General > General Technical Chat

What happens when you draw too much current from a usb port?

<< < (5/5)

janoc:
I have seen USB port controllers or even entire motherboards fried by overcurrent/short on USB ports, so don't assume they are protected against it or that you can blindly draw more than 500mA. Even that is often too much on older devices, esp. anything powered from batteries (older laptops).

Also assuming that all manufacturers of every gadget or gizmo having a USB port actually follow the standard and sane design practices instead of pinching pennies on overcurrent protection is foolish ... 

E.g. if the only protection is a polyfuse then you are at risk - that is a rather slow device and a large current spike may kill something on the board before the fuse has time to react and disable the power.

And powering anything like a motor, servo or solenoid from a USB port of a laptop is asking for a very expensive repair. The issue isn't only the current but also the high voltage spikes due to the back-EMF from these devices - and USB ports are rarely protected against them. A lot of people have fried ports or entire laptops like this. Just don't do this.

If you really must deal with a project like this, use at least a disposable USB hub between your laptop and the project, so when something gets fried, it will be likely the easy to replace hub and not your computer. Even better would be to use a cheap USB isolator and power the load from an external supply instead.

chrisb741:
I tested with a resistive load. I can't find any 100mA limit.
I'm not testing to failure or destruction

janoc:

--- Quote from: chrisb741 on December 26, 2023, 01:25:23 am ---I tested with a resistive load. I can't find any 100mA limit.
I'm not testing to failure or destruction

--- End quote ---

100mA is not a hard limit but a specification. A bus powered device on an USB A port is not allowed to draw more than 100mA before enumeration and asking for more via the descriptor (USB C has the PD spec and negotiation but that's a different system). Most devices will tolerate drawing up to 500mA without negotiation but it does violate the USB spec. E.g. if you will try to get the USB certification because you want to carry the official logo or apply for a VID, then you will fail it if the device behaves like this.

That you didn't hit that limit on your device or nothing happened (port switching off, etc.) doesn't mean that it is OK to do this or that it won't happen on another hardware.

Usually on a old style USB 1.x ports the limit is around 500mA - if you exceed it the port will most often cut out and your OS will report an error, often indicating that a device has malfunctioned or that there was an overcurrent event. Desktop computers may deliver more current even though spec only says 500mA - but one can't rely on that, it is far from universal.

On more modern machines some USB A ports can handle up to 3/5A. However,  without enumerating and getting it actually confirmed by the host that it really is capable of supplying that current (and it is not only an old school 500mA capable port) it is dangerous to attempt to exceed those 100mA/500mA - there are plenty of poorly designed ports around and you can easily blow them out.

Navigation

[0] Message Index

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod