Honeywell bendis king KLN 94 Maintenance Manual page 24

BENDIX/KING
If too much current flows, R6037 will begin to turn on Q6011 which then cuts back gate drive to
Q6009-B, thereby limiting the current.
When the unit is turned off, the +5V power applied to the gate of Q6010 will be so low that Q6010
will be off. This causes R6036 and R6035 to set the gate-source voltage of Q6009-A at such a
small magnitude that R6028 and R6030 no longer provide current. Only R6029 provides current
which is just enough to provide dimly lit buttons. CR6011, C6013, and R6032 make up a timing
network to make sure on/off transitions are handled smoothly, eliminating flashes of light.
Resistor R6031 provides a load to make measurements easier during troubleshooting. Bypass
capacitors C6012 and C6052 reduce noise. R6020 is a current sense resistor that generates a
current sense voltage that is filtered by R6026 and C6009 and provided to U6001 for current-mode
regulation.
4.3.1.3.11 Display Backlighting Supply
The LEDs of the display assembly's backlighting must be supplied with a stable current source to
regulate brightness and heat generation. Q6022 is the pass transistor providing this current
source. Resistors R6088 and R6089 apply gate-source voltage to Q6022 to turn it on.
During the portion of the PWM cycle when the LEDs are conducting, current flows from L6006
through current limit resistors R6093, R6092, R6091, and R6090. This current then flows through
Q6022 and on to the front panel via J6005-6. If too much current flows, R6087 will begin to turn
on Q6027, which then cuts back gate drive to Q6022, thereby limiting the current.
Resistor R6069 provides a load to make measurements easier during troubleshooting. Bypass
capacitors C6043 and C6048 reduce noise.
4.3.1.3.12 +9V and -15V Voltage Regulators
Two regulators provide Zener diode controlled voltages. In the +9V regulator, the base voltage
(and therefore the emitter voltage) of Q6013 is limited by the voltage rating of CR6012. Resistor
R6042 provides base drive current as well as Zener current. Bypass capacitor C6044 reduces
high frequency noise. In the -15 regulator, the base voltage (and therefore the emitter voltage) of
Q6024 is limited by the voltage rating of CR6030. Resistor R6081 provides base drive current as
well as Zener current. Bypass capacitor C6049 reduces high frequency noise.
4.3.1.3.13 LCD Bias Supply and Timing Control
The supply for the display assembly's LCD bias voltage is derived from the voltage supplied by
rectifier CR6023. First, the voltage is regulated. Then, a timing control makes sure that the LCD
bias is applied to the display only when +5V is present. This prevents damage to the LCD display.
The voltage present on CR6023's cathode is applied to the nominal +20.5V regulator consisting
of Q6019, R6056, CR6022, and CR6021. This regulator is very similar to that used in the +9V
regulator described elsewhere.
Under normal conditions, the reset controller's output pin, U6004-1, applies base drive to Q6017,
causing it to be turned on fully. This causes resistors R6058 and R6059 to apply base drive to
pass transistor Q6020. In this condition, Q6020 is fully on, and the V_LCD_DRIVE signal on
Q6020's collector is essentially the same as the voltage present on regulator Q6019's output. By-
pass capacitors C6050 reduces noise.
During power up, the reset controller's output pin, U6004-1, stays low for a short period of time
after a valid +5V supply level is detected. While it is low, Q6017 doesn't conduct which causes
Q6020 to also be turned off. Resistor R6057 keeps the level of the LCD Bias Supply at 0V while
the +5V supply voltage rises to normal levels. When the time period is complete, the timing control
assumes normal operation as described above.
Rev 0, Sept/2000 15599M00.JA
KLN 94
Page 4-25