# USB current limiting

#### DJ_AA

Joined Aug 6, 2021
267
Hi

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?

#### Hymie

Joined Mar 30, 2018
1,273
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.

#### DJ_AA

Joined Aug 6, 2021
267
Great, its simple circuit, instead of some type of electronics controller.

#### nsaspook

Joined Aug 27, 2009
12,841
Current limiting will also be in the 12-24V DC to 5V DC converter for USB power. Most have short-circuit/over-current protection.
Select the needed amperage for the application.
500mA example: https://www.pololu.com/product-info-merged/2843

#### paulskirk53

Joined Sep 8, 2021
45
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....
Paul

#### crutschow

Joined Mar 14, 2008
34,063
Below is a relatively simple two-transistor, two-resistor current-limit circuit, if you want to consider an electronic solution:

#### paulskirk53

Joined Sep 8, 2021
45
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,
Paul

#### crutschow

Joined Mar 14, 2008
34,063
So R1 = 0.6 / 0.1 = 6 Ohms?
Correct, if the load is less than 100mA (say 50mA for margin).
(Note that R1 does cause a small voltage drop to the load of R1*Iload.)
I'm not sure how to work out R=21 - time.
That was just to simulate a varying load.
It's not something done with a real circuit.

#### paulskirk53

Joined Sep 8, 2021
45
Great, thanks, I'll give it a try.

#### paulskirk53

Joined Sep 8, 2021
45
Great, thanks, I'll give it a try.
Just trying to source the IRF7406, but my searches show it as a surface mout device as below:

IRF7406PBF P-Channel 30 V 5.8A (Ta) 2.5W (Ta) Surface Mount 8-SO

Can I use an alternative P-Channel MOSFET with a similar spec?

thanks.

#### crutschow

Joined Mar 14, 2008
34,063
Can I use an alternative P-Channel MOSFET with a similar spec?
Yes.
It doesn't need similar specs.
For the low voltage and current you have, just about any P-MOSFET with an on-resistance of <1Ω should work.