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

Mr&MrsAccount hbissell at wowway.com
Sat May 29 00:26:31 CEST 2021

Yes you are basically correct.
The "unpowered" regulator issue is misunderstood. It (almost always) can only source current, not sink it. So the excess voltage coming through the upper clamp diode never goes "back" into the regulator but "forward" into your circuit.  The current available is the difference of the input voltage to the gate, and the nominal supply voltage, across the input resistor (1K). so if you put 15V at the input, there is 10V across the 1K resistor = 10mA extra current.
This might even be enough to run your circuit (as long as the gate is high)
Another issue with putting the zener clamp across the power supply is tolerances.  The 5.1V zener might always draw a little current from the supply if the supply is on the "high side" and the zener clamp is on the "low side".   In addition, if the supply were to go high because of a malfunction, the zener could get very hot unless it were able to dump all the excess power as heat.
Another technique is to use the zener on the input right after the input diode.  For a positive voltage it will clamp at the zener voltage, and the inherent reverse diode will stop a negative input from going more than 0.7V below ground.
If you expect unpredictable input voltages this is the way to go.  If you want to protect all the inputs at once, try using the "high side" clamp diode but instead of running it to the power supply, run it to a single zener instead. Then you can share the single zener between all inputs.  But watch out, if ALL the inputs are overvoltage, the input resistors end up in parallel and contribute to the power the zener needs to dump.  Doing it this way means you never risk an overvoltage on your system power supply.
The two clamp diodes to the power supply rails is more intended to prevent against a transient overvoltage only.  In this case the energy from the overvoltage transient dumps into the system power supply capacitors, which are usually chosen large enough that the spike doesn't raise the supply voltage too much, thinkof dumping a glass of water into an already full bathtub.  Not much change is going to happen to the water level.
----- Original Message -----
From: ShedSynth via Synth-diy <synth-diy at synth-diy.org>
To: synth-diy at synth-diy.org
Sent: Fri, 28 May 2021 12:57:33 -0400 (EDT)
Subject: [sdiy] Will clamping diode work when power is off?


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-->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.pngI 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 – home-knitted eurorack 
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