Powering arduino and circuit in a parallel configuration from USB


I have a few IR LEDS, a 3.3V Arduino Pro Mini with a few sensors and a FTDI adapter. As I need 100 mA just for my LEDS which remain lit constantly, I want to feed them not trough my Arduino but directly from the USB plug.

So I plan to solder a split USB cable with an A plug on one side, a mini-B plug in parallel with the + and gnd on the other side.

However do I need to protect the USB connection in any way ?

Thank you in advance.

Always use a POWERED hub for this type of experiment. DO NOT connect directly to your laptop or other computer!

So there's no safe solution without a USB hub ?

Hub's are cheap, what's the issue?

It's for a live music setup, so the less gear I have to carry the better.

USB port is good for 500mA, I'd think you'd be okay. Can put a resettable fuse in series like the arduino has coming in from the USB connector.

Resettable fuses are said to be slow. Maybe I could use a specific IC. Any suggestion ?

Fast blow fuse? Check maxim-ic.com, maybe power management ICs that can limit total current flow. They have chips that limit current flow into LiPo batteries to 100/500mA, similar for total current flow should be possible too.

Thanks CrossRoads.

In between I have found this page with a schematics to build a current limiter with a few transistors. It could be a cheap solution.


What do you think about it ?

That's the idea - for that particular circuit: The output voltage is 2V roughly lower than the voltage of supply.

Find one that is MOSFET based so there is less voltage loss. USB out is 5V +/- 5%, so on the low side its 4.75V already. And its rare to see it up on the 5.25V side.

Your original proposal was unnecessarily involved - the output from the FTDI adapter is the 5V directly from the USB connector and as best I can see, it is un-fused. (How do I know? Well, the Polyfuse would have to be the same size as the one in the Uno and there simply is not enough space on the FTDI adapter.)

You just take power from the same connection that feeds the Pro Mini. If you need overload protection (and once the system is fully developed and built, you have to ask: why would you need such protection?), you use a Polyfuse device for exactly the same reason it is used in the Uno - because it performs the task correctly and simply.

Resettable fuses of this sort are indeed slow. They are thermal devices. That is so that they do not actuate on overloads which are too short to do any damage. The USB port from which this is all operated has current limiting (though this may not be so evident if you use a power supply instead of a computer) and will not be damaged by overloads long enough to actuate the Polyfuse.

With the circuit's part besides the Arduino consuming something like 100 mA, it seems to operate without any issue. Also I was convinced by Paul__B's post and removed the polyfuse.