USB current limiting

Thread Starter


Joined Aug 6, 2021

I am working on circuit that will be powered using 12-24V for various FANS . I would also like to introduce 5VDC modules for the internal electronics but also to provide a USB charge port. What would be a good method to introduce a current limit on the USB ports to insure there is no shorts or high draw that could effect the remaining part of the circuit?


Joined Mar 30, 2018
Most USB outputs are current limited using PTCs or polyfuses. The advantage is that they are self-resetting (once they cool down) such that an overload does not require equipment repair.

You just need to decide how much current your USB ports are to be capable of supply and select an appropriate value PTC.


Joined Sep 8, 2021
On an intel NUC 6cayh, I just destroyed all the USB ports. There is NO effetive current limiting. I don't know if Intel uses polyfuses on this NUC, but if they do, they didn't work. So I am now exploring myself about how to limit the current drawn by a USB connection. My NUC is now an ornament....


Joined Mar 14, 2008
Below is a relatively simple two-transistor, two-resistor current-limit circuit, if you want to consider an electronic solution:



Joined Sep 8, 2021
Thanks for this, so if I have it right, I use I_limit ~ 0.6/ R1 to work out the value of R1 required in order to limit the current to say 100mA Is that right? (So R1 = 0.6 / 0.1 = 6 Ohms?)

The load in my case is going to be an FTDI board (with usb connector), so I'm not sure how to work out R=21 - time.

thanks very much for your help,