Prototyping question

Thread Starter


Joined Apr 19, 2008
Hello. I am thinking about prototyping using the +5V from the USB red/black power leads. I'm just wondering whether this source needs regulation or not?


Joined Mar 17, 2011
The USB's own 5volts DC supply should work fine without any additional regulation. I am doing precisely the same thing as I cobble together a USB-powered throttle quadrant for my desk-top flight sim flailings. The USB 5 volt supply evidently does have a 500 mA current limitation, which bears keeping in mind as your design progresses.


Joined Jul 17, 2007
The 500mA limit is for tower/desktop computers; it's lower for laptops, so keep that in mind.

You might consider using a "Y" Molex adapter from the main supply instead. The Y adapters are cheap, and you can use the main supply +5v (red to black) and +12v (yellow to black) instead of loading your USB port.


Joined Mar 17, 2011
Forgot to mention that many cell-phone chargers are rated for 5 volts DC, with varying current levels, making them convenient to use as a substitute, or in parallel with the USB supply.


Joined Feb 1, 2010
Depending on what version of USB your host (I assume it's a PC?) is, technically you're only supposed to draw something like 100mA and "negotiate" with the host to ask for the 500mA, or 900mA for USB 3.0

Here's some related quotes from the wikipedia article:

prior to the Battery Charging Specification, the USB specification required that devices connect in a low-power mode (100 mA maximum) and communicate their current requirements to the host, which would then permit the device to switch into high-power mode.

A unit load is defined as 100 mA in USB 2.0, and 150 mA in USB 3.0. A device may draw a maximum of 5 unit loads (500 mA) from a port in USB 2.0; 6 (900 mA) in USB 3.0. There are two types of devices: low-power and high-power. A low-power device draws at most 1 unit load, with minimum operating voltage of 4.4 V in USB 2.0, and 4 V in USB 3.0. A high-power device draws the maximum number of unit loads permitted by the standard. Every device functions initially as low-power but the device may request high-power and will get it if the power is available on the providing bus.[35]

In my experience I haven't found any PC hosts that actually have required the negotiation to draw the 500mA but if you run into a current or collapsing voltage issue this is probably why.
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Joined Feb 11, 2008
And the regulation of USB 5v is quite poor, you can expect it to vary from 4.7v to 5.2v at any time depending on your load and what your PC is doing.