[sdiy] Protecting microcontroller inputs from - voltage?

[Bret Truchan]

Hi Harry,

Do you mean something like this?

                R1       R2
I/O pin] -----/\/\--+--/\/\----------< to whatever
                    _|_
                    /_\
                     | anode
                     |
                    gnd


R1, R2 = 200 ohm ~ 500 ohm

My current design already looks similar to that, except that I use a
5.1v zener diode between the two resistors.  How could I incorporate
both diodes into my design?  Would this work?

                R1                   R2
I/O pin] -----/\/\--+-----------+--/\/\------< to whatever
                    _|_          |
                    /_\         _|_
                     | anode    /_\  = 5.1 zener
                     |           |
                    gnd         gnd


>>Hah... a trick question
>>
>>Putting the diode right AT the chip will probably not help you, unless you
>>use shottky diodes.  A diode at the chip is a race... will the external 
>>diode
>>conduct before the internal diode ?
>>
>>I'd even suggest using two resistors in series... with the diodes 
>>connected at
>>the
>>center of the resistors.. It would protect in both directions and not 
>>bother
>>either
>>the chip or the external input/output with too much current draw in a 
>>fault.
>>
>>Chip protection should be in proportion to the chances that someone will 
>>connect
>>
>>something unusual / stupid to it... almost a sure bet in a modular :^P
>>
>>H^) harry
>>
>>Ken Stone wrote:
>>
>> > >>   Use a diode clamp.  That is, a diode from the I/O pin to ground, 
>>with
>> > >> the cathode on the I/O pin.  Check the microcontroller electrical 
>>spec,
>> > >> but most will tolerate -1V or so.  If even less drop is needed, use 
>>a
>> > >> Schottky diode.  In practice, a 1N4148 will work fine.  Caveat: put 
>>a
>> > >> small resistor in series with the I/O pin and whatever it is 
>>sensing.
>> > >
>> > >On which side of the resistor would you put the diode ?
>> >
>> > The diode goes directly to the IC pin. The resistor goes between the 
>>pin and
>> > whatever you are connecting to. The purpose of the resistor is to limit 
>>the
>> > current flow through the diode in the "protected" condition. If you put 
>>the
>> > resistor on the other side, you'll probably blow the diode up when the 
>>input
>> > goes negative, or at very least, you will be sending whatever is doing 
>>the
>> > driver to it's current limit.
>> > _______________________________________________________________________
>> > Ken Stone   sasami at hotkey.net.au
>> > Modular Synth PCBs for sale <http://www.blaze.net.au/~sasami/synth/>
>> > Australian Miniature Horses & Ponies <http://www.blaze.net.au/~sasami/>
>>
>

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