[En-Nut-Discussion] Hardware and fuse info

[Harald Kipp]

Hi Bas,

At 22:50 24.05.2006 +0200, you wrote:
>i just unwrapped an ethernut 2.1 board, and hooked it up according to the 
>manual. I used 9v w/ the barrel connector. The manual says it is protected 
>by a fuse (F1), which blew after about 10 secs. So i replaced it with a 
>spare fuse, and started from 3v to 7.5, after which the red led became 
>active again. But now the replacement fuse shines a little bit, and i dont 
>too much about hardware, to determine if that is normal operation 
>procedure   :)

At egnite the boards are automatically tested up to 12V DC,
measuring the current.

I'll explain the Ethernut 2 power supply in detail, as there
are obviously uncertainties and many people asked related
questions in the past. In general, the same applies for
Ethernut 1. But not for Ethernut 3, which has an advanced
switching regulator with a wide input range of 5-24V.

Please check the schematic page 4, starting at the barrel
connector J2. This power supply input is connected via AC1/2 to
JP3 (on page 3), which allows to alternatively supply the board
via the two spare wire pairs of the Ethernet TP cable (10/100 MBit
needs  two pairs of the eight wires only). By default, jumpers at
JP3 (located near the Ethernet connector) should be removed.
Ethernut 2 doesn't support standard power over Ethernet. It's
more like poor man's PoE and requires a special injector box.

Back to page 4 of the schematic. Power then flows through the
fuse (1A, fast acting, LittleFuse Part # 0453 001) and passes
D3. This is a bidirectional Transil Diode, which is used as
an overvoltage protection. If the supply raises above about
15V, this diode shorts the power supply within very short time.
As a result the fuse will blow. This is a very reliable
overvoltage protection, however, at the expense of having
to replace the fuse.

Next the power is routed through D4..D7, which provide a
regulator bridge. Because of this bridge, the polarity at
the barrel connector doesn't matter. The rectifier bridge
will make sure, that the positive voltage always appears
at the regulator input of IC8 and the negative voltage
will be at the board's ground layer. One disadvantage of
the diodes is, that each will drop the voltage by about
0.7V. At 9V input, IC8 will see 7.6V only, because each
power supply line passes two diodes.

The two capacitors C19 and C33 will keep the supply quiet.
However, C19 is two small to allow Ethernut 2 being supplied by
AC. Only DC input will work. So don't get confused by the
rectifier bridge or the misleading signal names AC1 and AC2.

IC8 is a linear regulator, which requires a minimum difference
of 1.2V between input and output in order to keep the regulated
5V output within the required tolerances. Adding the 1.4V
voltage drop caused by the diodes, results in a minimum input
of 7.6V.

In theory IC8, the linear regulator, offers up to 1.5A current.
The bad thing with linear regulators is, that they keep
drawing the same amount of current, no matter how high
the input voltage may be. At 15V input at J2, the regulator
will still see 12.4V, making an input/output difference
of 12.4V - 5V = 7.4V. Ethernut 2 may draw about 250-300 mA,
which means, that the regulator has to destroy
7.4V * 0.3A = 2.22W. This power is converted to heat and
believe me, this is a lot. Not for heating up rooms, but
for heating up enclosures or burning your fingers.

The final result: 7.6V is the minimum, 9V is just perfect,
voltages above 12V should be avoided and anything above
15V will blow the fuse.

Btw. newbies typically make the mistake to place the board
on the conductive shipping bag. This is a bad idea. Putting
the board on an electrostatic surface is even worse. Never
ever do it like Adam:
http://www.sics.se/~adam/img/contiki-ethernut-server.jpg
Adam is of course no newbie, but many pros simply ignore
the danger caused by electrostatic discharge.
Best use a sheet of paper, raw wooden surfaces are perfect
too. Or mount some legs, so the board will not touch any
surface.

Beside all that, you board might got broken after it passed
the final test before shipping. egnite will replace it free
of charge.

Harald