Philips PM2521 Service Manual page 36

36
The
principle of the
l-V
convertor
is
explained
in
the functional description.
The
operational amplifier
A201
is
a
shunt feedback
amplifier that converts the input currents
into
output
voltages
between 0
to
0,1
V
and 0
to
1
V.
RANGES ADC
SENSITIVITY RELAYS
MODE
SIGNAL
2 aiA
IV
20
mA
0,1V
K202
200 mA IV K202
2mA
0,1V K201 + K202
20mA
IV
K201 + K202
200mA
0,1V
2
A
IV A
BUS
20
A
0,1V
The
feedback
resistors
are selected according to the
table
by
reed
relay
switches
K201 and K202.
The two
transistors
V203,
V204
take over the extra current from
the operational amplifier A201,
when
the
voltage across
R220
exceeds
600mV.
The
protection
circuits for the
l-V
convertor
are:
—
Transistors
V209, V210, which
provide overload protection.
Normally
the input of A201
is
at virtual earth,
but
excessive negative inputs are prevented by
V208,
and
excessive positive inputs are prevented
by V207.
—
Zener diodes V257,
V258
wich provide current protection.
—
Protection diodes V253,
V254
for the shunt feedback path
in series
with
V251
,
V252, which
are included
to
block the
leakage current of the protection diodes.
The
output
from
the
l-V
convertor
is
passed
via
the
A
.rr
2 function
selector
switch to the
active
filter
and
the
ADC.
1.4.
2.4.
Alternating current measurements
)
For
alternating current
measurements,
the
voltage
output
of
l-V
convertor
is
applied
via
the function
selector
switch
A~
2 to the impedance
convertor
and
the R.M.S convertor before being passed by
reed relay switch
contact to the
active
filter
and
ADC.