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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">It will clamp. But it can also sometimes form a relaxation oscillator with any capacitance on the output if there is an external voltage there.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> Synth-diy [mailto:synth-diy-bounces@synth-diy.org]
<b>On Behalf Of </b>ShedSynth via Synth-diy<br>
<b>Sent:</b> 16 June 2021 17:52<br>
<b>To:</b> synth-diy@synth-diy.org<br>
<b>Subject:</b> Re: [sdiy] Will clamping diode work when power is off?<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoPlainText">I hope you won't mind me exhuming my old question again.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">My friend Tim suggested using a BC557 to actively clamp the input to the supply rail:<o:p></o:p></p>
<p class="MsoPlainText"><img width="345" height="183" id="Picture_x0020_1" src="cid:image002.jpg@01D762D9.48F41520"><o:p></o:p></p>
<p class="MsoPlainText"><a href="https://shedsynth.files.wordpress.com/2021/06/adsr-a-schematic-gate-to-d2-2-2.png">https://shedsynth.files.wordpress.com/2021/06/adsr-a-schematic-gate-to-d2-2-2.png</a><o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">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.<o:p></o:p></p>
<p class="MsoPlainText">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).<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">I’d be interested in comments, particularly about behaviour when the circuit isn’t powered – the original question.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">Thanks,<o:p></o:p></p>
<p class="MsoPlainText">Al<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">-----Original Message-----<o:p></o:p></p>
<p class="MsoPlainText">From: ShedSynth <mailto:shedsynth@gmail.com> <o:p></o:p></p>
<p class="MsoPlainText">Sent: 30 May 2021 09:16<o:p></o:p></p>
<p class="MsoPlainText">To: mailto:synth-diy@synth-diy.org<o:p></o:p></p>
<p class="MsoPlainText">Subject: RE: [sdiy] Will clamping diode work when power is off?<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">Thank you to everybody for the replies.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">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.<o:p></o:p></p>
<p class="MsoPlainText">I'll experiment with clamping each input through diodes to a Zener to 0V, if I understood that suggestion correctly.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">I'll experiment to measure current through a Zener across 0V and +5V when the module is powered normally.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">I hadn't understood the reason for the diodes across voltage regulators.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">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.<o:p></o:p></p>
<p class="MsoPlainText">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.<o:p></o:p></p>
<p class="MsoPlainText">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%).<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">I want to mitigate against likely mistakes in the expected range -12V to +12V.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">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.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">Spotting the issue, and your replies, has helped me understand some fundamentals.<o:p></o:p></p>
<p class="MsoPlainText">Thank you all again,<o:p></o:p></p>
<p class="MsoPlainText">Al<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">-----Original Message-----<o:p></o:p></p>
<p class="MsoPlainText">From: Synth-diy <mailto:synth-diy-bounces@synth-diy.org> On Behalf Of MTG<o:p></o:p></p>
<p class="MsoPlainText">Sent: 29 May 2021 17:46<o:p></o:p></p>
<p class="MsoPlainText">To: mailto:synth-diy@synth-diy.org<o:p></o:p></p>
<p class="MsoPlainText">Subject: Re: [sdiy] Will clamping diode work when power is off?<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">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?<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">On 5/29/2021 5:10 AM, S Ridley via Synth-diy wrote:<o:p></o:p></p>
<p class="MsoPlainText">> <o:p></o:p></p>
<p class="MsoPlainText">> Another option is to invert the gate in the software, use the pin's
<o:p></o:p></p>
<p class="MsoPlainText">> internal input pull-up and use an npn transistor to switch it to 0v.<o:p></o:p></p>
<p class="MsoPlainText">> I suppose current might flow through the BC junction if the power is
<o:p></o:p></p>
<p class="MsoPlainText">> off, but you'd be using a much higher base resistor on the input so
<o:p></o:p></p>
<p class="MsoPlainText">> current through that route can be minimised.<o:p></o:p></p>
<p class="MsoPlainText">> <o:p></o:p></p>
<p class="MsoPlainText">> Steve<o:p></o:p></p>
<p class="MsoPlainText">> <o:p></o:p></p>
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