[sdiy] Will clamping diode work when power is off?

[John Luciani]

Rather than a transistor + resistor I would use a diode connected
between +5V (cathode) and Arduino input (anode). I also like to
split the series resistance so that one resistor will limit the
current from the input through the diodes and the second limits the current
from
a clamped condition into the Arduino input.

John L

On Wed, Jun 16, 2021 at 12:53 PM ShedSynth via Synth-diy <
synth-diy at synth-diy.org> wrote:

> I hope you won't mind me exhuming my old question again.
>
>
>
> My friend Tim suggested using a BC557 to actively clamp the input to the
> supply rail:
>
>
> https://shedsynth.files.wordpress.com/2021/06/adsr-a-schematic-gate-to-d2-2-2.png
>
>
>
> If the voltage on the emitter goes above the voltage on the base (+5V when
> the module is powered, 0V when not powered) the transistor will conduct to
> ground, clamping the input.
>
> I believe it will clamp to Vbe (about 0.7V) above the base voltage, so I
> planned to use a preset resistor P1 to set the clamp within the safety
> limit of the Arduino inputs (VDD + 0.5V).
>
>
>
> I’d be interested in comments, particularly about behaviour when the
> circuit isn’t powered – the original question.
>
>
>
> Thanks,
>
> Al
>
>
>
> -----Original Message-----
>
> From: ShedSynth <mailto:shedsynth at gmail.com>
>
> Sent: 30 May 2021 09:16
>
> To: mailto:synth-diy at synth-diy.org
>
> Subject: RE: [sdiy] Will clamping diode work when power is off?
>
>
>
> Thank you to everybody for the replies.
>
>
>
> I have previously experimented with a 5.1V Zener across Arduino inputs:
> GATE inputs triggered successfully but CVs appeared not to be read linearly
> by the analog inputs.
>
> I'll experiment with clamping each input through diodes to a Zener to 0V,
> if I understood that suggestion correctly.
>
>
>
> I'll experiment to measure current through a Zener across 0V and +5V when
> the module is powered normally.
>
>
>
> I hadn't understood the reason for the diodes across voltage regulators.
>
>
>
> The use-case is that I have two boxes which share the same 19V DC supply
> and are normally switched on and off together as a single unit.
>
> I noticed the issue when I switched off the second box to fiddle with a
> module under test, while it was still connected to the CV from an LFO
> module in the first box.
>
> To me this seems like a common risk when fiddling, less so when actually
> playing any music (and for me that ratio is about 90/10%).
>
>
>
> I want to mitigate against likely mistakes in the expected range -12V to
> +12V.
>
>
>
> So far I seem to have got away with it, although I'm now trying to
> remember when and how I broke the voltage regulator in the only Arduino
> that has failed on me so far - that still works on USB power, not on +12V.
>
>
>
> Spotting the issue, and your replies, has helped me understand some
> fundamentals.
>
> Thank you all again,
>
> Al
>
>
>
>
>
>
>
> -----Original Message-----
>
> From: Synth-diy <mailto:synth-diy-bounces at synth-diy.org> On Behalf Of MTG
>
> Sent: 29 May 2021 17:46
>
> To: mailto:synth-diy at synth-diy.org
>
> Subject: Re: [sdiy] Will clamping diode work when power is off?
>
>
>
> This is a cool idea, but I'm curious about the OP's problem. What's the
> use-case here where part of the rig is powered? Is this just power-on-off
> timing situation or is powering part of the system a normal part of using
> it?
>
>
>
>
>
> On 5/29/2021 5:10 AM, S Ridley via Synth-diy wrote:
>
> >
>
> > Another option is to invert the gate in the software, use the pin's
>
> > internal input pull-up and use an npn transistor to switch it to 0v.
>
> > I suppose current might flow through the BC junction if the power is
>
> > off, but you'd be using a much higher base resistor on the input so
>
> > current through that route can be minimised.
>
> >
>
> > Steve
>
> >
>
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