ConMed ExcaliburPLUS PC - Page 48

Eco ec NS
PAD/DUAL PAD and MONITOR SET POINT
buttons to determine when a RETURN FAULT
condition exists. The ASSIST Resistance
Indicator is also driven by software to indicate the
value of the measured DUAL FOIL resistance in
the 10 to 150 ohm range.
Portions of this function are implemented on the
A3 Controller PWB and on the A5 Output PWB.
150-
1304
1104
904
704
50”
RESISTANCE
(OHMS)
304
10. T T T T T
2 4 6 8
RESISTANCE BARS
FIGURE 3.4
Figure 3.4 shows the approximate resistance vs.
number of illuminated bars in the resistance indi-
cator. If no bars are lit, then the resistance is less
than approximately 10 ohms, if 10 bars are lit,
the resistance is greater than approximately 150
ohms. It is not possible for just 1 or 9 bars to be
lit, unless a bar segment has failed.
Schematic 4.6 contains the ARM Circuitry on the
A5 PWB. It is comprised of an oscillator section
and an isolation section. The isolation section
employs a shielded toroidal transformer, T5, to
couple the impedance presented at return elec-
trode plate jacks, A7J8 and J9 to the ARM oscil-
lator, while isolating that circuit from the effects
of applied RF electrosurgical current and voltage.
Capacitors C44 and 45 split the return current
evenly between the two legs, thus minimizing the
RF voltage appearing across T5 windings. T5
also acts to step up the return circuit impedance
by about 10:1. The shield serves to prevent the
RF stray magnetic field generated by the
monopolar output transformer, T3, from
interfering with the ARM circuitry during RF
activation.
The A.R.M. oscillator generates a low-power
sinewave voltage of about 36 KHz. This fre-
quency is determined by the inductance of T4 in
parallel with the capacitance presented by C21 -
23, and that of C44 and C45 reflected through
T5. Transistors Q2 and Q3 are cross-coupled via
R7 and R8, so that when one transistor is con-
ducting, the other is fully turned off due to lack
of base drive. The conducting transistor turns off
at the next zero-crossing of the sinusoidal voltage
on the primary of T4. This allows its collector
voltage to rise and thus provide base current to
the other transistor to turn it on. In operation,
the collector voltages appear like half-wave recti-
fied ac, with each collector 180 degrees out of
phase with the other. See Figure 3.7.
The A.R.M. oscillator is powered by a constant
0.5 mA de current driven from the A3 PWB via
the VARM signal line. This current feeds into
the center tap of T4 primary. The voltage on the
center tap is the average of the two collector volt-
ages, so it appears as a full-wave rectified sine
wave with a peak amplitude of one-half that on
either collector. Inductor L1 helps hold the cur-
rent fed to T4 constant regardless of these voltage
variations, while C15 serves as a bypass to limit
the noise conducted from the A5 PWB up the
VARM line to the A3 PWB.
The A.R.M. oscillator is a de-to-ac power con-
verter, with its major losses appearing as resistors
in parallel with the resistance of the return elec-
trode circuit, RI, transformed up through T4 and
T5. In effect, the A.R.M. oscillator transforms RI
into an equivalent de resistance, Rin, appearing at
the VARM input to the circuit. Thus when RI is
very high, as when no connection is made to the
Return electrode jacks, Rin is maximum, allowing
the VARM voltage to rise to +2.3 - 3.0 Vdc.
When RI falls into the 10 to 150 ohm range nor-
mally encountered with a properly applied dual-
foil electrode, Rin also drops and VARM falls
into the 1.0 to 2.5 V range. If RI is very low, as
when a single foil electrode is connected, VARM
drops to about +0.8 Vdc. Resistors R11 and
R12 serve to set a lower limit to the resistance
applied across T4’s secondary. Without this lower
limit, the effective short circuit presented by a sin-
gle foil return electrode would reduce the Q of
the 37 kHz tuned circuit to the point that the
oscillator would behave erratically. Thus VARM