MW1008P, user manual
17
Bias current is I
o
= VBIAS/(R2+R3+r)
When using this circuit one will avoid to disconnect the inductor while a current is flowing
into it. VBIAS voltage should be decreased down to 0V in order to discharge the coil.
Opening the circuit can introduce a high voltage with high energy which can be dangerous
id the inductance or the current is high.
Resistors
The unit of measurement for resistance is the Ohm. Of the three basic circuit components,
resistors cause the fewest measurement problems. This is true because it is practical to measure
resistors by applying a dc signal or relatively low ac frequencies. Resistors are usually measured at
dc or low frequency ac where Ohm's Law gives the true value under the assumption that loss
factors are accounted for.
Model
For low values of resistors (below l kΩ) the choice usually becomes a low frequency
measurement in a series equivalent mode. Series because the reactive part most likely present in a
low value resistor is series inductance, which has no effect on the measurement of series R. For
high values of resistors (greater than several MΩ) the best choice is usually a low frequency
measurement in a parallel equivalent mode. Parallel because the reactive part most likely present
in a high value resistor is shunt capacitance, which has no effect on the measurement of parallel R.
Some high precision resistors are winding resistors, and the number of turns can be quite large for
high value resistors. In this case the series inductance can be quite large and measurement should
be done at low frequency to avoid errors caused by the inter-winding capacitance.
MEASUREMENT
OF
INTERNAL
RESISTANCE OF BATTERIES
The internal resistance of batteries can be measured
using circuit diagram shown here. The dc voltage is
isolated by capacitor C1 = 47µF. The internal
resistance will generally be measured at 1 kHz.
Before doing the measurement itself, it is necessary
to perform a short-circuit calibration by replacing
the battery with a short. The measurement is done
with no current. It is possible to connect shortly a
parallel resistor in order to drain some current. It is
only necessary then to do a little calculation to find
the actual internal resistance.
