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Because patients' and return site resistances vary
over a considerable range, it is not safe to assume
that any in-range resistance indicates safe elec-
trode attachment. For example, a poorly placed
electrode on a well-perfused site can show the
same resistance as a safely attached electrode on
adipose tissue. Yet the poorly placed electrode
could still result in a burn due to low contact
area. The clinical staff is responsible for the final
judgement of safe return electrode placement.
The unit powers up in whichever pad mode was
last used and the Return Fault is ON awaiting a
new set point. The Return Fault will also be lit
whenever Dual Foil is selected after being in
Single Foil Mode. This prevents a poor single foil
contact from being mistaken for a good dual foil
contact. Normal single foil electrode resistance is
well below the range acceptable for dual foil oper-
ation.
3.2.3 Continuity Detector
The Continuity Detector provides footswitch and
hand switch isolation by both magnetic and opti-
cal coupling. Figure 5.7b includes a schematic of
the continuity detector. A 90 KHz oscillator,
A4U2 generates a 20% duty-cycle rectangular
wave drive to FET A4Q10, which drives the reso-
nant primary circuit of a toroidal isolation trans-
former, T2. Figure 3.3 shows typical waveforms
in the Continuity Detector Isolated Power circuit-
ry. The energy coupled to the secondary wind-
ings is rectified and filtered to produce an isolated
3 to 4 Vdc source for hand switch and foot
switch isolation circuitry. The Monopolar hand
Cut switch continuity detector will be used as an
example, since all sections are identical. When this
switch is closed, dc current limited by R63 sets
the emitter voltage of Q14. If the switch resis-
tance is less than about 400 ohms, Q14 will draw
base current via voltage divider R60-61. The cor-
responding collector current flows through the
LED in optical isolator U5. This produces a
beam of light which falls on U5 phototransistor
causing it to draw collector current. This current
pulls the signal line /H1CT to a low voltage, and
this state is interpreted by the microprocessor as a
closure of the switch. When the hand switch is
released, Q14 emitter voltage is pulled high, caus-
ing Q14 to turn off.
3-8
The LED goes dark causing the phototransistor
to cease conduction and allow the signal line to
be pulled to a high state by resistor network
A3RN1. C68 bypasses any RF currents around
the accessory switch than may occur due to
reverse accessory connection while blocking dc
from the Q1 emitter when the accessory switch is
open.
3.3 POWER AMPLIFIER
The Power Amplifier (PA) is a hybrid cascode
amplifier made of high voltage bipolar transistors
and low voltage power MOSFETs. The schematic
for this section is included in Figure 5.7a. Figure
3.4 shows the basic hybrid cascode amplifier con-
figuration. The combination of Q1 and Q2 make
up a fast, high-voltage amplifier that can be con-
trolled by the combination of the dc voltage
VBASE, and the fixed amplitude, variable pulse
width signal, VGATE.
Rb
VBASE
VGATE
Figure 3.4 Basic Hybrid Cascode
Configuration
In the OFF condition, VGATE is near ground,
turning off Q2 so that no drain current can flow.
Thus no base or emitter current can flow in the
bipolar transistor. Since V+ is always greater than
VBASE, the collector base junction is reverse-
biased, so no collector current will flow and no
power is delivered to the load. Turn-on com-
mences with VGATE rising rapidly to about
+10V . This results in a large pulse of base current
flowing in Q1 from Cb, quickly turning Q1 on
which delivers power to Z1. After the turn-on
transient, Q2 will be conducting hard and Q1
V+
Zl
D1
Q1
Cb
Re
Q2
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Troubleshooting
