Bias Voltage/Current; Automatic/Manual Range Selection; Automatic Range Selection; Manual Range Selection - Hameg HM8118 Handbuch

M e a s u r e m e n t s w i t h t h e P r o g r a m m a b l e L C R B r i d g e H M 8 1 1 8
impair the accuracy by a factor of 2. The following table lists the
impedance ranges for constant voltage operation:
Range Source impedance
25 Ω
1
25 Ω
2
25 Ω
3
25 Ω
4
25 Ω
5
25 Ω
6
The following table lists the impedance ranges when the constant
voltage mode is switched off (CST V OFF):
Range Impedance of the component
Z > 3.33 Ω
1 to 2
Z > 400 Ω
2 to 3
3 to 4 Z > 6.67 kΩ
4 to 5 Z > 100 kΩ
5 to 6 Z > 2.22 MΩ
Z < 2.7 Ω
2 to 1
Z < 324 Ω
3 to 2
4 to 3 Z < 5.4 kΩ
5 to 4 Z < 81 kΩ
6 to 5 Z < 1.8 MΩ
There may be conditions which cause an "OVERRANGE" display.
This may happen if both constant voltage and MANUAL operation
is selected. In order to sidestep this problem choose manually
a higher range or switch to AUTO mode.

BIAS voltage/current

It is possible to superimpose a DC voltage or current on the
AC measuring signal. Components such as electrolytic capa-
citors require a positive bias voltage in order to obtain correct
measurement results. The internal bias voltage of 0 to +5 V
with a resolution of 10 mV or an external bias voltage of 0 to +40 V
allow you measurements under realistic bias conditions. The
internal bias voltage may also be used for some semiconductor
measurements. Press the button BIAS
bias voltage on, the button will light up. Pressing the button
again will turn the bias off, the button will extinguish.
It is also possible to superimpose a DC current on an AC measu-
ring current. The internal bias current can be set from 0 to +
200 mA with a resolution of 1 mA. It allows you to measure the
dependence of inductances on a DC current. In order to turn
the internal bias voltage or current on press the button BIAS
26 which will light up. If the button is pressed again the bias
voltage/current will be turned off. The voltage/current amplitude
can be set by using the arrow buttons
Warning:
Unipolar capacitors must be discharged before
correct connection, i.e. the positive terminal must
be connected to the left contact and the negative
terminal to the right contact. The bias voltage is
STOP
only available for capacitance measurements.
Warning:
Inductors must be discharged before disconnec-
ting! This means that after switching off of the bias
current a moment of waiting is necessary so the
current may decay to zero. The waiting period is
STOP
indicated in the display by „Please wait...". The bias
38
Subject to change without notice
Impedance of the
component
3 Ω
10 μΩ to
3 Ω to 100 Ω
100 Ω
to 1.6kΩ
1.6 kΩ to 25 kΩ
25 kΩ to 2 MΩ
2 MΩ to 100 MΩ
DC
26
to turn the internal
7 and the KNOB 6 .
current is only available for inductance measure-
ments.
Warning:
For all measurements with bias voltage or current
the constant voltage (CST V) must be turned on!
STOP

Automatic/manual range selection

Range selection may be automatic or manual. use the button
AUTO/HOLD 17 to alternate the mode between Auto and ma-
nual.

Automatic range selection

If autoranging is chosen the HM8118 bridge will automatically
select the optimum range for precise measurements of the
DUT.
If the result becomes < 22.5 % of a range the next lower range
will be switched in, if it exceeds > 90 % of a range the next
higher range will be switched in. The following table shows the
levels at which ranges will be changed (the constant voltage
CST V must be turned off):
Measurement range Impedance of the DUT
3 Ω
Z >
1 to 2
Z > 100 Ω
2 to 3
Z >
3 to 4 1,6 kΩ
Z > 25 kΩ
4 to 5
Z >
5 to 6 1 MΩ
2.7 Ω
Z <
2 to 1
Z < 90 Ω
3 to 2
Z <
4 to 3 1.44 kΩ
Z < 22.5 kΩ
5 to 4
Z < 900 kΩ
6 to 5
Please note:
When measuring certain inductors it may occur
that the bridge continuously changes the range.
The source impedance depends on the range used,
hence it may happen that the measurement
STOP
results differ more than the hysteresis of 10 %
when the range is changed, causing this effect.
It is then advisable to switch to manual range
selection.

Manual range selection

As the impedance of capacitors is inversely proportional to fre-
quency larger capacitances are measured in the lower impedance
ranges. Hence the proper range may change if the frequency
changes.
In case several similar components are to be measured in turn
the measurement time can be shortened if the fi rst component
is measured with autoranging on, then the button AUTO/HOLD
should be pressed to change to manual range mode and hold it,
the button AUTO/HOLD will extinguish.
In order to prevent false operation it is always best to use the
AUTO mode. If manual upranging is desired, press the UP button,
if downranging is desired the DOWN button.