I am about to unleash 50 students with mbed boards in our lab class here. It's just a matter of time until they short things out. So exactly what happens when they short 5V (or 3.3V) to GND on the protoboard. Does a PCB trace or some tiny SMT fuse blow out on the mbed or PC's USB hardware or are you OK?

We had some big protoboards in the past and had to put replaceable fuses on them to cut down on the smoke in the lab.

Someone out there has probably already done this and knows the answer.

I found the info. http://www.ce-mag.com/archive/03/ARG/mikolajczak.html has the answer - should be OK per USB standard with resettable polymeric positive- temperature-coefficient (PPTC) devices (a fuse that cools off and resets itself) , but others on the web claim to have blown out their USB ports somehow. I guess time will tell! Sounds like some hubs have a power switch circuit instead, but it is optional on PC.

"PPTC circuit-protection devices are made from a composite of semicrystalline polymer and conductive particles. A small surface mount PPTC is seen above. At normal temperatures, the conductive particles form low-resistance networks in the polymer. However, if the temperature rises above the device's switching temperature (T_Sw), either from high current through the part or from an increase in the ambient temperature, the crystallites in the polymer melt and become amorphous. The increase in volume during melting of the crystalline phase causes a hydraulic separation of the conductive particles and results in a large nonlinear increase in the resistance of the device.

The resistance can increase by three or more decades. This increased resistance protects the equipment in the circuit by reducing the amount of current that can flow under the fault condition to a low, steady-state level. When the device transitions to the high-resistance state, it is said to have "tripped." The device will remain in its tripped (high-resistance) position until the fault is cleared and power to the circuit is removed—at which time the conductive composite cools and recrystallizes, restoring the PPTC to a low-resistance state in the circuit and returning the affected equipment to normal operating conditions.

PPTC devices are typically used in USB hosts and in USB self-powered hubs. The resettable current-limiting devices are placed in series with the output power port of a USB device. Current limiting in a fault condition helps prevent circuit damage and systemwide voltage droops. With individual port protection, it allows the remainder of the USB bus to continue functioning even if one port is short-circuited. "

In my experience, it differs between USB hosts (computers). Some (most?), especially generic desktop PCs seem not to cut out until 2A or more - presumably they have one larger fuse/PTC per bank of ports to save a few pence, rather than one per port. 2A is more than enough to do damage to small PCB traces! On the other hand, laptops and more power-conscious hosts, will enforce the 500 mA per port limit. I've noticed my MacBook Pros USB ports will strictly enforce the 500 mA limit, and show a message dialog and cut off the power if you go over that.

There is also the issue of how much current the USB device negotiates for during enumeration. If you're doing your own USB device (using the mbed D+/D- pair rather than via the mbed interface chip), then you can change this. The max is still 500 mA though. Finally, a compliant USB device is allowed some amount of current (100 mA I think) before enumeration, but if it wants more it has to negotiate for it. (In this respect, many, *many* devices are not compliant.)

There are also all manner of new USB sub-standards for things like OTG power, battery charging, USB car kits for mobile phones, and so on.

Summary: not as simple as it should be. Don't rely on the PTC/polyfuse protecting you (unless you know it wasn't cut out of your machines to make them cheaper :-). Best do a check in your own lab with your own machines.

Thanks - I was wondering if you could count on the motherboard to meet the USB standard. Too bad you can't get a schematic for your PC motherboard and check it!

http://www.usb.org/developers/whitepapers/power_delivery_motherboards.pdf has more details on how the power and PPTC should work. Sounds like a PPTC has to be bigger than 500MA because of in rush current. The Mac must use a power switch. PPTC data sheets also seem to mum on how long to cool down and reset and how many lifetime trip/reset cycles they have. The power switch would be safer and I wonder which PCs you could count on to have it.

Does the built-in mbed USB setup for 500MA after enumeration?

Noticed USB 3.0 is going to be 900MA. Just saw some of the new 3.0 cards at a computer store.

Just saw your other post about USB 3.0 - is there any way for a user to hack the mbed for 900MA?

Hi All,

We designed the mbed to be as nice to the PC as we could to protect it from abuse it might take, so we've got protection for the USB power in the form of a FPF2123 Current Limiter; see the schematic on the mbed NXP LPC1768 page. This will kick in at around 460mA.

If you do manage to do something silly, this should stop you drawing too much current from the PC, and in some cases that means it gets very hot; once it has cooled back down, usually everything is back to normal (I think we may have seen one or two sacrifice themselves for the benefit of the PC! But failures have been very rare). In addition, most PC USB ports have protection, and many will cut themselves off for a while or until a reboot as part of their protection. So there is protection built in to the mbed, and most PC USB interfaces.

As you guessed, the mbed interface negotiates with the PC to get 500mA.

However, despite this protection, the reality is you just have to be careful!

Thanks,
Simon