6300
Note that there are two classes
of
line voltage operation, i.e., 110v and
220v.
When a
particular
disk
drive is configured for operation
on
nominal line voltages between
9Sv
and
12Sv,
P403
is connected to J403 on the Motor Control PCBA, and jumpers W1, W3,
W4,
W5
are
installed.
P403
is the plug from the
motor
capacitors and
P405,
P406
are the plugs from
the disk drive motor. For operation on nominal line voltages
of
190v
to
250v,
P403
connects to J404, and jumpers
W2
and
W6
are installed. Nominal line voltage
of
95v
to
125v is defined as
110v operation classification; nominal line voltage
of
190v to
2S0v
is
defined
as
220v
operation classification. Refer to Paragraph 4.8
for
the functional details
of
11
Ov
and
220v
operation of the drive motor.
Other than the interconnection arrangement previously described, all Circuitry on the
Motor Control PCBA is dedicated to switching the triac and suppressing any transients
that result from
this switching. A
dc
power supply voltage is required to provide current
to
the gate of the triac and must be developed with respect
to
the line voltage common. This
is a function which is performed by a special power supply made up
of
CR1,
R8,
and
C1
(zone 0,
6,
7). The voltage is negative
dc
with respect to the ac common (pin 3
of
J402). An
ac
Signal of approximately
8v
rms is provided
to
pin 5
of
J402 from a separate secondary
winding of the main transformer mounted on the power supply chassis. The voltage
provided by
this
winding is rectified by
CR1
and charges
C1
to
provide a relatively steady
dc voltage; current
limiting
is provided by
R8.
The trigger from the Servo PCBA is supplied to the Motor Control PCBA at
J401
(zone E7).
This trigger is the output of the Motor
Speed Control circuitry on the Servo PCBA and is, in
essence, a transistor switch closure to ground when the trigger is asserted.
When the transistor switch on the
Servo PCBA is conducting and the trigger is asserted,
current from the
+
Sv
supply flows through
R1
(zone
E7)
and a Light Emitting Diode (LED),
which is part
of
U1.
U1
is
an
optical isolator consisting
of
a light-emitting diode optically
coupled
to
a photo-transistor. When current passes through the LED
it
causes the
LED
to
emit light; this light is coupled
to
the photo-transistor causing
it
to
conduct. Conduction
of the photo-transist9r in
U1
causes a base current to flow in 01 (zone
E6)
which, in turn,
causes a base current
in
02.
02
conducts and provides the current to triac
SCR1
(zone
ES).
Gate current flowing
in
SCR1
results in the triac conducting current through the motor
winding. Conversely, when the trigger signal is absent there is no current
flow
through the
LED in
U1
; therefore, the voltage across
R2
causes 01 to turn
off
and the voltage across
R7
causes
02
to turn off. Therefore, there is no current into the gate
of
the triac and
conduction will stop as soon as the current through the triac passes below the holding
current value, a characteristic
of
the device.
Resistors
R3, R6, R4,
and
R5
provide the proper values
of
current in the collectors
of
transistors
01
and
02,
and into the gate
of
the triac.
R9
ensures that the gate current is
sufficiently low that the triac
will stop conducting.
C2
and
R10
(zone
OS)
provide a network
for compensating for the fact that the drive
motor
is
an
inductive load. This means that at
the time the current falls below the holding current value and the triac ceases
to
conduct,
there will exist a certain voltage across the triac.
If this voltage appears too rapidly, the
triac
will
resume conduction and control is lost.
In
order to achieve control with such
an
inductive load the rate
of
rise in voltage
(dv/dt)
must
be
limited by a series
R-C
network
across the triac. The capacitor will then
limit
the rate
of
change
of
voltage across the triac.
The resistor is necessary to
limit
the surge
of
current from the capacitor when the triac
fires and to damp the resonance
of
the capacitor with the load and circuit inductance.
S-44
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