USB to 3V help

[ptrout]

Hello

First post, and thank in advacne for any help.

I have a clock circuit, that runs on 12V and uses 3V battery backup.

Power consumption:
3.36W (280mA@12V) from wall plug power supply (all digits to display)
2.3mW (0.75mA@3V) from backup battery ( 3V CR2032).

I want to rewire this to run off my computer power supply.

I am going to wire the main directly to a 12V rail via molex connector to power plug.

The battery is only good for 2-3 days according to specs to keep accurate time. I want to replace the battery CR2032 with constant 3V. I know the I can tap the USB header on the motherboard for 5V from the 5VSB. Look for step down circuit or resources to make this happen, any links or websites with needed parts would be appreciated. This can work I hope.

Thank you

 

[mneary]

The 3V shouldn't have a drain on it while the 12V is available.The battery would be just fine. When the power goes away, the USB disappears at the same time as the 12V.

 

[ptrout]

On my particular motherboard, the +5V standby is always on to supply power to USB devices such as mouse and keyboard to enable "power on" from these devices. So I know even when the computer is off, and 12V rail off, there is still +5V on the USB. The problem being I needed to step down the 5V to 3 or something near it.

I found these two products, I hoping they will do the trick

SparkFun Electronics - Breadboard Power Supply USB - 5V/3.3V

or

SparkFun Electronics - Logic Level Converter

Thanks for the help

appreciated