(i) The blanking signal from the timing generator also is fed to the ramp
generator. After the blanking signal is negated, the ramp generator begins to output a
positive-going ramp with a linear slope. The ramp continues to rise until the next blanking
pulse arrives and starts a new cycle.
b. Signal Paths in the Defibrillator.
Generation of the trapezoidal defibrillator pulse.
(a) The cathodes of two high voltage and high current capacity silicon
controlled rectifiers (SCR) are connected to each other and to the negative terminal of the
capacitor bank through resistor R1. The anode of the dumper SCR connects directly to
the positive terminal of that bank. The fire SCR anode is connected through resistor R2 to
the same point.
(b) One paddle also receives this voltage directly once the fire button
located on its housing is depressed. The second paddle, through a similar switch
arrangement, takes its signal from the anode of the fire SCR.
(c) As long as neither of the SCR is triggered into conduction, no voltage
differential exists between paddles. As soon as the fire SCR starts conducting, its anode
falls to the negative polarity of the capacitor bank. This bank starts discharging into the
load, patient, or test resistor.
(d) The current which is now flowing through R1 is integrated. Once the
selected amount of joules (watts/second) is detected, the dumper SCR is fired removing
the current from the load and discharging the capacitor bank completely. By integrating
the current, a 4:1 load resistance ratio (25 ohm to 100 ohm) can be covered with
substantially the same energy delivery by simply changing the pulse duration. This feature
is called the transthoracic load compensation.
Main power invertor, energy selector, hi voltage regulators.
(a) The energy selection is primarily obtained by charging the capacitor
bank to a preset voltage which, as a consequence, stores approximately 50 percent more
energy than needed for each watt second setting. This technique conserves the battery
charge and still produces an output pulse with reasonable droop.
(b) As soon as the capacitor bank reaches the corresponding voltage,
the main power invertor shuts off, and the ready line informs the rest of the circuitry that
the defibrillator is ready to be fired.