How Do I Bypass BA9741F (Chip On CCFL-Driver)? - Page 1
Forum section: electronics / projects

author: Undweeber
author_rank: Regular Contributor
timestamp: November 27, 2015, 01:29:21 am
content: I found a PC monitor that i took apart, i wanted to turn it into a wall lamp after removing the LCD, but it was too dim so i took off all lens filters ended up with 4 CCFL tubes in pairs of two, i figured out the positive negative to the board right away, problem was when i connected 12 V nothing happened, so i started shorting the BA9741F pins and randomly when i shorted pins 9 and 10 which are OUT2 and VCC, the lamps fired up, but when they warmed up, one set of 2 lamps was brighter than the other one, when i touched the metallic casing of the lamp i felt buzzing and when i touched the metallic casing of two lamps - there was a tiny sparking arc. it stopped the annoying buzzing the lamps were producing, but half of them were still dim.My questions:1. What pins should i short to bypass this pesky chip,do I need to short a third one to the two I shorted? or should I short totally different ones?I need to know for sure what to short before I use my $1.50 soldering iron that falls out of the power socket and which has a wobbly nose, and takes 15 minutes to heat up.and here is the chips assigned pins

author: Tim F
author_rank: Regular Contributor
timestamp: November 27, 2015, 05:03:45 am
content: Why do you think you need to bypass it? It is a DC to DC converter which is providing the high voltage output needed to power the CCFLs. By shorting the 12V to the output of the DC-DC IC you may have damaged something.Given that there are more than 2 wires going to the board, it is likely that some of the other pins are enable pins or control the brightness, so you need to drive those with appropriate signals for the DC-DC converter to work.

author: AustinEngy
author_rank: Regular Contributor
timestamp: November 27, 2015, 05:10:17 am
content: I mean, I don't know a whole lot, but it looks like to me if you wanted to bypass this chip for whatever reason, you would determine the input to the chip and short it to the output. It looks like maybe the inverting inputs to the op amps controls the state of the output (also dependent on the clock cycle). So if you were to short that it would skip over the chip. But I don't know about whether or not shorting this thing would be a good idea or not.

author: Tim F
author_rank: Regular Contributor
timestamp: November 27, 2015, 06:09:40 am
content: The IC produces a pulse width modulated output which drives the transformers and produces the high voltage AC needed to run the CCFLs. If the output is just shorted to a DC supply rail nothing will happen, some components might even get hot and fail. The operation of the DC-DC IC is critical for the circuit to work at all.I think what the OP has managed to do by randomly shorting pins together is strike the CCFL lamps to start them. There would surely be an inbuilt mechanism to do this, perhaps and enable pin that needs to be pulled high or low to strike the lamps.

author: DanielS
author_rank: Frequent Contributor
timestamp: November 27, 2015, 02:00:37 pm
content: If the lamps were working fine before you took the LCD apart and started poking around the CCFL driver, you may have busted a shunt resistor or clamping diode in the open-lamp detection or current feedback circuit.

author: Undweeber
author_rank: Regular Contributor
timestamp: December 01, 2015, 10:45:18 pm
content: back, sorry been away, i switched the lamps places and same exact story, so my suspicion is the lamps are fine and the driver is either busted, OR i am not bypassing that chip correctly, i might have gotten lucky by shorting those two legs, but its not completing the story, but merely giving me "the end"ill refer to the schematic posted and see what else i can short to make them work better and come back with resultsPS. thing says warning high voltage, can 12V power adapter really zap me? it has a half amp and one amp mode inside, but its chinese so you can pretty much half that, if the board upps the voltage to 200-300v to drive the lamps, with super low current, will it be any worse than piezoelectric lighter shocker that has like 10kV? or will it be more like a tazer?pps. the monitor was never on i found it outside and took it apart, dave taught me not to turn it on but to take it apart, then turn it on