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WARNING: Disconnect and lockout AC primary power prior to servicing
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9.2.4.3 Fault Protection Circuits
a. Over-voltage Protection: A sample of the output voltage of the supply is
compared with REF#2 by U8-C. If the output voltage exceeds 57 volts, the opamp
and Q21 turn on SCR U40. This turns on the over-voltage indicator LED, DS1.
The current flowing through CR32 and CR55 will shut off the 24 volt regulator,
turning off the supply.
b. Over-current Protection: The current shunt scales the current to 50 mV at 300
amps. U3B amplifies the voltage drop across the shunt, R75 is adjusted to
calibrate the current metering sample to be 1.00 volts per 100 amps. The
following opamp compares this voltage with a reference and will turn on CR41,
the indicator for an over-current fault. The fault current set point is 335 amps.
Over-current fault will shut off the 24 Volt regulator and illuminates DS2.
c. Over-temperature Protection:
When the heat sink temperature reaches 100 degrees centigrade, the thermal
switch will close, firing the SCR, removing the 24 Volts and illuminating DS3.
When the switch cools to about 65 degrees centigrade, it will reopen, allowing the
power supply to be reset.
9.2.4.4 GO / NOGO Circuit
The output is also monitored by a GO/NOGO circuit, when the output voltage is higher
than 46 volts Q19 is turned on, this sends current trough an opto-isolator in the PA
cabinet controller telling that logic that the supply is working. When this signal is not
present, the PA cabinet controller card reports a power supply fault.
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