Philips PM2517E - Page 32

24
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3.3.2-3- D.c,
current
Measurements
(Fig.
l4)
-
The input
circuit for d.c. current measurements is
shown in figure 14.
-
Range 100
mA-.
ln the
function mA:-:: only
100m,A range is available. The 100 mA range is
determined by
shunts R
603
and
R
'1.
Resistor R
605
is
used
for
adjusting the 100 mA range.
-
Protection
The
'100
mA
range is
protected
by fuse F1
(3'15
mA
quick).
When
in case of measuring in
the
voltage function,
is switched
over to the
(m)A
function
with the voltaqe
still on
the
input
terminals, then due
to
low resistance
values of the shunts a high
current is switched.
The
high current
can damage the
function
switch. To
prevent
this the
(m)A
ranqe is also
protected
by
means
of a switch
position
(m)
A'-.
which is situated between
the
positions
of the
iunctions
oC
and
(m)A-
in
the
function
switch. In
this
case
the
input
is
first
connected with NTC resistor R
601.
When
the input voltage
at the input is too high
then
fuse F1
will blow.
-
Range
'10
A-
The
10A range isdetermined
by shunt R1
(12
m Q).The 10 A shunt is adjusted by resistor
R608.
-
In
the
digital
section ofthe
PM
2517switchinq
from
the
,]00
mA to the 10
A range is effected in
the
following
way:
When
a banana
plug
is inserted
in the 10 A socket
the displav is switched from 100 mA
to
10 A. This
is
done
by means of the 10 A
switch
which
is buiit in
the 1O A socket.
Range Input FD
oo 0055
100 mA
.10
A
0
1
-
General
Resistor R
610 is used
to
get
an equal load in
the
lm)A-
and
(m)A-
function.
'
The ADC has
an
input
sensitivity
of 0.1
V
in case of dc
currenr measuremenrs.
3.3.2.4.
A.c. current measuremenTs
(Fig.
l5)
-
The input
circuit
for
a.c. current measurements
is
shown
in fjgure
15.
-
The a.c. input current
range 100 mA
is shunted
and adjusted in the same
wav as d.c. currents
(refer
to
3.3.2.3.). The voltage from
the shunts
(0.1V
end of range) is supplied to the Vin 1 inpur
of the a.c./d.c.
convertor
0O0061 via switch mA- and
C601. Input
Vin
2 of the 000061 is disconnected from the
input
circuits, by means
of connecting
-8.5
V
supply ro the
gates
of V
301 and
V
302. Signal
R NGB
has
no influence
{also
refer
to a.c. voltage measurements
3.3.2.2.).
RANGE
BNGB RNGA
Input
AC/DC
convertor
ln
put
ADC
X
X
0.1 v
The AC/DC
convertor
has an input
sensitivity of 0.1V in the
a.c. current ranges.
Protection
The
a.c. current ranges
are
protected
in the same way
as the
d.c. cuirent ranges.
Adjusting
The
a.c. current ranqes
are adjusted
by
means of influencing
the
sensitivity
of the ac/dc
convertor with
potentiometer
R304.