ConMed ExcaliburPLUS PC - Page 36

respectively. See Section 4.2.5.1 for instructions
on changing the factory supplied strap option.
POWER TRANSFORMER. Power transformer
A6T1 is a ferroresonant type. Also known as con-
stant voltage transformers, these devices provide
regulation against variations in mains voltage.
This feature is primarily responsible for the
Excalibur Plus PC™’s ability to hold RF output
power steady in the presence of mains voltage
variations over the specified range.
Ferroresonant transformers have some unusual
physical and electrical properties:
1. A special secondary winding (red leads 5 and 6
on A6T1) connects to an ac capacitor, A6C1. If
this capacitor is open or shorted, the secondary
voltage will fall well below normal.
>WARNING<
HIGH AC VOLTAGE IS PRESENT AT
A6C1 WHEN THE UNIT IS OPERATING.
USE CARE TO AVOID ACCIDENTAL
CONTACT OR NICKING OF
THE WIRE INSULATION WHEN
WORKING IN THIS AREA.
2. The secondary voltage waveforms are sguare
rather than sinusoidal in shape. Most ac volt-
meters are calibrated only for sine waves, so the
secondary voltage readings on A6T1 may vary
widely between different instruments. The recti-
fied de output voltage is the most repeatable indi-
cation of secondary ac voltage.
3. A secondary overload of greater than 200%
will cause the transformer to go into current limit
with very low secondary voltage and less than full
load mains current. Before diagnosing a defective
transformer, first eliminate the possibility of a sec-
ondary fault, such as a shorted rectifier or filter
capacitor. The low power secondary (leads 9 and
11) is provided with internal thermal fuses, since
it is possible to overload the winding so that the
insulation rating would be exceeded without
going into current limit.
4. The magnetic core saturates briefly every half-
cycle. This high flux density produces higher
than normal core losses, so the core runs fairly
hot to the touch even at idle. The device is
mounted using rubber grommets because high
core flux produces more audible hum than ordi-
nary transformers. When the core saturates, a
burst of magnetic flux can induce mains-frequen-
cy noise into nearby wiring. For this reason, do
not alter the harness dress near A9TI.
5. Conventional ferroresonant transformers, used
in all Excalibur Plus PC™’s specified for 50 or 60
Hz only, are sensitive to mains frequency varia-
tions. Units specified for use as 50-60 Hz
employ a special Controlled Ferroresonant
Transformer which electronically regulates de out-
put voltage. The latter are equipped with a
Control Module, which samples +16 VAC via
A4P4, and an Overvoltage Protection module,
which will drive the transformer to a minimum
output condition if +16 VAC or +110 VAC at
A6BRI or BR2 exceed preset voltage limits.
Both modules are contained in a potted housing
mounted atop the power transformer.
CAUTION: Never operate the unit at a nomi-
nal mains frequency other than that specified
on the Nameplate and A9T1.
RECTIFICATION. The secondaries of A9T1 are
terminated in terminal block, AGTB1 (the termi-
nal numbering on this block matches the lead
numbering on A9T1). Harness A6W2 connects
to the bridge rectifiers A6BR1 and A6BR2 and
carries the rectified power to the A4 Power
Conversion PWB via A4J3.
A6BR2 acts as a conventional full-wave bridge
rectifier for supplying +16 V, while simultaneous-
ly acting as a negative-leg full-wave center-tap rec-
tifier in providing +8 Vdc.
A4 POWER PROCESSING AND USAGE.
Refer to the A4 Power Conversion PWB
Schematic 4.5. Capacitors C11 and C20 filter the
+110 Vdc supply delivered at A4J3, while R12
acts as both a minimum load and a bleeder to dis-
charge the capacitors when power is turned off.
The negative leg of A6BR1 is connected to
ground via a 0.1 ohm current sense resistor,
A4R8. The signal -ISENSE is a negative dc sig-
nal used on the A3 Controller PWB to monitor
the supply current drawn by the Power Amplifier.