Isolated Handswitching/Footswitching - ConMed Sabre 2400 Operators & Service Manual

3.1.9 Isolated Handswitching/Footswitching

The circuit for the handswitched accessories are
electrically isolated using both magnetic and opti-
cal coupling. Refer to Schematic 4.7. A 90 KHz
oscillator, A1U2, generates a 20% duty-cycle rect-
angular wave drive to the FET transistor A1Q6,
which drives the resonant primary circuit of a
toroidal isolation transformer, T2. 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.
The secondary winding T2-8 and T2-7 combine
with D16 and C59 to produce an isolated dc
voltage. When the bipolar hand switch is closed,
the emitter of Q16 is pulled to the return of it's
isolated power supply, turning Q16 on. With
Q16 turned on, current flows through the LED
of the optocoupler U10, turning its output tran-
sistor on. This results in /BIP-H going low, and
signalling the microprocessor that the handswitch
is on. C60 bypasses any RF currents around the
bipolar accessory switch. R53, R54,and R63 bias
Q16 to prevent the circuit from turning on when
the impedance presented by the output switch
is >1000 ohms. This decreases the possibility of
accidental activations if blood or saline seep into
the switch. International units use similar circuits
(see Schematic 4.7) to provide footswitch isola-
tion.
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