse, an over-voltage crowbar and reverse
protection diodes. The fuse is the primary disconnect to protect against any of the fault conditions.
If the DC input is too high (>32 V), the over-voltage crowbar triggers and the SCR opens the fuse. If
the DC input polarity is incorrect, the input clamp diodes forward bias and open the fuse. If the input
converter fails and the output of the converter is above 12 V, the SCR triggers and opens the fuse. If
the DC input voltage is too low, the input converter is disabled until the input voltage is within the
allowable range.
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