Actually I took the 5V from in front of the main input fuse. A switching AC adapter will have overcurrent protection as well, anyway.jbeale wrote:It is also possible to make a "cheater" USB cable which takes a separate +5V power feed (essentially a poor-man's 1-port powered hub). That's almost what you did, except doing it on the board can present some other issues, eg. overloading the main input fuse and starving the Pi for voltage.
What a silly suggestion. The purpose of F1 and F2 is to trip the power to a peripheral without crashing the entire board. If they were as high a value as F3 then F3 would be most likely to open instead. You might as well do away with F1 and F2 as increase them.mahjongg wrote:In this case the best choice would probably be a 1A polyfuse, same as on the power input.
This seems like bad advice, as "lsusb -v|grep MaxPower" informs me that a tiny 8GB USB memory stick (image) has a max power of 500mA yet it works perfectly in a RasPi:jojopi wrote: If any of these are over 100mA then they are not suitable for direct connection to the Pi.
Code: Select all
Bus 001 Device 004: ID 18a5:023c Verbatim, Ltd Device Descriptor: bLength 18 bDescriptorType 1 bcdUSB 2.00 bDeviceClass 0 (Defined at Interface level) bDeviceSubClass 0 bDeviceProtocol 0 bMaxPacketSize0 64 idVendor 0x18a5 Verbatim, Ltd idProduct 0x023c bcdDevice 2.3c iManufacturer 1 Verbatim iProduct 2 micro USB Flash Drive iSerial 3 10082613030274 bNumConfigurations 1 Configuration Descriptor: bLength 9 bDescriptorType 2 wTotalLength 32 bNumInterfaces 1 bConfigurationValue 1 iConfiguration 0 bmAttributes 0x80 (Bus Powered) MaxPower 500mA
Well not quite, if you take into account that the input power supply voltage may be slightly lower than 5.0 V.mahjongg wrote:The USB2.0 spec allows low-powered ports to provide a Vbus of 4.4V minimum. So there is a reasonable margin for voltage drop on F1 and F2 at 100mA.
Exactly. It sounds like those polyfuses are speced way too small.mahjongg wrote:Protection from a (poly)fuse is intended as a safety measure, to protect from dramatic destruction of the device it protects (in this case the R-PI), not to secure normal operation
These are USB-2 ports, only USB-1 ports are expected to only support low power (low speed) devices.MrEngman wrote:Interesting.
Personally I am not convinced that there is any design flaw as such. Using polyfuses for protecting the USB is I believe a fairly common practice. There are, however; several issues that need thinking about the PSU being a significant one, but there are also others just as relevant.
I think a major issue, however, is that most people just know the term USB and don't realise there is a low power version (max current 100mA) and a high power version (max current 500mA) so people just plug in their devices and expect them to work.
Yes, the standard solution for USB protection is using a silicon (chip) current monitor, fault detector and current turn off solution. There is a plethora of such devices, but they all cost a few dimes, while a polyfuse costs mere cents. Also, a chip USB protector needs at least an input and an output GPIO, and software support (in the kernel).Dave_G_2 wrote:Below is a link to a pdf by Micrel which has some interesting info on USB power protection.
See first 2 pages:
Try here http://www.littelfuse.com/resettable-ptcs.htmlMorgaine wrote:+1 to mahjongg's last point.
Which takes me back to my initial quest for fully detailed information on the electrical characteristics of these polyfuses. That brief PDF I linked is far below what is considered detailed design information in the industry. I expect parametric curves for pre-trip operation at the very least, so that a designer can target a desired safe operating region without using guesswork. And the post-trip holding current curves would be useful too.
Thanks Dave, that is indeed an interesting article.Dave_G_2 wrote:Below is a link to a pdf by Micrel which has some interesting info on USB power protection.
See first 2 pages:
Thanks MrEngman, some good info there, if we can assume (for now at least) that their devices are electrically roughly similar to ours.MrEngman wrote:Try here http://www.littelfuse.com/resettable-ptcs.html