5V wallwart to 4 USBs and 3Pin fans

I resolved the problem, but it took re-manufacturing the board. The problem seemed to be with the routing and layer stack-up. The USB and Ethernet traces did not have correct impedance due to two reasons.

Firstly, the track geometry was not correct for 90 ohm (USB) and 100 ohm (ETHERNET) differential impedance. If your layout software does not support differential routing (eg: Altium does), then you will need to use something like SaturnPCB to calculate the track geometry and manually route the traces accordingly (I had to do this).

Secondly, and most importantly, the layer stackup was not correct to support USB and Ethernet high speed traces.The stack-up was TOP-INNER SIG-GND-BOTTOM and the high speed traces where routed on top layer. USB and ETH traces require a solid ground plane underneath and since this was not the case on my board it really messed up the signal integrity. Putting a solid ground plane (hence changing the stack-up to TOP-GND-INNER SIGNAL-BOTTOM) fixed the signal integrity issues.

I also cleaned up the placement of decoupling caps around the LAN9512 IC.

After these changes, the new board worked without problems. If you're using the raspberryPi to clock the LAN9512 (as B+ with LAN9514 does), then you also need to configure the Pi to output a clock signal. You can find info on this here: http://www.elektricks.net/raspberry-pi-compute-module-general-purpose-clock/ If you have a separate oscillator for the LAN9512 then ignore this paragraph.

Hope that helps!

I take it that you've used the multiplier factor of 3 over minimum hFE to ensure saturation. You seem to be missing the Vbe voltage, and to have slipped a decimal place, but the approach is valid.

The base resistor would be:

R = \$\frac{(V_{GPIO} - V_{BE})}{3\cdot 2.8mA}\$

Assuming the output voltage is 3V with that load, and Vbe is 0.7V

R~= 270\$\Omega\$

The Rpi has programmable output current capability of up to 16mA but I've not easily found the specification of minimum output voltage at 8mA source current. Assuming it's in the 3V range, your circuit should work fine as is, with a 270 ohm base resistor.

It might be a good idea to put a reverse-biased diode across the fan to deal with any inductance the motor windings have.

Edit: Here is a typical fan controller chip (inside your 12V fan) output section:

When your transistor turns off the energy stored in the inductance of the motor windings causes the voltage on the outputs to rise until the zener diodes conduct turning the transistors partially on, causing the chip to heat and wasting power. If you put a diode across the fan then more of the current will be used in the coils providing torque for the fan motor and resulting in less stress on the controller chip.