Philips pm2521 - 6.4. MEASURING; 6.4.1. Standard measurements

35
1.
4.2.3.
Direct current
measurements (A
—
)
Currents less
than
20mA
applied to
BU2
are routed via
the A
.tt
function
switch
contact
direct
to the
l-V
convertor.
Any
differences in
polarity for
zero input in the
low current
range
can be
offset by
the preset R227.
In the 0.2A to
10A
ranges,
the unknown current
source
applied to BUS
is
shunted by R1
and routed
via FET
switch V201
and
the resistors
R203, R204//R205 to
the
input
of the l-V
convertor
amplifier
A201
.
The initial offset
routine
for the high
current ranges
is performed
by
software,
controlling
the RNG C
input.
During
this
routine
the input current
on BUS is
isolated by a
logic 1 in
RNG C
which
switches
off
V205
and
consequently
FET
switch V201
.
The logic 1
is also applied
to the
gate of
FET
V202
to
earth
the input to
the
l-V
convertor
for the
zero offset
routine.
The high
current input
is
protected against overloads by
the diodes V259,
V260.
The BUS
input
connector,
when inserted, links
the input
socket
with the
base of
transistor V206,
which sends
a
logic
0
to
the A-bus
of the switch
decoding
multiplexer, to
signal
that the high
current
ranges have
been
selected.
l-V
CONVERTOR
V251
Fig.
25.
!-V
convertor