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Modifying a HDMI to VGA adapter cable for Raspberry Pi
By admin | January 12, 2015
I recently acquired a Raspberry Pi B+. It sports a HDMI video out port, which works fine when connected to my TV set. However, I wanted to connect it to a dust-collecting LCD VGA monitor. Therefore an HDMI to VGA cable adapter was ordered via Internet, arriving the next day. This is an active adapter, powered via the HDMI cable. Power is drawn from the HDMI port of the Rapberry Pi, that is not able to supply more than 200 mA, and even that with pain.
The HDMI organisation specifies no more than 50 mA should be drawn from the source. When measured, the adapter drew about 310 mA, way to much! Initial testing showed the adapter worked, although RiscOS halted halfway the startup. Scary tales about smoking Raspberries made me set out to modify the adapter to be self-powered. The information I needed was found on the Internet. Pin 18 of the HDMI connector is the +5V supply. A description of a modification of another adapter (one with a seperate audio out) was found here.
Opening the housing of the adapter was easy: just pry with a plastic prying tool or a small not to sharp screwdriver in the tiny crack between the housing and the front cover on the VGA connector side. With just a little bit of force, the front comes off.
The PCB can then be pushed out by pushing the cable out. The cable is soldered to the PCB board on both sides. The soldering is covered with hot glue. The pads to which the cable is soldered have numbers next to it. Measuring the resistance between pad 18 and connector pin 18 proved this was the +5V supply. The purple wire connected to the pad was cut, the part going to the solder pad removed and the cable part isolated with small shrink tube. Next I drilled a hole next to the cable entry in the housing, inserted a small rubber grommet as cable protector, and fed a USB cable through the hole.
The USB cable was chosen so it could be plugged in to the powered USB hub I needed for the Rapberry Pi anyway, but any power cable would do. The red wire of the USB cable needs to go to the +5V on the PCB, either to pad 18 or as I did to the plus side of C6. Make sure all the glue is removed from the pad before soldering! Also, cover the end of the USB cable with shrink tube to insulate the green and white data wires and the earth screen, and to act as a cable retraction safeguard. Better do this before soldering the red and black wire on, or you have to redo it as I had to.
The black wire is soldered to the ground plane near the edge of the PCB. A cutout for the grommet has to be made in the rubber gasket that seats the PCB. The PCB is then put back in the housing, gently tugging the cables and pushing the PCB. Once seated, the front cover can be replaced and the adapter tested. Plugging the USB cable in to the powered hub (not directly in to the Raspberry Pi), it now worked perfect; RiscOS started up without problems. It does get warm to the touch, though, about 35 – 40 degrees Celsius.
Good luck with your mod!
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