[sdiy] "Expression Pedal" output protection?

[Harry Bissell]

Reminds me of the time when an industrial customer wanted to add "protection"

There was a transformer overtemp circuit on 24VDC, sharing a cable with a 600V
1.2KHz square wave inverter output capable of delivering several hundred amps.

The cable would break due to wear at the end of a robotic actuator, as the cables
are expensive and time consuming to replace, the typical maintenance approach is to
cut the cable shorter at the end of the actuator and reconnect.

Each repair makes the cable a little shorter...

Eventually the robot makes a full length reach, rips the cable out and the 600V shorts to
the 24VDC circuit, blowing the entire thing to kingdom come...

Yes, if you wanted to, you could make it fail-safe. It would not, however, be cost competitive with
just replacing the cable correctly the first time. How to do it is left as an exercise for the reader :^)

H^) harry




----- Original Message -----
From: Tom Wiltshire <tom at electricdruid.net>
To: Mike Beauchamp <mikebeauchamp at gmail.com>
Cc: sdiy DIY <synth-diy at dropmix.xs4all.nl>
Sent: Sun, 25 Sep 2011 12:31:22 -0400 (EDT)
Subject: Re: [sdiy] "Expression Pedal" output protection?


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



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-- 
Harry Bissell & Nora Abdullah 4eva