27
Subject to change without notice
Voltage measurements
The maximum input voltage of HM8012
is 600 V
DC
when the COM terminal is
to ground potential, i.e. by connecting
HM8012 to the object to be measured,
the sum of the measurement voltage and
that of the COM terminal with respect
to ground shall not exceed 600 V
DC
. In
this case, the maximum voltage value
between the COM terminal and ground
is 250 V
DC
.
For AC voltages, the true rms value of the input
voltage will be measured, and the DC component
eliminated in AC mode. If possible, the COM ter-
minal
shall be connected directly to ground or
to the point of the measurement circuit having
the lowest potential. The 0.5 V and 5 V voltage
measurement ranges are protected from input
voltages to 300 V
rms
; all the other ranges are
protected to 850 V
rms
.
During measurements on circuits using inductive
components, inadmissible high voltages may
appear when the circuit is opened. In such cases,
take steps to prevent the destruction of HM8012
by inductive voltages.
Input impedance in the DC range
To make the most of the excellent linearity of
the measurement system, the input impedance
for voltage measurements is very high (1 G ) for
some ranges. For instance, this makes possible
to perform accurate measurements on ranges of
up to ±5 V, even when the internal impedance of
the source to be measured is high. For instance,
for the 500 mV range, an internal 5 M source resi-
stance will induce a maximum error of 150 μV.
Indeed, switching to the 50 V range will cause the in-
put voltage to drop because of the input impedance
of 10 M which can cause the multimeter to switch
to the lower range and so on. After some swings,
the instrument goes into MANUAL mode.
Current measurement
For current measurements, the connection of the
object to be measured is made across the mA
terminal or to the A
terminal for currents of up
to 10 A. The AUTO mode is inhibited because there
is only one range.
The HM8012 should be connected to the circuit
whose potential with respect to ground is lowest.
For safety reasons, the COM terminal
must
not exceed 250 V
DC
with respect to ground.
Current ranges are protected by fuse against over-
loads. If the fuse blows, the cause of the overload
must be eliminated. In case of an open fuse the
instrument has to be sent in for repair. A change of
the fuse by the customer is not permitted.
AC voltage measurement
The instrument measures the true rms value of
the input voltage with or without its DC component.
To measure low voltages, or in the event of high
noise, it is necessary to use a shielded cable.
Take into account the input impedance of the mul-
timeter. It is 1 M in the AC mode and 10 M in the
AC + DC mode. In addition, there is a slight measu-
rement difference between these two modes due
to the input circuits. If AC measurements without
a DC component are to be made, it is preferable
to use the straight AC mode.
When the multimeter is used in AUTOMATIC mode,
there can be continuous swinging between two
ranges for frequencies above 30 kHz, because
of the frequency response difference of the two
ranges. After some swings, the instrument goes
into MANUAL mode.
Measuring errors can be caused by an overload of
the measuring amplifi ers and the A/D converter.
For measurements of mixing tensions (positive or
negative DC voltage overlaid with an alternating
voltage) select a measuring range which is not
exceeded by the peak value of the alternating
voltage – the largest deviation from the reference
potential (normally 0 V). That applies also, if only
the DC voltage portion is to be measured.
A DC voltage of 350 mV is superimposed with a
sine signal with an amplitude of 200 mV
p
(400
mV
pp
). The highest deviation from 0 V amounts
550 mV
p
(350 mV
p
+ 200 mV
p
). For DC and AC+DC
measurements the 5 V-range must be selected.
AC measurements can be done in the 500 mV
range, because then a capacitor in the entrance
suppresses the DC voltage. It is reasonable, fi rst
to determine the height of the possibly existing
alternating voltage with AC measurement. Then
switch to DC or AC+DC function and consider the
F u n c t i o n s
