(2) This allows a chain of fire pulses to appear in the output of U14 and on the
primary side of torroid T5. These pulses subsequently appear at the gate of the fire SCR
through the current limiting resistor R69, and the discharge is initiated.
k. Functions of the Circuits in the Primary Dump Circuit.
(1) The load current is sensed by R26, the 4 ohm resistor. The voltage across
the resistor is used to charge the main timing capacitor (.47 mfd) C26 through a precision
resistor selected by another section of SW5. It appears on the non-inverting input of the
remaining section of U10.
(2) The inverting input of this comparator has a preset voltage on it which is
set at the 200j output by R112 and not readjusted again. This corresponds to a 12.0msec
pulse width into the 50 ohm load.
(3) When the voltage across the 4 ohm resistor charges the timing capacitor
to the same voltage as the preset input, the comparator outputs a high which turns on the
U11 chip. A chain of pulses then appears in the primary of torroid T4.
(4) The secondary side of the torroid couples the pulses to the gate of the
dump SCR through an "OR" diode CR40 and a current limiting resistor R50, firing the
dump SCR and terminating the output pulse.
(5) The current available for charging the timing capacitor depends on the
load current as sensed by R26 and the value of the precision resistor as selected by
energy switch SW5. This same current is capable of compensating for differences in load
(6) Since the current through R26 is the load current, a lower load impedance
will cause an increased voltage drop across the 4 ohm resistor due to increased current
flow in the load. This larger voltage drop gives a proportional increase in the current
available to charge C26, resulting in a shorter output pulse width. A higher load
impedance will act to increase the output pulse width. This is the principal of transthoracic
compensation which allows the defibrillator to deliver selected energy over a wide range of
Functions of the Circuits in the Safety Dump.
(1) An abnormally high resistive load or no load at all can cause the primary
dump circuit not to fire at the appropriate time. This indicates the paddles have a high
voltage on them decaying exponentially. An extra dump circuit is supplied to prevent this.
(2) When the machine is fired, the safety dump network (T3, C33, Q7, R65,
R61, R66 and CR41) is placed across the entire high voltage by virtue of the fire SCR
providing a low impedance path to the low side of the capacitor bank.