Additional Power Control

[ken.neiser@gmail.com]

I just implemented an ATXRasPi (5V buck converter supplied power to ATXRasPi) on my new RPi2, works great! I have one challenge though. For various reasons I have to implement a separate USB hub (powered by a separate 5V buck converter) which is connected to a laptop HDD. I want to be able to control turning the power off to the USB hub when the ATXRasPi is shutting down the RPi2. Do you have any thoughts on this?

[Felix]

Ken, thanks for your question.
Do you have any idea how much current that hub+HDD need, or would you be able to measure?

[ken.neiser@gmail.com]

At the moment no I don't. I know it's a D-link hub with a max current output on it's 5V adapter of 2A.

[Felix]

You might just be able to squeeze that through the ATXRaspi along with your Pi2.
The Pi2 is pretty efficient. So assuming it uses around 0.5A and another 2A for your hub, that might be possible through ATXRaspi, although it was not really designed to handle more than just a Pi so if you don't want to do this at your own risk then I will suggest something else:

- you can implement another circuit with a power mosfet to feed your hub/HDD. That will be activated by the output of the ATXRaspi. Since your HUB should only be powered ON as long as your Pi is ON, then your circuit can be driven by the 5V output of ATXRaspi (assume P mosfet driven through a NPN transistor).



Here the "12V" would be your 5V/2A permanent power. Then the gate of the transistor that drives the mosfet would come from the 5V output of ATXRaspi. When ATXRaspi cuts the 5V output off, then this circuit would turn off the LOAD (hub/hdd) as well. I think that should work.

[ken.neiser@gmail.com]

Thanx for the response!
A couple of questions:
1. Logic Input - where would this connect on the ATXRasPi?
2. GNDA - please elaborate.
3. 12V Supply - How would component values change if the supply was 19V?

[Felix]

1. Logic Input - where would this connect on the ATXRasPi?
    This connects to ATXRaspi's 5v output. As long as your Pi is ON this circuit will also be turned ON.

2. GNDA - please elaborate.
    This is the same as ATXRaspi's GND

3. 12V Supply - How would component values change if the supply was 19V?
    If your hub/hdd need 19V then 12V would become that. Just make sure whatever mosfet you choose will support switching 19V (actually give it a good margin above that).

When you have the parts and circuit built test it by turning on/off a small light bulb or something like that. Applying 5V to the "Logic Input" will turn the "Load" on (ie open the path from your 19V to it).

[ken.neiser@gmail.com]

The 19V supply is actually from an old power adapter that I have. The hub requires the 5V @ 2A max.. That is why I asked if there was much change in component values.

Thanx again!

[Felix]

Ok so then where and why would you use 19v?

[ken.neiser@gmail.com]

After some thinking, shopping, and tinkering I put together this, it was fairly easy to implement. I have attached a diagram.

The "+/- Contact" will be the ATXRaspi's 5v output connections.

Thanks for your help!

[Felix]

Nice work.
This could have also been implemented with a single power mosfet to allow a higher output power or even a higher voltage, without the need for relays and the resistors.