[sdiy] Protection Device?

[Jason Proctor]

fwiw, and remember this is #amateur_mode here, this is how i implemented
clamping for my Arduino module. i ran my rail to rail opamps (LT1013 IIRC)
at +/-5, scaled to +/-2.5v, biased up by 2.5v, so the uP saw 0-5v. the
firmware considered anything below 2.5v as "negative".

for digital ins i just used the same opamps in a straightforward comparator
config.



On Tue, Jun 19, 2018 at 2:48 PM, O Gillet <ol.gillet at gmail.com> wrote:

> > The inputs must be clamped that range.
>
> The circuit I have posted clamps all voltages below -8V to 3.275V, all
> voltages above +8V to 0.025V, and is linear in-between. You can change
> the resistor values to get other scale/offset ratios.
>
> Unless you're building something like a development board, I don't see
> a case for directly exposing MCU pins to the external world. What
> you're sending to those MCU pins is very likely to need some
> conditioning anyway (filtering, scale/offset) - so the idea is to use
> a single-supply rail to rail op-amp powered by the same voltage as
> your MCU to get the clamping "for free".
> On Tue, Jun 19, 2018 at 11:40 PM Tim Ressel <timr at circuitabbey.com> wrote:
> >
> > the issue is this: the uP input cannot tolerate voltages much beyond its
> > rails, so the max range is -0.3V to +3.6V (in my case). The inputs must
> > be clamped that range.
> >
> > --timmers
> >
> >
> > On 6/19/2018 2:22 PM, O Gillet wrote:
> > > The CV input circuitry of pretty much all my modules look like this:
> > > https://imgur.com/a/Xz5YN0X
> > >
> > > The op-amp has R2R outputs and is single-supply, powered by 3.3V.
> > >
> > > Benefits:
> > >
> > > - The V- input of the op-amp is a summing point - you can attach there
> > > as many CV inputs or pots as you want. It's very helpful in some
> > > situations to have both a CV input and a pot to control a parameter!
> > >
> > > - If you connect an external signal straight to an MCU ADC input, the
> > > range of the CV will be necessarily equal to the range of the
> > > microcontroller ADC input. It's probably OK if you use a +5V
> > > microcontroller and want 0V to correspond to the minimum value of the
> > > parameter, +5V to the maximum value of the parameter... But what if
> > > you want the range of the parameter to be -5V to +5V ; or 0V to +8V?
> > > Or what if your microcontroller is powered by +3.3V? The schematic I
> > > have posted covers all these cases - you just change the resistors to
> > > get different scale/offset values.
> > >
> > > - The capacitor acts as a 1-pole low-pass filter which removes some of
> > > the high frequencies in the CV signal - providing cleaner readouts.
> > >
> > > - ADCs do not like being driven from sources with high ouptut
> > > impedances. For example, the ADCs on the AVR want a source impedance
> > > of 10k or lower - to rapidly charge the S&H capacitor which is part of
> > > the ADC circuit. The output of the op-amp works as a very low
> > > impedance source for the ADC - and will simultaneously leave you in
> > > control of the input impedance of your module (100k is an implicit
> > > standard for Euro modules). Standardizing all input impedances to 100k
> > > is great - it allows consistant behaviour when using passive modules.
> > >
> > > - Of course the op-amp input will never "see" extreme external
> > > voltages, because you're not directly exposing any gate to the
> > > external world - every input voltage goes through the 100k resistor.
> > > If you connect +50V to the CV input, well, that's only 0.5mA flowing
> > > through the input resistor (and 25mW dissipated by the resistor).
> > >
> > > So yes, an op-amp might look a bit too much, but it kills a whole
> > > flock of birds with one stone.
> > > _______________________________________________
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> > >
> >
> > --
> > --Tim Ressel
> > Circuit Abbey
> > timr at circuitabbey.com
> >
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