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Control panel power via USB

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u_rebelscum:

To add to what MonMotha said, some hubs "take" 100 mA, leaving only 400 mA for the rest.  BTW, this is why most un-powered hubs are only 4 port, as there is only enough mAs left for four 100 mA ports.  And if any one USB device asks for 200-400 mA, that "removes" another port per extra 100 mA.

So if you didn't have the LED board, you might be okay (with the 2 u360s + the 1 mouse = 300 mA).  However, the 15 * 15 = 225, so the board has to ask for 300 mA minimum to work.  And 300 + 300 = 600; too high for the USB standard.

MonMotha:

Actually, a USB device gets to ask for power in 2mA increments, not 100mA.  However, there still has to be the 100mA reserved for enumeration on enabled but unoccupied ports.   I don't remember if the USB spec allows power for devices that ask for less than 100mA (if that's even allowed, which I recall it is) to be "pooled" back and allotted to other ports or not.  I never had reason to really read up on the hub spec.

richms:

I have severly overloaded USB ports and its fine - how do you thing the USB port trash products like hand warmers, coffee warmers, vacuum cleaners etc get away with it.

The big killer is voltage drop over the cables - most USB devices just have a couple of diodes to drop to 3.3v internally, and if the voltage sags then you will start to see erratic operation.

I would get a powered hub, since that will solve any enumiration issues, and then run a nice thick red and black cable back to a molex or somewhere to get a stable nice 5v to it.

No motherboard I have seen has actually done anything with the power to the USB ports other then connect it straight to 5v, possibly via a PTC for some protection, but not proper current limiting.

MonMotha:


--- Quote from: richms on January 21, 2009, 06:31:43 am ---The big killer is voltage drop over the cables - most USB devices just have a couple of diodes to drop to 3.3v internally, and if the voltage sags then you will start to see erratic operation.
--- End quote ---
Most that I've torn apart (and designed) have an LDO linear to go from the 4.5-5.5V Vbus down to a stable 3.3V.  However, if you go below about 3.8-4V, the regulator will start to drop out (same result).  I guess REALLY cheap USB devices may use diodes.  Makes it harder to do proper soft-start (which most devices don't do anyway), though.

Note that the USB standard accounts for most of this.  That's why Vbus can vary so much and why it has soft-start provisions.  Unfortunately, everybody ignores all this stuff, violates the spec, then wonders why things don't always work right.


--- Quote from: richms on January 21, 2009, 06:31:43 am ---No motherboard I have seen has actually done anything with the power to the USB ports other then connect it straight to 5v, possibly via a PTC for some protection, but not proper current limiting.

--- End quote ---
Many laptops do enforce the limits.  Many these days (especially small stuff like netbooks, tablets, and ultraportables) will enforce the actual allocated limit (i.e. if you draw more than you ask for, you'll get cut off), but some do just enforce a 500mA hard limit on every port.  Sometimes that's done with a PTC, and sometimes it's done with an actual monitor/sense circuit which provides reliable error flag and on/off operation.

Most desktop motherboards I've seen create a dedicated "Vbus" rail that is connected to a 5V via a PTC that provides a limit at ~10% over max combined draw since it would be downright dangerous to connect Vbus directly to the 5V rail off the PSU (often capable of sourcing >25A!).  So, if the mobo has 4 ports, there will be a ~2.25A PTC that attempts to protect the ports.  Kinda a minimal protection if you ask me, but I guess it's cheaper than doing it the right way.

Some desktop motherboards support a "wake via USB" feature.  These motherboards are capable of supplying Vbus via ATX +5VSB when in soft-off.  Many of these mobos do enforce limits (at least a 500mA hard limit) since +5VSB has limited current capability (often 1A at best).

Of course, your LED-Wiz and similar have no idea how much power you're going to actually use.  I would suspect those kind of devices just ask for the full 500mA.  I'm guessing that's documented somewhere.  If it does this, it may cause a failure to complete enumeration due to apparent lack of power unless you use a powered hub or plug it directly into your PC.

Bottom line: power your USB devices via USB like they are designed to be, and power your power loads (e.g. blinkenlichten) via a dedicated power line (from a suitable wall wart or your PC PSU).  If you need more than 100mA per port on a 4-port (or larger) hub, make sure you use a powered hub.  The power source for the hub is up to you: most come with a wall wart, and I'd recommend using it, but you could also run dedicated lines (I'd suggest 20AWG or larger) from your PC PSU if that fits your fancy better.

I would reccommend installing a 1A (or other suitable) fuse in your +5V source line for your power loads (blinkenlichten) for safety reasons.  Large PC PSUs are capable of currents suitable for welding!

richms:

The computer I found in the inorganic collection with a nice burnt trail from the melted front USB port, along the flyleads, to a charred header on the motherboard and some darkened areas all the way to the ATX connector shows that some do absolutely nothing for protection.

The thing I find laughable is the HDDs that come with a Y cable to get more power, when there is nothing to request the power from the second port, so they are violating spec anyway, and since they are feeding the power straight to a 2.5" HDD they cant use diodes to prevent issues with connecting 2 5v supplies together. A friend had mystery problems with their PC with that plugged in, turns out some USBs were jumpered for 5vsb, some not, and joining them made for an unhappy computer.

Its crazy that the USB association dont go after the pervayors of non compliant junk for saying that they are USB for the stuff like warmers and chargers etc which are probably responsible for a hell of a lot of blown USB ports on office PCs that "just broke one day"

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