Fluke 45 - Manual (Page 51)

45
Users Manual
4-4
CAL
ENABLE
V 10A
100
mA
600V CAT I
1000V CAT I
FUSE F1
500 mA
F 250V
POWER
AUTO
2ND
MN MX RATE
dB REL
REF# LOCAL
THRESH ADDR BAUD
HOLD
LO HI COMP
A
A V FREQ
DUAL DISPLAY MULTIMETER 45
V
!
REF
FUSED
COM
V
A
CIRCUIT
2ND
LOAD
aam17f.eps
Figure 4-2 DC Voltage and DC Current Measurement on Input Signal
The lead from the internal measuring circuitry of the meter to the COM binding post (on
the front panel) is the same for both voltage and current measurements. The resistance of
this lead is approximately .003
Ω
. If current is being measured, therefore, a voltage drop
will occur in the resistance that is common to both circuits. This internal resistance, when
added to the external resistance of the lead from the COM input terminal will affect the
accuracy of the voltage reading. For instance, if the external lead resistance is .007
Ω
, the
"total" common resistance is .010
Ω
. If there is 1 A of current, the voltage reading would
be affected by
(1 A x.01
Ω
) = .01 V or 10 m V.
Depending on the circumstances, this may be significant.
If you want to measure dc voltage on an input signal in the primary display and dc
current in the secondary display, proceed as follows:
1. Turn the meter on.
2. Press
Y to select the dc voltage function for the primary display.
3. Press
S, then press C to select the dc current function for the secondary display.
4. Connect the leads to the test circuit as shown in Figure 4-2 and read the
measurements on the displays. Although current will be displayed as negative, it is in
fact positive when interpreted according current flow convention.
Response Times
Response time is the time between a change in an input and when that change is displayed.
The meter’s response time depends on many factors: the measurement function selected,
number of measurements being made (single measurement when only the primary display is
used, or two measurements when both the primary and secondary display are used), the input