Err, I must admit that I wrote more in general... I haven't had a look at none
of the schematics or drawings...
First, to clear a bit confusion:
Schottky: A diode that is usually significantly faster than silicon diodes, and
has a lover voltage drop (~ 0.3V). Reverse polarity is however limited, 30V is
common. There exist types that can withstand higher voltage, but the speed
decreases due to the capacitance.
Zener: A silicon diode that conducts on straight polarity and insulates on
reverse. But from a certain ("breakdown") voltage upward, zener diodes start
conducting even in reverse polarity. This voltage is not very accurate, and
there is a quite soft bend in the curve, no corner, when it starts conducting.
It can be used to limit voltages, but in regulation it's usually not the best
choice.
>>>The trouble with polarity protection (this is what we're talking about isn't
>>>?) is the drop voltage involved with diode.
>>Only if you're using a diode in series. You could also use the fuse and a
>>reasonably strong schottky diode from ground. So, reverse polarity will not
>>go beyond 0.5V, and the fuse will blow when strong reverse power is applied.
>Thanks for the tip. Could you point to some direction about this diode ?
I was also thinking about using a schottky diode to regulate max 3.3V. I mean,
according to my last power supply schematic, if user select USB but no
regulation, xbee could be damaged for instance. Could a 3.3V schottky diode be
used for thi purpose ? (make sure it won't be more than 3.3V ?)
A resistor+zener diode for regulation eats a lot of power. If you need 100mA at
min. 3V while supply is 3.3V, the resistor can be at most 0.3V/0.1A=3 Ohm. If
supply is 5.5V then while the zener diode limits to 3.3V, the resistor conducts
(5.5-3.3)V/3 Ohm=733mA. That is 1.6 Watt for the resistor, and 2.5 Watt for the
zener diode. Quiescent power 4 Watt. Usable as heating, but no micro electronic.
Reverse protection using fuse and "crowbar" diode: I've not seen SMD fuses
below 500mA to easily come by. So I feel like protecting the computer-onboard
fuse can or can not work. Even a super fast acting fuse can hold double it's
rating for up to five seconds. It's pure chance which of two fuses in series
will blow. To make sure which fuse blows, you would have to use one rated at
most 300mA. But, reverse polarity should not be the main concern on USB, but on
other inputs. A 500mA super fast fuse will blow within 5s at 1A and within 1ms
at 5A. This is what the crowbar diode must cope with, only occasionally,
though. SS12L could be a candidate. It is only rated 1A, but a surge current of
30A for 8ms is specified in the datasheet. It is in DO-219-AB(SMF) package,
2x3mm in size.
Whoops... Looked in a PIC datasheet. Only -0.3V are rated for PICs as absolute
maximum. The SS12L will allow about 0.7V while conducting 3A. This might kill
the PIC. I do know that PICs die on reverse polarity. But does anyone know if
they die within milliseconds when applying only 1V in reverse?
Greets,
Kiste
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