Am Montag, 4. Juni 2007 15:42 schrieb Vortex:
>
> > The OP tried that already, and it worked. Therefore my advice.
>
> OP? Does that mean it's a known issue?
>
OP: original posting/poster.
> But i can't imagine that this
> usb-port which is capable of 500mA should have a problem with a few
> mA. Spikes and something like that on the power lines should be
> sufficiently blocked by Capacitors and RF-Chokes which i already
> placed there. And due to your recommends i increased the
> Capacitors now. Without any improvement.
>
Increasing the capacitors make things *worse*, not better! Empty capacitors
are like short circuits. You have to limit the current flowing into empty
capacitors by applying a resistor or (better) a choke coil to the supply
lines.
USB circuits are powered on on detection/driver-reload, so this power-on
problem is more inconvenient than with other devices.
Even a "quick melt" 100mA fuse (your other posting) sure doesn't melt if a 1A
current is flowing for a few µs. Melting is a energy -- not current --
related process, and ~1A*0,1V*100µs=10µJ is too few energy to melt even a
thin stanniol wire .There are fuse deratings to calculate the neccessary
current for a given time. This is why melting fuses are useless as soon as
high-speed electronic switches should be protected. Your fuse protects the
onewire cable, not the interface circuit.
Most USB interfaces in desktop computers doesn't have appropiate current
limiting switches (MIC2075 or TPS2024) to limit the current to the specified
500mA, but instead use a 1A fuse. Your laptop however, may use such a
switch -- because a molt fuse is inconvenient in a laptop -- and thus, cause
problems.
Kind regards
Jan
--
Microsoft and its customer are addicted to backwards compatibility
in a way that makes a heroin addict look silly.
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