o. Functions of the Safety Circuits.
(1) Numerous safety circuits are provided to protect the patient and the
operator from the possible effects of improper operation or equipment malfunction. Some
of these have been discussed already such as safety dump, low voltage fire safety (ready
light), energy protection discharge, and external/internal paddle charge lockouts.
(2) Another safety feature is two button firing where both paddle fire buttons
must be depressed simultaneously in order to fire.
(3) There is also an automatic energy dump. A power switch and load
resistor, R6 (500 ohm), are so arranged that when the cover of the defibrillator section is
removed, the capacitor bank is discharged through the resistor yielding a safe chassis for
(4) There is another circuit consisting principally of Q8 which gets turned on
by a fire pulse from U14. It acts upon Q9 to release relay K1 after each firing and requires
reactivation of the charge button for additional operations. This is the automatic shut off
(5) To better understand this AUTO/OFF feature, carefully examine the
schematic in Appendix G. It reveals the mechanism of this circuit which serves several
purposes. Observe the diode CR59 in series with the 4 ohm resistor and the dump SCR.
Each time that the dump SCR is activated, a current flows through the 4 ohm resister and
the diode creating a voltage drop of approximately 0.7 volt on the diode. The voltage is
impressed on the non-inverting input of U10, a comparator, which has approximately 0.5v
on the inverting input due to voltage divider R127(1K), and R128 (220 ohm).
(6) Normally, there is no output from the comparator, and hence the 4N36
(U13) is not in conduction. This Q18 2N4403 is conducts normally and provides the 12v
boost voltage from its collector to the pole SW5. Standing-wave ratio (SWR) supplies
power to the driver U3 to permit charging. Furthermore, Q19 (2N4401), is also conducting
by virtue of R131(3.3K) and R132(1.5K) supplying base drive.
(7) However, when the dump SCR is activated and voltage generates on the
diode CR 59(MR752), the comparator will reach the point where it outputs a high which
turns on the opto-coupler U13.
(8) When the output side conducts the 2N4403, Q18 is deprived of drive and
turns off, which removes power from U3 and stops the charge. It also stops Q19 (2N4401)
from conducting. As its collector pulls up, a pulse is output through the 1N4148 diode
which activates the auto-off circuit, opening relay K1, via Q9, the holding transistor.