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

[ShedSynth]

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 <synth-diy-bounces at synth-diy.org> On Behalf Of MTG
Sent: 29 May 2021 17:46
To: 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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