12v mod to LCD monitor. - Page 1
Forum section: electronics / beginners

author: Nigelmouse
author_rank: Newbie
timestamp: February 25, 2023, 09:17:42 pm
content: Hi,I'm new here so I hope I follow the etiquette correctly.I've got a Hanns.G LCD monitor that takes UK mains power and I would like to modify it to allow it to be used from a 12v supply in my shed. My shed has solar power and a LiFePo4 battery so it actually gets 10-14v (it's rarely under 12v though).The monitor I have is the Hanns.G HW191D and my first step was to look for a service manual and/or schematic for it. I didn't find them but I did find the service manuals for very similar monitors: Hanns.G HG191D and HannStar HW191A.https://www.manualslib.com/manual/1087178/Hanns-G-Hsd-Hg191d.html?page=21#manualhttps://www.manualslib.com/manual/1309432/Hannstar-Hw191a.html?page=24#manualBoth of these manuals have a flow chart for the monitors power board that shows the mains comes in and is converted to 5v and 12v DC. The 5v appears to head to the logic board and the 12v heads off to power the back light (I suspect the 12v might be stepped up again). I suspect these monitors power boards will be very similar to the one in mine, though from the connectors they don't appear to be identical.My thinking is that if I can find the points on the power supply board where the mains has been converted to 5 and 12v I could use a DC-DC power supply from my shed's 12v battery to provide the 5v, and the 12v for the backlight could be run directly from the shed battery if it tolerates up to 14v. If not, another DC-DC power board could be used to provide 12v.I have got as far as disassembling the monitor and removing the power board. The power board connects to a separate board that appears to have the microcontroller and low voltage logic, and the power board also has connections directly to the monitor backlights. When mains power is connected the board always provides 5v to the microcontroller logic board. When you turn the monitor 'on' from standby the microcontroller board sends a signal back to the power board to tell it to turn on the backlights. I've powered up the power supply board on it's own and I've found a number of pins on the connector to the microcontroller board carry 5.7v so I'm pretty confident I can inject the power needed for that at that point.What I'm not sure about is where the power to inject the 12v for the backlight circuitry. I've done some probing while the power board is in standby and I've not found a steady 12v.Hopefully this modification is possible and I just need to find the point on the power board to connect 12v to. Any tips on how to find a point to attach 12v power is much appreciated.

author: mariush
author_rank: Super Contributor
timestamp: February 26, 2023, 04:31:04 am
content: The connector going to the smaller board with the display processor will typically have 5v , ground, backlight enable , backlight brightness, sometimes 12v (for audio amplifier or headphones amplifier) and sometimes 3.3vIn your case, the backlight enable and brightness pins are most likely those two traces that go all the way on the left and bottom edges of the board, then the trace going on the bottom edge also connected to the mounting holes tells you it's a ground pin, then you have the thicker trace that's probably 5v and maybe other voltages (you can use a multimeter to measure the voltage across those electrolytic capacitors in the bottom right corner in third picture while the psu runs - but be careful and have the board on some insulated material, paper, cardboard, plastic, glass, wood desk, or mounted inside the actual monitor case. Use a marker or something to mark the pins of the capacitor, then flip the board and power it and measure the voltage across the capacitors careful not to touch anything on high voltage side.The power supply board has the high voltage backlight driver for the 4 CCFL tubes and typically the voltage going to that circuit is around 20v - if you want to you can measure it by checking voltage across those two capacitors on the bottom right corner of the high voltage transformer (to the right of the text saying 49.26)The best part is that you can buy universal backlight drivers which typically work with 8v...30-ish volts so you can replace that whole half of original power supply with one.Here's an example :https://www.ebay.com/itm/144764252017The pinout of the connector is printed on the back but it's only 4 wires .. ground, voltage (10v-29v) , enable and brightness adjustment (so you can connect the last two directly to the display board)Another design :https://www.ebay.com/itm/403013616579orhttps://www.ebay.com/itm/173692176465This one has more wires but pinout is in description : 2 input voltage wires (10-25v), enable, brightness adjust, 2 ground wiresYou will have to be careful how you mount these inverter boards so that the bottom is shielded and not touching metal case because it's high voltage in that area (basically i'd put some plexiglass - a bit of plastic cut from a cd case would work - or something under the pcb)Often the pinout of the connector going to the display board (the one with vga/hdmi etc inputs) is printed on the bottom of that board or somewhere on that board.BUT, should be easy to figure out even without info ... use a multimeter in continuity mode to find the ground, it will probably be continuity with the exterior of video connectors but even easier there's often a small electrolytic capacitor near that connector so you have the negative of that capacitor there. Most display boards have a linear regulator converting 5v from power supply to 3.3v or some other voltage - you can determine the regulator's pinout and then you know the input is 5v so you can follow the trace to the connector.The backlight enable/disable you can determine simply measuring voltage on pin when you try to turn monitor on and off - that pin will switch from 0v to 3.3v or higher. Brightness is a pwm signal..So ... the display board usually consumes very little power .. it should be possible to use a linear regulator to convert your unregulated 12v to 5v, just add a tiny heatsink on the regulator and it should be fine... but if it seems to hot during use, you could replace with a 5v dc-dc converter.Most display boards that also have pins for 12v or higher voltage will work without any voltage on that, especially if there's no audio amplifier on the board. They make models with and without audio but keep pinout the same.