Power - Fluke 93 Service Manual

3*34
CIRCUIT
DESCRIPTIONS
If
th
0 ScopeMeter
is
connected
to the
line
voltage
and
is
not operallonaJ, the flyback
converter
operates almost
wrthoiJt
a
load (only the NICd
battery pack).
This
irr^plies
that the current
floating
through windings L2504 and L2505
(averaged
In
time)
is
almost
zero.
Because
of
this,
the voltage on
CL+
Is
about 30
mA
and
the voltage
on CL-
Is
about 170 mV.
The
battery pack
will
be charged
with
170
mA.
If
the flyback
converter
Is
operated normally
(ScopeMeter
"ON"), the voltage on both CL- and CL+
will rise
and
the charging current
will
decrease
to
100 mA.
Battery
charge
protection
To prevent charging
of
non
-rechargeable
batteries,
a
special protection
circuit
Is
provided. For
safety
reasons,
this circuit consists
of
two cascaded
sections.
When
the
ScopeMeter
Is
"ON", the flyback
converter
will
be
operative.
The
produced
voltage
POWER-ON
will
drive both
Feld
Effect Transistors
V2537 and V2538 Open
(conductive)
via
R2568
and R2569.
Now
the battery plus contact
Is
connected
to
the
ScopeMeter
clrcuH ground,
if
line
voltage
Is
present,
the voltage
-VCH
produced by
the flyback converter
will
drive
V2534
and V2536, which
prevent
transistcfs
V2537
and V2638
from
conducting.
The
battery plus contact
is
disconnected from ground.
Power
ON/OFF
circuitry
During normal operation
the
POWER-ON
signal
Is-hSV.
Transistor
V2542
Is
opened
(conductive),
so
•Vbat/s
equals -V.CH.
If
me
ScopeMeter
Is
operating
and
me
RESPOWHT
( 'reset
power
si^>ply")
becomes
"high".
V2541
will
conduct
and V4542
will
stop conducting. This
will
disconnect -VbaVs
from
•V_CH.
3.4.9
POWERSUPPLY
•
Introduction
See
figure
3.19.
Different supply voltages are
needed
for various
ScopeMeter
sections.
A
second
flyback converter
Is
used
to
convert
-V Dal's to supply voltages
of -30V,
-5V and
-fSV.
This
voltage,
-VbaVs,
is
made
by
the
first
flyback
converter
(In
the battery charger
section) or
comes
from
the
batteries. -Vbat/s
Is
5V
If
operated
with
NICad
battery
pack,
and
8
V
if
operated from
line
voltage.
-
Detailed
circuit description
See
figure 3.19
and
circuit
diagram
A2c
(figure 10.7).
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