[sdiy] "Expression Pedal" output protection?

[Tom Wiltshire]

On 24 Sep 2011, at 20:50, Mike Beauchamp wrote:

>> What are you worried about? :-)
>> (Smoking your pots?;)
> 
> Haha, thanks Mattias :)
> 
> Just like Rumsfeld, I'm worried about the known unknowns and the
> unknown unknowns... if that helps.

I always thought Rumsfeld got an unnecessarily hard time over that quote. It was one of the few times I agreed with him.

I guess in this case the 'unknown unknowns' are the cases of somebody doing something unutterably stupid like plugging their lightning conductor into their synth, in which case, yes, something/body is going to get smoked and there isn't going to be much you can do about it. Or you *could*, but unless you're building a mil-spec synth, I wouldn't bother.

Instead, you should plan for the known unknowns - things getting shorted to other things (can every pin survive a short to every other?) or unexpected signals getting fed in (can every pin survive a +/-15V rail? An LFO? An oscillator?). If it survives that, it'll be fine for typical use, and it'll cope easily with people being occasionally absent-minded, confused and/or drunk.

With your design, the various pins can all be shorted safely:
22K between +15V and gnd is ok
100K input resistor to +15V is ok
100K input resistor to ground is fine.

And you could feed in any signal up to the rails without any problem beyond perhaps clipping the op-amp. So you're pretty safe to +/-15V. People using the socket to inject signals onto the power rails might be a pain. You could perhaps use an op-amp buffer to provide the +15V signal you use for the pot.

There's always some imaginative idiot who can come up with a brilliant new way to wreck your lovely equipment, but I don't see why we should all design stuff just for them. "I wanted to modulate the sound with a 50Hz sine wave, so I plugged the expression socket to the mains..." doesn't justify (for me) including mains protection on every jack input/output.

Tom