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The third sub-circuit monitors the MOSFET / lamp current and adjusts voltage to
maintain a constant operating current for consistent brightness. This last sub-circuit
also informs the microprocessor if a defective (unlit) lamp is not drawing current.
• High Voltage Output - The PWM output from the MOSFETs Drivers is boosted to
over 20KVac by a special high voltage transformer (T003). Notice that this
transformer normally tests open circuited between the lamp terminals. The external
spark-gap device (Z001) is connected to a tap in T003 to protect the transformer
from damage if the high voltage is excessive.
2. Inform the microprocessor that the lamp is on and
3. Internally monitor the lamp current:
Normally the HV rises until the lamp ignites and draws the normal range of lamp
current. Lamp current is monitored by the voltage drop across parallel ground resistors
R133, R134, and R136. This voltage is fed to the control circuit MC001 (MC002 is for
MOSFETs drive only). Once MC001 determines the lamp is within the normal current
range (lamp is operational), the ballast control circuit informs the microprocessor by
using PC002 to ground out the voltage from the microprocessor (input B2/pin 3). When
this pin is grounded, the microprocessor knows that the lamp is lit and changes the
ballast-on signal (into B2/pin 1) from a High to low going 4Vp-p pulses.
4. Shut off HV to the lamp if it does not ignite:
Normally at start up, the lamp HV voltage rises until lamp current is detected. However
if the lamp is not present or if it does not ignite, the HV climbs until the spark gap arcs,
drawing higher current than that of an operating lamp. Under these abnormal
conditions, the ballast board stops the HV ballast operation, despite the applied ballast-
on microprocessor command (B2/pin 1).
Within the ballast circuit, if excessive current drawn by the arcing Z001 spark gap is
detected (by control circuit MC001), it stops providing PWM and MOSFET drive,
stopping HV internally. Meanwhile, if the microprocessor issues a ballast-on (B2/pin 1 =
Figure 18 - Ballast Block Diagram
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