Protection of analog pins from over voltage

I have been reading that it is possible to add a 5.1V zener diode and a 220R resistor to the analog pins to provide extra over voltage protection.

It there any reason why you couldn't use a 'rubber zener' / 'Vbe multiplier' in place of the zener diode?

'rubber zener' / 'Vbe multiplier' ??

Rubber zeners, hmmm --> ???

According to info Crossroads Bob posted in the past, the internal clamping diodes
in the ATmega chips can handle at least 5mA. Therefore, another possibility might
be to use just a 4.7K series-R in the ADC lines. This would protect to over 25V,
and shouldn't affect the ADC readings since the datasheet says

RAIN Analog Input Resistance 100 Mohms

A rubber zener will protect against overvoltage, but unlike a real zener will not protect against reverse voltage, and it will not react immediately to fast transients because of the collector-base capacitance and the switching speed of the transistor.

Unless you are measuring signals that change rapidly, a series resistor is all you need. For example, 22K will protect against about +/- 40V. The important thing is to limit the current into the internal pin protection diode to 2mA or less (I normally design for a few hundred microamps at most).

[EDIT: oric_dan got there first. Not sure where the figure of 5mA came from, I've seen 2mA in an Atmel application note.]

Yeah, I seem to recall Bob saying 5 mA in the past, but maybe the memory is
fluttering here too, LOL.

Yeah, I seem to recall Bob saying 5 mA in the past, based on some appnote, but
maybe the memory is fluttering here too, LOL.

I don't think I've ever posted 5mA. I would never post that one should rely on using the internal clamping diodes as protection.

Voltage should not exceed Vcc by more than 0.5V, or go below Gnd by more than 0.5V.

That's why we always quote what the "other" guy said, ;-). Works better if he's
not around too. I did a little searching and came up with this doco, which says
1 mA at the top of page 4.

'rubber zener' / 'Vbe multiplier' ??

One of these things:

Given this from the Ruggeduino site:

I was thinking of simply replacing the 5.1V zener with a Vbe multiplier since I have a heap of 2SC1815s and trim pots I could use.

For reverse current protection, what about a 1n5819 in series with the resistor, I also have a heap of those.

I was surprised to find that zener diodes required a certain minimum amount of current flow across them before they would limit voltage levels.

For reverse-polarity protection, the zener will turn on in the forward direction,
but will have no current-limiting with the ckt shown.

The app note say 1mA, and recommends 1M resistor.
I suppose for 240VAC, 240K would be okay then.
I think I'd still run mains thru a transformer, knock it down to some non-lethal voltage.