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

[S Ridley]

Many voltage regulators suggest adding a diode from output to input to
protect them against a higher voltage on the output than the input.  The
LM1117 datasheet (used in Arduinos) has this (on page 14):

https://www.ti.com/lit/ds/symlink/lm1117.pdf

but Arduinos don't have this AFAIK.

If you only have a problem with one input, you could try a 4.7v zener after
your 1k input resistance - that would allow sufficient voltage to read as
high, but shouldn't have the possible leakage current problem of slapping a
5.1v zener across the 5v supply.

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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On Fri, 28 May 2021 at 17:59, ShedSynth via Synth-diy <
synth-diy at synth-diy.org> wrote:

> I have an Arduino-based module, with a GATE input protected by 1K input
> resistor then clamping diodes to 0V & +5V.
>
>
> https://shedsynth.files.wordpress.com/2021/05/adsr-a-schematic-gate-to-d2.png
>
> I find that with power off to that module any GATE or CV to the input can
> pull up the module's +5V through the diode and even power the whole case's
> +5V rail.
>
> The clue was seeing the case’s +5V LED flashing in time with the GATE
> input.
>
>
>
> And I think that if the input is more than +5V, the un-powered voltage
> regulator will not be able to regulate so digital components (and perhaps
> the voltage regulator itself) might be damaged.
>
> Have I understood this correctly?
>
>
>
> I'm wondering whether to put a 5.1V zener across the module's 0V and +5V
> to catch any over-voltage.
>
> Have I understood THAT correctly?
>
>
>
> Thanks in advance,
>
> Al
>
>
>
> SHEDSYNTH <https://shedsynth.wordpress.com/> – home-knitted eurorack
>
>
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