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PICList Thread
'[EE:] Pin protection'
2003\10\24@183648 by Mike Singer

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Pedro Drummond wrote:
>  Starting a new old thread, how you folks usually protect
> I/O pins in a noisy environment ?
...
>  But how about a lighter problem, just a 10-inch wire to
> a pushbutton (in the same noisy environment) ?


There was a thread:
"[EE]: Foolproof input line" with excellent Russell's comments.


Just my 2 cents.

Mike.

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2003\10\25@113207 by Pedro Drummond

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Mike, thanks for pointing out.
I have been looking, that was a good thread (exactly one year ago). But I
wouldn't say that nice circuit is a very good alternative for noisy
environments.
I may be wrong, though.



{Original Message removed}

2003\10\25@141419 by Mike Singer

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Pedro Drummond wrote:
> But I wouldn't say that nice circuit is a very good
> alternative for noisy environments.

Incomplete sentence: alternative to what?

Mike.

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'[EE:] Pin Protection'
2004\07\16@150512 by Shawn Yates
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What would you recommend for protecting a PIC pin used to detect switch
closure at the end of a long run of wire?  I have had no problems, but
since I am relaying the board anyhow, I thought I would ask for input.
The remote switch is always a switch to ground (its either floating or
grounded).

Here is whate I have now:

        +5v                                             +5v
         |                                               |
        10 K                                       6.8V TVS (bidir)
         |                                               |
PIN  ------- Anode -- Cathode ---- 1.0K ----+--- to long wire pair
                   914 Diode                     |
                                                   6.8V TVS (bidir)
                                                         |
                                                         |
                                                        Vss        



Would anyone suggest anything more or different?

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2004\07\16@151135 by David VanHorn

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At 02:59 PM 7/16/2004 -0400, Shawn Yates wrote:

>What would you recommend for protecting a PIC pin used to detect switch
>closure at the end of a long run of wire?  I have had no problems, but
>since I am relaying the board anyhow, I thought I would ask for input.
>The remote switch is always a switch to ground (its either floating or
>grounded).

Me, I would put a series resistor, 1.5k or more, between the pin and the outside world. On the outside of that, a pair of diodes, shunting any excess or <0 voltage to VCC and ground, and outside that, another 1.5k to limit the current that could possibly be dumped into VCC.

If I were really paranoid, at the I/O pin, a neon lamp.

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2004\07\16@154339 by Eisermann, Phil [Ridg/CO]

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pic microcontroller discussion list wrote:
> At 02:59 PM 7/16/2004 -0400, Shawn Yates wrote:
>
>> What would you recommend for protecting a PIC pin used to detect
>> switch closure at the end of a long run of wire?  I have had no
>> problems, but since I am relaying the board anyhow, I thought I
>> would ask for input. The remote switch is always a switch to ground
>> (its either floating or grounded).
>
> Me, I would put a series resistor, 1.5k or more, between the pin and
> the outside world. On the outside of that, a pair of diodes, shunting
> any excess or <0 voltage to VCC and ground, and outside that, another
> 1.5k to limit the current that could possibly be dumped into VCC.
>
> If I were really paranoid, at the I/O pin, a neon lamp.

This topic periodically comes up. Isn't there something on the
piclist FAQ? If there isn't, there should be.

i do much the same as Dave suggested. I often add a 100nF cap after
the series resistor to double as a low-pass filter. Placing the
diodes in front of the resistor allows said resistor to limit the
current to the PIC pin in case they do conduct. It depends on
my level of paranoia in the particular application/end usage, how
critical the protection, etc. Always there are trade-offs.

Out of curiosity, why is the TVS by the switch? Is there
additional circuitry that needs protecting? Or is the
switch not a mechanical switch like I am thinking?


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2004\07\16@162549 by Shawn Yates

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Sorry,  the TVS is on the circuit board very close to the pin where the
100ft+ wire to the switch is connected.  My reading showed that the TVS
should be as close to the source of potential problem as possible.  The
potential source, as far as I know, is the wire.

It sounds like you are all saying things very similar to what I already
have.  I like the CAP idea though.

Thanks for the input.  (no pun intended)

Shawn
{Original Message removed}

2004\07\16@165139 by Eisermann, Phil [Ridg/CO]

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pic microcontroller discussion list wrote:
> Sorry,  the TVS is on the circuit board very close to the pin where
> the 100ft+ wire to the switch is connected.  My reading showed that
> the TVS should be as close to the source of potential problem as
> possible.  The potential source, as far as I know, is the wire.

But if its a mechanical switch, it doesn't really need the protection.
It's wasted on the switch. The 100ft wire is a great antenna for
picking up noise and bringing it into your circuit regardless of the
TVS at the switch. If you must/want to have it, the proper place is
on the circuit board, before the current-limiting resistor. I think
the diodes are likely enough, unless you are trying to protect
against lightning.

> It sounds like you are all saying things very similar to what I
> already have.  I like the CAP idea though.

execpt the suggestion by David VanHorn was not to place the diode
in series with the input, but to use two diodes to shunt excessive
voltage to power and ground. You undoubtedly were aware of that, I
just wanted to emphasize the difference.

> Thanks for the input.  (no pun intended)

glad to be able to help once in a while.

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2004\07\16@173851 by Shawn Yates

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The TVS are on the circuit board with the PIC.  The switch is at the
other end of the long esd/lightning antenna as you pointed out (100ft
wire).  I was under the impression they (the TVS) would act like the
diodes suggested by David and shunt voltage which is more than 6.8 above
ground or more than 6.8 below the 5V (up to 1.5 KW worth).  Isn't that
they way they work?

Hope I am not being obtuse here.  I want to understand what your saying
so I can take full advantage of it.

Shawn

{Original Message removed}

2004\07\16@233831 by Dwayne Reid

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At 12:59 PM 7/16/2004, Shawn Yates wrote:
>What would you recommend for protecting a PIC pin used to detect switch
>closure at the end of a long run of wire?


                                         +5v
                                          |
                                         2K2
                                          |
PIN  -------+---------+-------- 10K ------+------+------- to long wire pair
            |         |                          |
           100n      2K7 (only if pull-up       10n
            |         |   is fed from +16V)      |
            |         |                          |
           Vss       Vss                        Vss

My standard input circuit is above.  I normally drive my input contact
closures from the unregulated supply, hence the need for the 2K7 shunt
resistor.  Omit if the pull-up is driven from +5V, change to 4K7 if PIC pin
has a schmitt trigger input (I try to avoid the schmitt trigger inputs
because the minimum logic HI voltage is so close to Vdd.  Seems
counter-intuitive but avoiding substrate current is important when using
a/d or comparitor inputs.)

The 10n RF bypass cap is as close to the input terminal and ground plane as
practical.  The 100n cap is sized as required for response time.


The above should answer your specific question but I'd like to take this
opportunity to ramble on about interfacing PICs to the outside world.

My multi-purpose PIC cards have holes for all the above components on
several port pins.  In addition, each pin's pull-up resistor can be
installed in one of 3 sets of pads for: Vdd, Vunreg, Gnd.  The input series
resistors are 1/4W resistors bent to 0.4" hole spacing - this leaves room
for the 3 voltage busses between the leads.  The pull-up or pull-down
resistor footprints all overlap with 0.1" offset - this allows each of the
3 footprints to pick up the appropriate voltage buss.  Obviously, you would
install a resistor in only one of the footprints.

If the pin is to be used as an output, omit unneeded resistors and
capacitors, change the 10K series resistor to 100R or whatever, change the
10n RF bypass cap to a MOV or TVS.

I'm a huge proponent of using the unregulated power supply for as many
things as possible.  Off-board switches, LEDs, relays, lamps,
whatever.  This keeps the regulated supply requirements to the bare minimum
and lets me use a LP2950 for both Vdd and A/D & D/A references.  Most of my
systems use only 15mA or so @ 5V even though the unregulated supply might
have to supply several Amps for the various loads.

Another advantage of using the unregulated supply for the off-board switch
pull-ups is greatly improved noise immunity.  The logic HI threshold for
most PIC pins is about 1.3V when operating the PIC from 5V.  With a 10K
series resistor and 2K7 shunt resistor, the unregulated supply can sag to
9V without any problems, it can surge to 22V without any problems.  That
odd voltage of 16V is the result of full-wave rectifying and filtering a 12
Vac transformer - it might get to 20V if lightly loaded and the line
voltage is high but it rarely drops below 12V even with heavy loading.

Anyway, enough rambling for now.  Hope this helps.

dwayne

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2004\07\21@141149 by Eisermann, Phil [Ridg/CO]

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pic microcontroller discussion list wrote:
> The TVS are on the circuit board with the PIC.  The switch is at the
> other end of the long esd/lightning antenna as you pointed out (100ft
> wire).

i lost the original message, but I seem to recall you had the TVS drawn
on the other end of the wire, by the switch. Hence my question. They
are correctly placed on the PCB near the PIC as you state.

> I was under the impression they (the TVS) would act like the
> diodes suggested by David and shunt voltage which is more than 6.8
> above ground or more than 6.8 below the 5V (up to 1.5 KW worth).
> Isn't that they way they work?

yes, that is the idea. Like a fast voltage clamp specifically designed
for short bursts such as ESD. There is usually 'stuff' after the TVS
since the clamp voltage is generally greater than 5V.

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