Date: 10/02/98 R762716
EHT Module
R762716
11
Technical description
Introduction
Technical Description EHT module R762716
On this board, the EHT drive pulses for the EHT power supply are generated. The
primary circuit for the EHT power supply receives its 300VDC supply from the
Mains. In the event of a failure, either because the EHT is too high, too much current
in the EHT circuit or a horizontal or vertical scan failure condition exists, the EHT
voltage is discontinued.
We will discuss the generation of the EHT pulses, its regulation, and, the different
protection circuits.
The EHT multivibrator is set up around two Schmidt Trigger NAND gates in IC2. The
frequency of this oscillator is typically around 100 kHz. Two time constants are
involved in this circuit : C14 / P3 + R22 and in the feedback loop C15 / (R21 + P4/
/(R12+R13)) voltage regulated.
The first time constant is invariable and determines the OFF time of the power switch
Q12 and is tuned with P3 to 6.2 µS. The second time constant is variable and
depends on the voltage level of IC4 (pin1.) By a change of the bias voltage on the
base of transistor Q2, this time constant can be regulated.
The buffer receives at its base the FBHV voltage (feedback voltage from the divider).
This is the EHT voltage divided by a factor 1000 on the splitter. The pin 5 of IC3 is
set at a reference of approxi10.2 volts by zener IC1. The duty cycle or the
on/off time of the power switcher Q12 is consequently regulated by the voltage
difference detected by the regulator circuit.
The squared waveform at pin 4 of the NAND gate is, via a fast switching and inverting
FET Q1, sent to the opto-coupler IC6. This opto-coupler is necessary because the
remainder of the circuit is supplied with the +17M and the +300M which are not
isolated from the Mains. The +17M voltage is obtained from a special winding on
the SMPS and the +300M is the rectified mains voltage (GNDM is mains or hot
ground).
Caution: Any servicing on a board that uses both a Mains Ground and a
Chassis ground should involve the use of an Isolation Transformer, espe-
cially when using an oscilloscope, or other equipment connected to the
main AC source. Do not connect the Main and Chassis Ground together at
any time.
As this board is supplied with the +300M as soon the Mains switch is pushed, it
is not recommended to remove this board any more even when the projector is in
a stand-by position. This can damage the contacts of the board.
The output of the opto-coupler IC6 drives the push-pull stage Q10/Q11. The pulses
are capacitively coupled with C32 to the gate of Q12. The 20 volts zener Z5 has two
purposes.
The negative level of the pulses is clamped at -0.6 volt, and on the other hand the
gate-source voltage is limited (protected) to 20 volt DC, in order to protect the
switcher Q12.
The drain (DHV) of the power switcher is connected with the primary winding of the
EHT transformer. Transformer and quadrupler are in the same unit.
The +300M enters the board and is passing a filter L1/C47 and a fuse before it
supplies the Transformer / Quadrupler unit.
DC controlled multivibrator
Summary of Contents for GRAPHICS 1209S
Page 4: ...BARCO PROJECTION SYSTEMS GRAPHICS 1209S 90 00972 230V AC 90 00977 120V AC SAFETY NOTICE...
Page 9: ...BARCO PROJECTION SYSTEMS GRAPHICS 1209S 90 00972 230V AC 90 00977 120V AC GENERAL INFORMATION...
Page 28: ...BARCO PROJECTION SYSTEMS GRAPHICS 1209S 90 00972 230V AC 90 00977 120V AC SERVICE SHEETS...
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