ConMed sabre 180, Service Manual

Page: 32 / 72

3-6
The A.R.M. oscillator is a dc-to-ac power con-
verter, with its major losses appearing as resistors
in parallel with the resistance of the return elec-
trode circuit, RL transformed up through T4 and
T5. In effect, the A.R.M. oscillator transforms
RL into an equivalent dc resistance, Rin , appear-
ing at the VARM input to the circuit. Thus when
RL 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 RL falls into the 10 to 150 ohm range
normally encountered with a properly applied
dual-foil electrode, Rin also drops and VARM
falls into the 1.0 to 2.5 V range. If RL is very
low, as when a single foil electrode is connected,
VARM drops to about +0.8 Vdc. Resistors R33
and R34 serve to set a lower limit to the resis-
tance applied across T4's secondary. Without this
lower limit, the effective short circuit presented
by a single foil return electrode would reduce the
Q of the 37 KHz turned circuit to the point that
the oscillator would behave erratically. Thus
VARM varies directly with the resistance appear-
ing in the return electrode circuit. The relation-
ship is essentially logarithmic, with increases in
VARM becoming vanishingly small as RL rises
above 1000 ohms. This means that VARM will
change by a nearly constant voltage for a given
percentage change in RL anywhere in the 10 to
150 ohms range.
The balance of the A.R.M. Circuitry resides on
the A3 Controller PWB, Figure 5.6b. Diode VR2
is a +1.235 V regulator whose output voltage
appears across the 2.49K resistor R47, thus dri-
ving 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 U19, 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 10mV. This
value is then processed along with the VARM
values for 10 and 150 ohm return circuit resis-
tances stored in the NOVRAM during the last
Pad Calibration to evaluate the current RL.
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 RL is
10 ohms or greater. The Resistance Bargraph is
always dark in this mode.
In Dual Foil Mode, the Resistance Bargraph will
be illuminated to indicate RL in the range of 10
to 150 ohms. At just over 10 ohms, the two left
bars are illuminated. As VARM increases, addi-
tional bars are illuminated in proportion to
VARM, progressing to the right, until RL
approaches 150 ohms, where eight bars are illu-
minated. When RL exceeds 150 ohms, all ten
bars are illuminated. Whenever a Return Fault
condition exists, all illuminated bars will flash, but
RL is still displayed as above. Table 3.1 illustrates
the relationship between RL and the Resistance
Bargraph bars.
In Dual Foil Mode, the microprocessor declares a
Return Fault if RL 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 Monitor Set Key is
pressed, and the present value of VARM is stored
for reference. A new Return Fault will be declared
if RL rises about 20% above this stored value or
goes out of the allowed range.
A rise of approximately 20% over the resistance
of a Return electrode in full contact with a patient
indicates significant electrode detachment. A
Return Fault declared in this case will NOT auto-
matically be cleared if the patient resistance drops
back to near the stored value.
The Monitor Set Key must again be pressed to
register the staff's satisfaction that the electrode
attachment is safe before turning off the alarm.