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G8MNY > TECH 26.05.15 10:03l 65 Lines 3040 Bytes #999 (0) @ WW
BID : 31564_GB7CIP
Read: GUEST
Subj: Laptop Fluorescent Tube Circuit
Path: IW8PGT<CX2SA<GB7CIP
Sent: 150526/0835Z @:GB7CIP.#32.GBR.EURO #:31564 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Update Nov 06)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
Unlike a simple single ended 12V fluorescent lamp system, that effectively only
puts DC pluses on the tube, that reduces the tube life time by half. This one
uses a much smaller cold cathode tube that needs very high voltage to start.
After reverse engineering the circuit for an old DEL laptop, I was surprised
just how complex it was..
SIMPLIFIED SCHEMATIC
ÚÄÄÄÄÄÄ¿ COLD CATHODE TUBE
³BRIDGE³ /³\ 80V - 1kV /³\
Tube Ú<Ä´& LOADÆÍ<ÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ)ÍÍÍÍÍÍ» ³
current³ ÀÄÄÄÄÄÄÙ À((((() ((((()Ù
feedback³ ===== =============
³ 12VÄÄ(((()ÄÂÄ´>ÃÄÄÂÄÄÄÄÄÄ¿ T
+5V ³ ÚÄÄÄÄÄ¿ ÚÄÄÄÄ¿ L ³ ³ (((() ÚÄÄÄÄ¿ ÚÄÄÄÄÄÄ¿ ÚÄÄÄÄ¿
Brill À>Ä´Pulse³ ³Push³ ³ C === ³ ³ ³Push³ ³ ö2 ³ ³RAMP³
Pot<ÄÄÄÄ>Ä´Width³ ³pull³ ÚÄÁÄ¿ ³ ÚÁÄÄÄÁ¿ ³Pull³ ³LOGIC Ã<´OSC ³
Á ÚÄ>Ä´Comp Ã>´buffÃÄ>´FET³ Á ³FETS Æ<µbuffÆ<µ40% ON³ ³ ³
ÚÄÄÄÄÄÄÁÄ¿ ÀÄÄÄÄÄÙ ÀÄÄÄÄÙ ÀÄÄÄÙ 12-80V ÀÄÄÄÄÄÙ ÀÄÄÄÄÙ ÀÄÄÄÄÄÄÙ ÀÄÄÄÄÙ
³RAMP OSC³ STEP UP CONTROL EFFICIENT INVERTER SYSTEM
ÀÄÄÄÄÄÄÄÄÙ
HOW IT WORKS
STEP UP
The left ramp osc is compared with both the brilliance pot & feedback to
produce a variable pulse width drive that feeds a complementary buffer before
driving a power FET (100V 5A). The FET puts extra current into the L choke &
when the FET turns off C is charged to a higher voltage (e.g. 80V to strike)
INVERTER
The tube inverter also has a ramp osc, & this is level compared to give 40%
on & 60% off & then divided to give 2 outputs of 40% on. These are both
buffered with complementary pairs to drive to higher voltage FETs (250V 2A).
These drive T the push pull step up ferrite transformer. T has a split high
voltage secondary, that connects to the tube, but the centre tap feeds a
bridge rectifier.
FEEDBACK
The bridge rectifier feeds a small capacitor & load R. The voltage across
this is proportional to the tube current & is used to control the overall
brilliance.
On power up there is no tube current so the step up runs flat out to produce
high voltage for the inverter to run on. With this the inverter produces
something like 1kV balanced AC to the tube. On striking, tube current flows &
the tube voltage drops to about 80V. The step up circuit now controls the
supply voltage to the inverter to maintain the current (brilliance) chosen.
OTHER BITS
Not shown here is the fuse, thermal trip link, & there are loads of protection
diodes across the FETs etc. Also the whole circuit is all SMD about 2cm x 4cm
double sided components & the diodes are in the same packages as the SMD
transistors making testing horrid! The origan fault was short circuit FETs, non
of the tiny SMD components was faulty. :-)
Why don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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