6-20
value can be locked into memory and the Return
Fault condition will be cleared.
6.11.3 RF Leakage Monitor
The System 7500™ actively monitors alternate
path RF leakage currents caused by stray parasitic
capacitance and limits the available open circuit
voltage in response to prevailing leakage condi
-
tions. This circuit is primarily active in Spray
Coag mode, but is available in all modes of opera
-
tion except ABC™.
The balun transformer description was covered in
the RF Output Assembly discussion, where it was
stated that the magnetic field is equal and oppos
-
ing if the source and returning RF currents are
equal. If the source and returning currents are not
equal, alternate leakage paths exist and the balun
transformer field becomes unbalanced, resulting in
restriction of the available output current due to
the inductive impedance of the transformer. The
balun transformer has a single turn winding that
has currents induced within it when a magnetic
field of the balun is unequal due to RF leakage.
The RF Leakage sense winding connects to J16
on the HS/A.R.M. assembly. The signal at J16
is an AC voltage that is converted to a DC volt
-
age at U2, which is an RMS-DC converter. The
true RMS voltage is compared with a reference
DC voltage representative of maximum allow
-
able alternate path leakage current at U1B. If
the leakage exceeds the limit value as set by R15,
J1-5 slews positive. The positive signal at J1-5
is supplied to the HV/FLOW Control Assembly
to limit the High Voltage Power Supply output
(HVDC). Limiting or reducing the High Voltage
Output results in reduced RF Leakage.
6.11.4 Transformer Select Relays
The ABC™ and Spray Coag transformers, along
with the associated tuning capacitors, are coupled
to the Single Ended Amplifier (SE Amplifier) by
relays K1 and K2 on the HS/A.R.M. assembly.
These relays switch into the circuit the primary
of the appropriate transformer, depending on the
mode requested.
The default or normally closed position of the
relays selects the ABC™ transformer. The relay
driver is located on the Power Control Assembly.
When Spray Coag is activated, J2-5 is pulled low,
allowing current to flow through the relay coils to
energize them.
6.11.5 Arc Sense Transformer
The Arc Sense Transformer, connected with the
-TANK terminal in ABC™ mode, sends a propor
-
tional sample of tank ring-down voltage to the
Power Control assembly. This sample signal is
used to determine if the ABC™ amplifier is loaded
(a heavily damped output signal) or open circuit
(lightly damped output signal). With a lightly
damped output signal, a peak detector on the
Power Control assembly provides a switching sig
-
nal for each peak that exceeds a predetermined ref
-
erence. The circuit monitoring for loaded or open
circuit outputs will assume an open circuit if four
peaks within each drive cycle exceeds the limits.
The output signal voltage and duty cycle is depen
-
dent on whether the output is loaded or open.
The transformer is switched into the circuit by
the relays K1 and K2, and only for ABC™ mode.
When Spray is activated, the relays remove the
Arc Sense Transformer from the circuit.
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