ConMed ExcaliburPLUS PC - Page 49

K
varies directly with the resistance appearing in the
return electrode circuit. The relationship is essen-
tially logarithmic, with increases in VARM
becoming vanishingly small as RI rises above
1000 ohms. This means that VARM will change
by a nearly constant voltage for a given percent-
age change in RI anywhere in the 10 to 150 ohm
range.
The balance of the A.R.M. Circuitry resides on
the A3 Controller PWB, Figure 4.4b. Diode
VR2 is a +1.235 V regulator whose output volt-
age appears across the 2.49K resistor R47, thus
driving a constant current of 0.5 mA into the
VARM line. R48 allows the circuit to operate to
ground, while R40 and C38 act as a low-pass
noise filter. The voltage BVARM at U21-8 is
essentially equal to VARM, since that opamp is
connected as a high-input-impedance voltage fol-
lower.
The digital-to-analog converter, DAC U1, is dri-
ven by the microprocessor to produce 0.0 to
+2.55 Vdc at U19-16. This voltage is compared
to BVARM by comparator U23 to drive ARM-
COMP at U23-13. Every 12 msec, the micro-
processor reads the ARMCOMP line in response
to a sequence of DAC voltages, determined by a
successive approximation algorithm, to measure
the VARM voltage to a precision of 10 mV. This
value is then processed along with the VARM val-
ues for 10 and 150 ohm return circuit resistances
stored in the NOVRAM during the last Pad
Calibration to evaluate the current RI.
The Return Fault process works on a 50-point
(0.6 sec) average VARM value. If Single Foil
Mode is selected, the microprocessor will declare
a Return Fault when VARM indicates that RI is
10 ohms or greater. The ASSIST Resistance
Indicator is always dark in this mode.
In Dual Foil Mode, the ASSIST Resistance
Indicator will be illuminated to indicate RJ in the
range of 10 to 150 ohms. At just over 10 ohms,
the two left bars are illuminated. As VARM
increases, additional bars are illuminated in pro-
portion to VARM, progressing to the right, until
RI approaches 150 ohms, where eight bars are
illuminated. When RI exceeds 150 ohms, all ten
bars are illuminated. Whenever a Return Fault
condition exists, all illuminated bars will flash, but
RI is still displayed as above. Figure 3.4 illustrates
the relationship between R1 and the ASSIST
Resistance Indicator bars.
In
Dual Foil Mode, the microprocessor declares a
Return Fault if RI is less than 10 ohms or greater
than 150 ohms. If VARM is within the allowed
range, then the Return Fault Indicator will turn
off when the Alarm Set Point Key is pressed, and
the present value of VARM is stored for refer-
ence. A new Return Fault will be declared if RI
rises about 20% above this stored value or goes
out of the allowed range.
This rise indicates significant electrode detach-
ment. A Return Fault declared in this case will
NOT automatically be cleared if the patient resis-
tance drops back to near the stored value. The
Alarm Set Point Key must again be pressed to
register the staff’s satisfaction that the electrode
attachment is safe before turning off the alarm.
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
judgment of safe return electrode placement.
3.9 Continuity Detector
The Continuity Detector provides isolation by
both magnetic and optical coupling. Schematic
4.6 includes a schematic of the continuity detec-
tor. A 90 kHz oscillator, A5U4, generates a 20%
duty-cycle rectangular wave drive to transistor
A5Q1, which drives the resonant primary circuit
of a toroidal isolation transformer, A5T2. See
Figure 3.7. The energy coupled to the secondary
windings is rectified and filtered to produce an
isolated 3 to 4 Vdc source for each of the three
separate RF output circuits (BIPOLAR, H1, and
H2). The bipolar hand switch continuity detector
will be used as an example, since all sections are
identical. When the bipolar hand switch is
closed, de current limited by R1 flows through
the LED in the optical isolator U1. This pro-
duces a beam of light which falls on the photo-
transistor in U1, causing it to draw collector cur-
rent. This current pulls the signal line /HBP to a
[2 3-15