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Trying once more, this time an attached JPG.


Em 6/1/2012 16:07, Isaac Marino Bavaresco escreveu:
>
>
>
> Point A is the input, point C is connected to the PIC.
>
> V(A) may be any voltage;
> -0.7V <=3D V(B) <=3D Vz (if zener parameters not violated);
> -0.7V <=3D V(C) <=3D Vcc (if Vz < Vcc).
>
> Current through R1 may be any value.
> Current through R2 will be between -((Vcc+0.7)/R2) and (Vz/R2).
>
>
> With good values chosen for the resistance of R1 and the power of zener,
> point A may be connected to mains and the only damage would be R1 burned.
>
>
> Best regards,
>
> Isaac
>
>
>
>
> Em 6/1/2012 12:28, Byron Jeff escreveu:
>> On Fri, Jan 06, 2012 at 11:00:23AM -0200, Isaac Marino Bavaresco wrote:
>>> Em 6/1/2012 10:26, Byron Jeff escreveu:
>>>> On Fri, Jan 06, 2012 at 07:20:08AM -0200, Isaac Marino Bavaresco wrote=
:
>>>>> Em 06/01/2012 06:34, William "Chops" Westfield escreveu:
>>>>>>>> Is there anything I can add to protect the chip from unwanted nega=
tive voltage?
>>>>>> Well, to start with, there should be a substantial resistor; about 1=
0k.
>>>>>> This will rely on the internal protection diodes to clamp the voltag=
es to levels that won't cause problems.  There is plenty of reason to assum=
e that this is insufficient, but microchip does publish app notes where thi=
s is done (apparently successfully.)
>>>>>> ...
>>>>> I prefer to use an external Zener diode plus two resistors to clamp t=
he
>>>>> voltages, forming a "T" with the Zener as the "leg" of the "T".
>>>> What's the purpose of the resistor between the zener and the PIC input=
?
>>> If the RS-232 is connected when the PIC is not powered, the pin will
>>> have 4.7V and the board will be powered by the PIC's internal parasitic
>>> diode and large currents may flow through it, perhaps violating some
>>> parameter.
>> The series resistor will limit the incoming current. With a 4.7V zener a=
nd
>> a 4.7K series resistor, the maximum current the unpowered PIC will see i=
s 1
>> mA. All the rest of current from the higher incoming voltage will be
>> shunted to ground by the zener.
>>
>>> With the additional resistor (say 1K) the current will be limited to a
>>> few mA, well inside the allowed range.
>>>>  For
>>>> quick and dirty testing, I use a 4.7K resistor in series with the RS-2=
32
>>>> output and a 4.7V zener. For more permanent circuits I use this as the=
 base
>>>> input to a NPN transistor inverter with a pullup resistor to protect t=
he
>>>> PIC's input from the -0.7V the zener allows when the RS-232 output swi=
ngs
>>>> negative.
>>> When using an NPN transistor I use a small signal diode from ground to
>>> base to protect the base-emitter junction from reverse biasing. The PIC
>>> will never see the -0.7V because the base-collector junction will be
>>> reverse biased, but the transistor's base-emitter junction may be
>>> damaged as they are usually rated for -5V.
>> The zener serves the purpose of that small signal diode with the input
>> voltage is negative as the zener is now forward biased. So the base-emit=
ter
>> voltage is never more that -0.7V which is nowhere near the reverse bias
>> breakdown voltage for the transistor.
>>
>> I haven't had any problems so far, and it's a heck of a lot easier than
>> trying to track down a MAX232 or equivalent. I haven't seen a USB serial
>> adapter yet that swings any more than +/- 5V, and every one so far works
>> fine with 0-5V going into their inputs. With the near universal death of
>> dedicated serial and parallel ports on PC's, USB serial adapters are my
>> current bit wiggling interface.
>>
>> BAJ
>>
>>> Best regards,
>>>
>>> Isaac
>>>
>>> =20
>>>
>>>> The last time this discussion came up, Russel noted to me that allowin=
g the
>>>> negative voltage to the PIC input, even though it's clamped by the
>>>> protection diodes, causes the part to operate outside of it's range
>>>> specified in the datasheet (-0.3V minimum). Hence the transistor in th=
e
>>>> permanent circuit. I'm unsure what if any damage that 150 uA of curren=
t at
>>>> -0.7V can do to the input.
>>>>
>>>> BAJ
>>>>
>>> --=20
>>> http://www.piclist.com PIC/SX FAQ & list archive
>>> View/change your membership options at
>>> http://mailman.mit.edu/mailman/listinfo/piclist


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