Valleylab SurgiStat II, Service Manual, Page: 50 / 82

4-4 SurgiStat II Service Manual
Patient Return Electrode Sensing Circuit
The patient return electrode sensing module senses and sends signals to the
system logic, which displays which type of patient return electrode is attached to
the patient.
When you connect a single-plate patient return electrode to the unit, the pad
sensing module will detect if the resistance is below 5
Ω
. If it is, the SurgiStat II
will display the green single-plate LED on the front of the unit.
When you connect a split-plate patient return electrode to a patient and the pad
sensing module detects a resistance between 10 and 135
Ω
, then the SurgiStat II
will display the green split-plate LED on the front of the unit.
The pad sensing module constantly monitors the patient contact quality. If the
impedance changes by a specific amount, then the unit displays an alarm and
immediately deactivates the RF output power.
RF Amplifier Circuit
The RF amplifier circuit generates the RF output energy that is delivered to the
patient. It is a single-ended power amplifier, incorporating three power MOSFETs
and two toroidal step-up transformers.
The digital PWM circuit and the system logic unit generate the initial RF drive
pulse. When the RF drive pulse turns the power MOSFETs “On,” current flows
from the high voltage supply through one of the output transformers, depending
on which mode the unit is in, through the clamping diodes, and then through the
MOSFETs to high voltage ground.
The energy developed by the “On” time is stored in an LC tank circuit. When the
MOSFETs are off, the energy is delivered to the patient through the output
capacitors. A longer “On” time develops more energy in the LC tank circuit;
therefore, more energy is delivered to the patient.
Monopolar Select Circuit
The monopolar select circuit switches the SurgiStat II between each of the four
monopolar modes. Matrix switches on the front panel allow mode selection. High
voltage relays switch and isolate the four monopolar configurations.
Monopolar/Bipolar Select Relays
The monopolar / bipolar select relays change which output transformer is used to
deliver the RF output to the patient.
Warning
Patient return electrode contact quality is only monitored when a split-plate
patient return electrode is attached to the patient.