Fluke PM3082 Service Manual page 365

5 .6-4
Secondary output circuitry
The secondary output circuits consist of rectifier diodes and buffer capacitors, followed by chokes and
capacitors for ripple suppression. The output circuits are protected against overload by the under
voltage protection.
Over and under-voltage protection
To protect the oscilloscope circuitry against over-voltage and the supply circuits against overload, the
power supply is provided with a protection circuit. As, due to the multiple output principle, the output
voltages are interdependent, it is sufficient to check only one voltage. In case of overload, the output
voltages will decrease and this will be detected by the under-voltage detection, V1241, which
monitors the -12 V. This will cause the collector of V1241 to be LOW. In case of over-voltage, the over­
voltage detection detects a too high +12 V or +5 V and this will cause pin 13 of N1236 to be LOW.
Due to this LOW signal, the intervention circuit V1241-1242-1243 will cause a current, the TPDOWN
signal, to V1213. This will switch ON this thyristor and this causes switching off the converter by
decreasing all output voltages to a very low, safe value.
Powerfail circuit
In normal cases, about 250 ms after switching on, the signal POWER HT will become HIGH and stay
HIGH. In case of an over- or under-voltage failure, the signal will become LOW due to V1242. In case
of a too low mains voltage, i.e. less than 80 V (AC) or 100 V (DC), the signal will become LOW due
to N1236 pin 2. The signal POWER HT is a logic signal, it will not switch off the main convertor circuit,
but it will shut down the EHT-convertor. POWER HT signals to the microprocessor unit A3 that power
is going down. This gives the processor the opportunity to save important data.
Temperature protection.
To protect the circuits against too high temperatures, the oscilloscope is provided with an overheat
shutdown circuit. The temperature of the power supply printed circuit board is monitored by NTC
resistor R1231, which is located on the PCB. At temperatures higher than about +80 °C, pin 8 of
N1236 will become HIGH and this will cause pin 14 to be LOW. Due to this, the TPDOWN signal
becomes active and the converter is switched off by triggering V1213. This temperature protection is
only meant for the power supply.
+10 V reference circuit
For application in the power supply and at other places in the oscilloscope, a stable +10 V reference
voltage is needed. This voltage is made by N1226A/1226 in the power supply. The voltage is not
adjustable. The accuracy is ±5 mV. Temperature coefficient is ±0,001 °K. The load of the +10 V is
about 10 mA.
5.6.2 Signal name list A6
Note: In the signal name list you find the itemnumber of the component that is source or destination.
Behind this itemnumber (separated by
source/destination can be found.
NAME
FAN0
FAN1
ILLUM0
ILLUM1
LINE
POWER-HT
SCL
SDA
TROTO
TROT1
MEANING
FAN SUPPLY 0
FAN SUPPLY 1
GRATICULE ILLUMINATION 0
GRATICULE ILLUMINATION 1
LINE/MAINS TRIGGER SIGNAL
POWER UP INDICATION SIGNAL
SERIAL CLOCK
SERIAL DATA
TRACE ROTATION 0
TRACE ROTATION 1
you find the number of the diagram where the
SOURCE DESTINATION
S-12V:02
L1146:02
S-18V:02
V1149:02
N1046:01
N1236:03
X1002:02
X1002:02
EARTH :02 X1002:02
V1146:02
UNIT DESCRIPTIONS
X1004:02
X1004:02
X1002:02
X1002:02
X1002:01
X1002:03
V1111:02
N1141:02
N1141:02
X1002:02