Erbe ICC 200, Service Manual

Page: 161 / 266

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3 CIRCUIT DESCRIPTION
Art. No.: 801
16-201
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4
The symmetry of HF current on the halves of the neutral electrode is measured by a symmetrical transformer,
the output voltage of which is a function of current symmetry.
Implementation: The circuitry is found on the Senso-board. The oscillator is designed as a frequency-
selective feedback amplifier: The amplifier IC6 actuates the push-pull stage, consisting of the transistors T4
and T5, and passes the amplified output signal to the frequency-selective serial circuit made of capacitor
C20, coil L1 and the inductivity from the primary side of transformer UE4.
One part of the output signal at the transformer UE4 is picked up via the resistor R18 and fed to the active
bandpass from the operation amplifier IC5 and its wiring. The center frequency is equal to the resonance
frequency of the previously mentioned serial circuit.
One part of the output signal from the active bandpass is fed again in proper phase to the amplifier IC6,
such that the entire arrangement fulfills the oscillation condition and oscillates at the resonance frequency
as an oscillator.
The oscillator can be switched off via the transistors T7 and T8 by means of the disable signal OSZI_DIS
from the processor.
The resistance of the neutral electrode is determined by loading this oscillator with the resistance to be
measured. The voltage collapses on the internal impedance of the oscillator according to the load. Thus
every oscillator voltage can be assigned an appropriate resistance value.
The oscillator voltage is present at the input of the active bandpass and can be picked up at the output of the
operation amplifier IC6.
After peak rectification by diode D30 and capacitor C34, the measurement voltage U_NESSY can be
picked up at the isolation amplifier IC9 and moves via the adjustment on the control board (trim potentiometer
TP9 ) to the processor to determine the resistance of the neutral electrode.
The high-frequency output voltage for the neutral electrode is balanced by the capacitors C2 and C3 on
both lines to the neutral electrode, i.e. divided equally.
Whether the current on both lines to the neutral electrode is now of equal size, depends on whether the two
electrode halves of the split neutral electrode are applied with the same surfaces to the patient. By measuring
the symmetry of the current and the resistance, it is therefore possible to receive information as to whether
the electrode is applied well to the patient with this entire surface.
The high-frequency current flows via the two primary windings 1 and 2 of the transformer UE3 in such a
manner that the resulting magnetic flow is cancelled out by equally strong currents. Thus in this case, no
voltage is induced at winding 3 of the transformer UE3. The reverse is true: The more unsymmetrical the
current flow is, the greater the voltage induced in winding 3.
The high-frequency output voltage, which is a measure for the unsymmetry of the current flow, is rectified
using diode D9 and filtered with capacitor C57, then amplified using the operation amplifier IC9, so that a
signal U_SYM is available which moves via the control board to the processor for evaluation.
The current density is calculated mathematically from the following dimensions, now known:
• High-frequency current,
• Symmetry of the high-frequency current at the neutral electrode,
• Contact resistance of the neutral electrode to the body of the patient.
Senso-board
Slot J7