HOWTO: Power a USB Hub for the G4
Depending on the number and type of devices you want to connect to the G4 you may need to use a USB and hub and if one or more of the devices consume a fair bit of power (i.e. external sound card, external hard drive), you may need to provide a power to the USB hub for it to work properly. If you don't power your USB hub, your devices may act strange when connected to it. They may not work at all, or they may appear to work, but cause errors or have erratic behavior.
Powering a hub in the car is actually a trivial problem with a non-trivial answer. Unfortunatly, you can not simply hook up a the unregulated 12 volts readily available in the car directly to the hub. USB hubs require regulated 5 volts and what is more, they should be powered whenever the computer is on. This is actually different from when the car is on. Since the computer must go through a shutdown sequence when it goes to hibernate or standby and this sequence is initiated by turning the key off in the car and doesn't end until a number of seconds after the car has lost power, you can not simply provide power when the accessory power of the car is on. If you do, your hub will lose power before the computer shuts down and windows will detect the forceful removal of the devices and it will wreak absolute havok on the device drivers. This will result in bad things happening the next time the computer is started.
Simply put, the solution is to provide power to the USB hub always and only when power on the USB bus from the computer is on. This can be implemented with a 5volt SPST relay that is triggered by the USB bus and turns on a 5 volt Point of Load (POL) power supply that in turn provides power to the USB hub. This may sound complicated, but really its pretty easy to do, you just have to make sure you connect the wires to the right places.
- Wire Strippers
- Soldering Iron and Solder
- Heat Shrink Tubing
- Wire 18 gauge or higher
Before you begin, you should first familiarise yourself with the Carnetix POL power supply manual, particularly section 2.2.2 on use as a Stand-Alone POL regulator.
First we shall make the trigger for the 5 volt relay. To do this, cut the USB extention cable completely in half at the center. You will notice there are 4 wires inside and the wire is surrounded by shielding wire. Strip both ends of the cut cable about 2 inches or so being carful to leave the metal shielding wire intact. Then strip the ends each of the 4 small individual wires on each wire.
Now get 2 6 inch lengths of wire and strip both ends of them. Twist one wire and the two red ends of the usb cable back together and the other wire and the two black ends (negative) of the usb cable back together. Twist the grend ends together then the white ends back together of the usb cable. These are the data lines and are not used. They were just cut to make striping the wire easier. Also twist the wire shielding from both ends of the cable back together. Use solder on all 5 wire twists, place and heat the heat shrink tubing over all of them.
The relay should have 2 poles on one side and 3 on the other, place the heat shrink tubing over solder the two loose wires coming from the USB cable you just made.
On the side of the relay with the 3 poles, you should connect the cars unregulated 12 volts to the input pole, usually the stand alone one and the other to the pole that is active only when the 5 volt trigger voltage is applied should be soldered to the green wire from the POL power supply.
Finally hook up the rest of the wires on the point of load power supply. Providing the cars unregulated 12 volts and ground where indicated in the manual and hook the red and black output wires to the positive pole and negative outside of the USB hub.