Nettled Shin wrote:interestedcp wrote:I think that devices themselves switches off for various reasons
But if the Kemo device is regulating effectively, the device connected to it should have no knowledge of the bicycle's speed, so would have no reason to switch off.
Could it be that the regulator overheats at higher speeds and goes thermal shutdown? Say a USB device usually draws 500mA@5V, and it is connected to a regulator set up to provide a constant 5V via a dynamo producing 6V. In this case, the regulator dissipates 0.5W. If the USB device only draws 400mA, a hub dynamo's output voltage could rise to 10V, say. In this case the regulator is dissipating 2W. This is all at a fairly average speed. If the bike is travelling fast, the the power the regulator is having to dissipate could be two or three watts higher. It would be interesting to see whether the regulator has a heatsink.
First of all, the Kemo may not be well regulated by design or by manufacturing faults. I am not defending the Kemo as such, but I suspect that the troubles some people experience with devices drawing a lot of power, isn't caused so much by the design of the charger, but more by the many quirks and pit falls that exist in both the USB specification and how naughty device manufacturers abuse the USB port, and of top of that, few if any, manufacturers design or test their devices to charging from a hub dynamo.
I won't go into detail, but the USB 2.0 specification had major revision when it came to charging, years after (2007) the original specification from 2000, and it keeps getting revised (latest oct. 2011), so USB device charging is a moving target with incompatible devices depending of manufacturing year. Usually, the end users doesn't feel too many problems, because most USB devices are either plugged into a dedicated charger from the manufacturer, or a PC, but when you stray from that path, you may experience problems.
The major reason for the patched USB standard was that smartphone and GPS manufacturers needed much higher power than the 500mA as the original spec stipulated. So they started doing naughty things with the USB port, like shortening D+D- to signify a dedicated charger. Some of these tricks was later incorporated in the USB spec where they may live a spectral half-life of not being directly prohibited by the standard while not being sanctioned by the official USB 2.0 standard either. See fx http://compliance.usb.org/index.asp?Upd ... t=Standard
So eg. smartphone manufacturers have several different proprietary standards for recognizing a "dumb"/dedicated charger; Some use a shortened pin 4&5 to signify a charger, others, like BlackBerry tie pin 4 to 5 (ground) using a 220k ohm resistor to indicate a "dumb charger", other like Garmin, tie pin 4 to 5 (ground) with a 17k ohm resistor to indicate a 1A charger, and tie it directly to ground (exactly like HTC devices) to indicate a 500 mA charger. (see http://garminoregon.wikispaces.com/Power )
So here is my take on what happened to the BlackBerry; At first it was just connected to the Kemo with a normal USB cable, (pin 4&5 not shortened). At 6 MPH the hub generated enough power through the Kemo that the BlackBerry discovered activity on the USB port; it then tried to digitally negotiate with the Kemo charger since it couldn't know it was a dumb charger. The phone may have charged a little (the free 100mA "no dead battery proviso"), but the negotiation failed of course, so the BlackBerry closed the connection with an error. The Brando adaptor solved the problem (making it less likely that problems was caused by the charger shutting up because of heat), since it can switch between normal (data) and charge mode, and could therefore tell the BlackBerry, the the Kemo was a "dumb" charger. Unfortunately I haven't seen any specs on the Brando, so whether it shorten pin 4&5 directly, with an resistor, or D+D- instead, is unknown. I guess one of the two latter options, since folks have reported the Kemo to work with Garmin devices when pin 4&5 was shorted. I guess the Garmin cut off at 16MPH for similar reasons; at a certain speed the Kemo generated enough power to trigger a threshold event at the Garmin device. It is possible that the Kemo was 100% within the USB power spec, while still being incompatible with some proprietary Garmin charging standard. The problems with the Garmin may be solved with a correctly shortened cable, or with a change in its setup (spanning mode, or "stay on when charging" or "don't attempt data transfer mode" or similar) or both.
The problem with dynamo hub chargers are that they are "dumb" chargers with highly varying output. That is quite an unusual device, and most manufacturers doesn't test for such marginal cases. So pioneering users are likely to hit all kinds of quirks and snags. OTHO, using the hub dynamo to charge those ubiquitous power hungry smartphones and GPS devices, is such a logical and tempting idea, that many more people will try it the next couple of years.