6-10
(AC Lo), and J1-4 (AC Com). We should note
that these AC signals are isolated from AC Mains
by means of an isolation transformer located in
the generator assembly. The AC signals are trans
-
former taps with AC Lo rated for 125V and AC
Hi rated for 185V with a nominal voltage input.
Referring once again to the schematic, Q3 & Q7
are triacs that are triggered by opto couplers U1
or U4 when the HVPS is enabled. Q3 is selected
when Spray, Pinpoint or ABC™ modes are acti
-
vated and Q7 is selected for Pure Cut, Blend, and
Bipolar modes. The resistor and capacitor that
are connected from MT2 & MT1 are snubbers for
switching energy and the remaining discrete com
-
ponents contribute to the generation of the trigger
pulse. The trigger pulse from the opto couplers
(U1 & U4) occurs at phase angles of approxi
-
mately 80° to 170° of the AC waveform.
Following the triacs are two separate bridge recti
-
fiers (B1 & B2). The rectified signal from the
bridges is then filtered by C13 & C14 which
results in a DC voltage. Resistor R18 is a bleed
resistor that will dissipate the energy from the
filter capacitors. The schematic shows two test
points (TP1 & TP2) that are labeled +HV and
-HV. The voltage on these two test points is the
regulated DC and is referred to as HVDC.
Connecting an oscilloscope to the +HV and -HV
test points would show a voltage with 120 Hz
ripple, and the magnitude of the ripple depends
on the load. A heavy load on the output means a
high ripple rate on the HVDC test points. These
test points have easy access and are a good place
to connect a DVM when troubleshooting a system
that does not have any RF Output. A system
failure detected by the monitor microcontroller
and some hardware circuits will typically inhibit
the HVDC as a means of shutting down the RF
output.
The high voltage triacs (U1 & U4) of this assem
-
bly are only enabled when the system is activated
for RF output, otherwise the voltage at TP1 &
TP2 is an idle voltage of 10V. Referring on the
schematic to TP1, a 33 ohm resistor and a fuse are
shown in series with a
MOSFET, Q8. This tran-
sistor is switched on following each RF deactiva
-
tion to dump the charge from the filter capacitors
(C13 & C14) and bring the HVDC back to, or
near, idle voltage.
The fuse would be the weak link of this power
supply. Each time an activation request is termi
-
nated, the voltage on HVDC is dumped through
this fuse. If the fuse opens up, then the only
means of discharging the HVDC is through the
bleed resistor R18.
6.4.3 HVPS Isolation Components
Digressing momentarily, this assembly has three
optically isolated triac drivers, one opto coupler,
and two isolation “sense” transformers. These
components isolate the High Voltage Output cir
-
cuit from the High Voltage Control circuit. The
System 7500™ has two intermediate circuits that
are both isolated from ground (chassis ground)
and both are also isolated from each other. When
using a voltmeter or oscilloscope on these cir
-
cuits, it is imperative that the ground reference be
connected to “-HV” of this circuit or the signal
ground of the control or low voltage circuit. The
signal grounds of the control circuit are not com
-
mon to the -HV ground of this circuit.
The triacs are fired when the cathode of the triac
drivers are pulled to signal ground. The diode of
these drivers is common with, and controlled by,
the HV Control circuit. A short pulse of current
through the diode of the triac driver switches on
the gate of the triac at a phase angle that is greater
than 80°. The triac will remain on until the AC
signal is at approximately zero volts.
The optocoupler (U5) is controlled by the HV
Control Circuit and it switches on Q8 when the
cathode of the diode is pulled low. When U5 is
energized, the emitter goes high which switches
on Q9; switches off Q10; allowing approximately
14V to switch on the gate of Q8. Pulling the
cathode of U5 back high switches off U5, allow
-
ing Q9 to be switched off and Q10 to pull the
charge from the gate of Q8 and switch it off.
Following each RF activation (system is unkeyed),
Q8 is switched on for approximately 100mS to
allow R33 to dissipate the energy stored in the
filter capacitors.
The two sense transformers (T1 & T2) transfer a
proportional ratio of the HVDC across the isola
-
tion barrier to the HV Control circuit. T1 pro
-
vides what is called the HV sense for control and
T2 provides the HV sense for monitoring. Both
circuits and transformers are the same and the
ratio of voltage transferred is the same. The pur
-
pose for the monitoring circuit is a means of veri
-
fying the controlling circuit is operating properly.
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