ConMed sabre 180, Service Manual

Page: 28 / 72

3-2
3.2.2 Aspen Return Monitor (A.R.M.)
Circuitry and Software
The A.R.M. Circuit converts the electrical resis-
tance appearing in the return electrode circuit into
a digital value which can be processed by the
microprocessor A3U3. Software processes use
this value in conjunction with the PAD SELECT
and MONITOR SET buttons to determine when
a return fault condition exists. The Resistance
Bargraph is also driven by software to indicate the
value of the measured DUAL FOIL resistance in
the 10 to 150 ohm range.
The A.R.M. oscillator generates a low-power
sinewave voltage of about 37 KHz. This frequen-
cy is determined by the inductance of T4 in paral-
lel with the capacitance presented by C33-34.
Transistors Q5 and Q6 are cross-coupled via R19
and R20, so that when one transistor is conduct-
ing, 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 volt-
age to rise and thus provide base current to the
other transistor to turn it on. Figure 3.2 shows
typical A.R.M. oscillator waveforms with a 50-
ohm return circuit resistance applied.
The A.R.M. oscillator is powered by a constant
0.5 mA dc 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 cen-
ter tap is the average of the two collector voltages
so it appears as a full-wave rectified sine wave
with a peak amplitude of one-half that on either
collector. Inductor L2 helps hold the current fed
to T4 constant regardless of these voltage varia-
tions, while C59 serves as a bypass to limit the
noise conducted from the A5 PWB up the
VARM line to the A3 PWB.
Resistance Typical
Bargraph Midrange
BARS Resistance
0 <10
2 18
3 31
4 43
5 60
6 83
7 118
8 145
10 >150
TABLE 3.1
Portions of this function are implemented on the
A3 Controller PWB, Figure 5.6b and on the A4
Output PWB, Figure 5.7b.
The A.R.M. circuitry on the A4 PWB comprises
an oscillator section and an isolation section. The
isolation section employs a shielded toroidal
transformer, T5, to couple the impedance present-
ed at return electrode plate jacks, A5J4 to the
A.R.M. oscillator, while isolating that circuit from
the effects of applied RF electrosurgical current
and voltage. Capacitors C71 and 72 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 cir-
cuit 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 A.R.M. circuitry during RF
activation.