SECTION 4
CIRCUIT DESCRIPTION
A. G E N E R A L
This section describes the electrical operation of the circuit.
First the principal operation is described by means of a block diagram .
A detailed des cription of the blocks follows . Also refer to the complete
diagram , section 9 in this manualo
B . B L O C K D I A G R A M
The complete block diagram of the power supply is shown in figure 19 .
The line delivers power to the transformer, where it is trans formed to a
suitable voltage. In the block " Controlled rectifying bridge" the voltage from
the transformer is rectified.
The SCR's of the " Controlled rectifying bridge" are fired at such a phase
angle by the firing system, that the DC pulses fed into the filter come out
as a DC voltage after the filter, which is just a few volts above the desired
output voltage.
The sampling element is designed so, that the input voltage to the " CV error
amplifier" is zero if the output voltage is correct. If for example the output
voltage is lower, the error is amplified in the " CV error amplifier", the
"OR-gate" and the "Driver amplifier" . The phase angle of this chain is such
that the series element is controlled to decrease the voltage across itself.
As this happens , the output voltage comes back to its correct value .
To make sure that the output current will never be excessive, RAC PAC is
equipped with a current limit system . The output current is monitored
through the resistor R70 .
When the voltage across R 70 is higher than the voltage at the wiper arm of
P9 3, the " C L amplifier" comes in through the "OR gate" and the "Driver
amplifier" and controls the "Series element" in such a way that the voltage
across R70 does not exceed a predetermined value.
The "Reference bridge" together with the " Reference bridge amplifier"
supplies an extremely constant reference voltage across
Zl.
This circuit
also supplies voltages for the other amplifiers in RAC PAC .
The "VM range selector circuit" monitors the output voltage.
When the output voltage is low (below 6 , 5 V for 60 V models and below 8 V
for 32 V models) the "VM range selector circuit11 closes the switch T325,
giving the voltmeter full scale deflection of 6 , 5 V for 60 V models and 8 V
for 32 V models . For higher output voltages T325 is opened and the voltmeter
has full scale deflection of 65 V for 60 V models and 40 V for 32 V models.
The range in use is indicated by the lamps I321 and I322 .
The "AM range selector circuit'1 measures the output current by monitoring
the voltage across R70 . In a similar way to the "VM range selector circuit"
it determines in which range the output current is and selects proper full
scale deflection for the ammeter by opening and closing the switch T307.
The ammeter range in us e is indicated by the lamps I301 and I302.
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