ESD protection for microcontroller pin that is to be touched

Dnia 09-10-2012 o 21:35:39 Rene napisa=C5=82(a):

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Given that the wheelchair is battery powered and the person in it is =

isolated from the ground, where do you expect to get your 50Hz from?

The touch is likely to be detectable anyways, since a static charge you'= re =

so afraid of will flow into the pin and a person is also an antenna. But= =

the original idea of using a real capacitive sensor gave you isolation -= I =

wouldn't give up on it so quickly. If the AVR is too much hassle, use a = =

PIC and the CVD method (Microchip's AN1298).

Regards, Andrzej Ekiert

I would put 1M from the sensor plate to a junction of a couple of Schottky diodes, one to ground, one to processor Vcc,then another resistor, maybe 10k or so, to processor pin. That way the external diodes carry the static discharges away, and the processor pin only sees the residual.

The PIC CVD method as suggested could also be adapted to AVR.

Static discharge is still an issue with capacitive sensors, the coupling can still transfer high voltages.

--
Regards, 

Adrian Jansen           adrianjansen at internode dot on dot net 
Note reply address is invalid, convert address above to machine form.

I'll second that. This is the way it's done right. The pull-down if needed can also be from that node. If this touch-stuff goes through the ADC then one can employ filtering to notch out 50, 100, 150Hz and so on. or in America 60, 120, 180Hz. If the uC runs out of MIPS sometimes the clever selection of the sampling intervals suffices.

Especially on a dry winter day ... bzzt ... pop.

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Regards, Joerg 

http://www.analogconsultants.com/

As a 'rule of thumb' I always encourage my staff to use buffers to the outside world. Several reasons, buffers tend to be more robust, when hit, they tend to not do much because they're not exercising a program; and being somewhat sacrificial, don't mind changing them out if they die.

Next, barring lightning, you can pretty much keep ESD out and do what you want. BUT you have to think in terms of bandwidths up into the GHz. just design the protection filter to function over that bandwidth and you have some hope.

*IF* you insist on direct connection to the uC; I'd put a nice cap adjacent to the uC pin right to its GND, put a series resistor as large as you can stand; which takes you over to the connection point and then put in the three fast tranzorbs, or components like that out at the pin to be touched between Vcc and GND, pin and GND, and pin and Vcc. AND right between that pin and the person add that series resistor to limit current. If components are SMT; you may wish to put several R's in series so their body capacitance doesn't feed thru/arc over too much and definitely connect right at the edge of the board, through a separate connection. There's a lot more to do to keep the onslaught of planar fields across the PCB and keep out any ESD induced magnetic field injections, too, but those are a start.

Have you thought? Using 50/60Hz AC mains pickup works like a charm UNTIL there's a power outage. Then not so good. If the Operator doesn't need this to work when there's no AC power, or operate at remote outside locations, then go ahead and use the technique. If I were you, I'd make this self-contained -- isolate the pin, drive an outside ring using the uC's DAC output, or sq wave output will also work. and sense the feedpath provided as a finger gets in there. All self contained, requires NO direct connection to the uC and will operate like a proximity switch. Doing such, you can put the ring and center pickup BEHIND a plastic membrane and keep a lot of unwanted stuff out, including dirt.

I'd use a unipolar TVS diode after a resistor. I don't want the ESD surge going into my supply, if I can help it, either. A capacitor at that node helps a lot, too.

But mind that TVS have tons of capacitance. With 1M series resistors I don't see how an ESD hit could possibly endanger a well-bypassed power rail. If still concerned make a "waste dump sub-rail" instead.

[...]

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Regards, Joerg 

http://www.analogconsultants.com/

Agreed, with one caveat:

Using a series 1M resistor against schottky diodes is iffy. Only 2.5uA of unbalanced leakage and your logic level is wrong.

I'd use 1N4148 instead. They'll still fire before the MCU input protection diodes, and their greater peak current handling capacity allows you to use a much smaller series resistance without endangering the MCU.

Tim

--
Deep Friar: a very philosophical monk. 
Website: http://webpages.charter.net/dawill/tmoranwms

You really should read and understand the manual first. The electrode is not to be touched! Qtouch is a fancy word for capacitive touch sensing which works because a finger changes the dielectric constant and thus the measured capacitance. You shouldn't use a scope on the pins because you won't see anything.

--
Failure does not prove something is impossible, failure simply 
indicates you are not using the right tools... 
nico@nctdevpuntnl (punt=.) 
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The capacitance, like everything else, depends on the part used. TVS diodes can be had

Wow, I had expected this many replies, thank you all for your efforts. I think I will go the route described with an extra node with protection diodes and an extra series resistor. Maybe I'll add an extra resistor.

Thanks again!

Yours sincerely, Rene

I have read it so many times that my eyes starting hurting ;-).

I know, it is a capacitive system. However, what I wanted to make after giving up on Qtouch, is a sensor that /is/ to be touched.

That is not correct. The changed capacitance is detected because there is a process of charging and discharging of the electrode going on (the timing in that process is a function of the capacity) and this can be seen on a scope (which is also mentioned in the manual).

Yours sincerely, Rene

To be honest, I had not thought of that. But fortunately that will not be a problem.

What you describe is what is called the Qmatrix method in the Qtouch library, there is also the Qtouch method which uses only one electrode.

I think first I will try to make things work with physical touch and later I will sink my teeth into capacitive sensing (and that cursed library ;-)) again.

Thank you!

Yours sincerely, Rene

I would be very gratefull if you could explain to me what a well-bypassed power rail means, I do not know what it is.

Thank you!

Yours sincerely, Rene

Lots of distributed capacitance to ground. The capacitors should be of different sizes (small for high frequency and larger for low).

?? "The capacitors should be of different sizes..." Did the industry change their thinking here?

I have NOT found that technique effective unless done with EXTREME care. Using HFSS to analyze.

Rather than risk your electronics, just use the IEEE model for a 'personal' ESD event and model the circuitry using LTspice, or such.

In other words, pretend you've just attached a 100pF capacitor charged to 18kV, and ask yourself, "I wonder what THAT does?" be sure to consider all those pesky parasitic paths, too. Oh, and also ask yourself what that resulting spike of current might induce into your circuit someplace else.

With a full plane you can live with a few sprinkled 0.1uF plus on 10uF or so. Because the plane itself is a small and very good distributed capacitor.

That was in the last century :-)

Rene, plus one very important thing in addition: If you get a barrage of hits and your system is extremely low in power consumption you might want to have a limiter that bleeds excess charge. This is because regulators can typically only source current but most (except for some sync bucks) cannot sink. A TLV431 is nice for this, just set it a few hundred mV above your nominal rail voltage. High enough that tolerances won't make it come on but low enough so it does come on before chips go kaputt.

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Regards, Joerg 

http://www.analogconsultants.com/

Don't use Schottkys for this.

The BAV199 is very low leakage and "budget-priced".

--
Regards, Joerg 

http://www.analogconsultants.com/

"Few" is left to the engineer.

Not so much. Still gotta supply current. n

It's easier just to dump through a TVS directly to ground.

Or guys like Muntz :-)

That's what the power supply is for. Nowadays many 1uF ceramics are good up into the gigeehoitzes.

That no workie. A TVS has huge tolerances and a large spread between when it starts conducting and then conducting some more. On the usual logic rail, however, you typically have only 10%-15% margin between operational at full clock speed and *PHUT*

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Regards, Joerg 

http://www.analogconsultants.com/