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

ShedSynth shedsynth at gmail.com
Wed Jun 16 18:51:40 CEST 2021

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:



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.





-----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,





-----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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