Expression pedal socket question - preventing/protecting against shorts

[ElectricDruid]

Hi all,

I'm thinking of adding an expression pedal input to my FilterFX box that I'm developing over in this other thread:

http://www.diystompboxes.com/smfforum/index.php?topic=119927.0

Now, this shouldn't be too difficult since the STOMPLFO chip accepts 0-5V control voltages to control the parameters. Normally you just wire a pot between 0V and 5V and feed the wiper output to the STOMPLFOs pins - simple.

I was thinking I would be able to use a TRS/stereo jack to interrupt this normalled connection and allow an expression pedal to take over from the internal knob. Something like this:



With nothing inserted, the ring provides 5V to the top the pot, the sleeve and the bottom of the pot are grounded, and the signal goes back to the LFO from the tip.

This ought to work well. But then I thought: what if someone inserts a mono jack by accident? The jack plug sleeve will short the sleeve and ring on the expression input, basically shorting my +5V supply to ground. Ouch!

What do I do about this? Sticking some resistor in series with 5V to limit current will also limit the upper range of the expression pedal, depending on the pedal's value (worse with 10K pedal than 100K, for example).Is there some way I can only supply the 5V if the pedal resistance is sufficiently large? Or can I limit the current to just a fraction of a mA without limiting the voltage?

Ideas please! I'd like to have this expression input, but I'm a bit stuck.

Thanks,
Tom

[Digital Larry]

Series resistor is the simple way to go, I do 47 ohms on a 3.3 v box.  3.3 ^2 / 47 = 0.23 watts, OK you might want to bump it up a little to give some margin on full blown power dissipation for 15 minutes while you or person scratches their head in wonder as to why it isn't working. 

100 ohms on the end of 10,000 isn't giving up much.

There's probably a way to do current limiting with less drop but that's beyond me at the moment.  Op-amp with small current sense resistor and FET with really low Rds(on)?

[ElectricDruid]

Hi Larry,

Ok, I think you've convinced me. I should have done the sums before posting...

You're dead right though. I'm on 5V so I've got Watts = V^2/R = 5^2/100 = 0.25W, which isn't bad. 220R for utterly bulletproof. Even at that, I only lose 2.1% of the top the range with 10K. So that's fine.

Thanks very much, I knew someone round here would have done this before.

Tom

[electrosonic]

If the pedal is powered by 9v, why not use an lm358 powered by the 9v supply to buffer the 5v that is supplied to the expression pedal. That limits the short circuit current to something like 15mA and prevents the user from shorting  out the 5v supply to ground.

Andrew.

[anotherjim]

You definitely have to fit a current limit, although there is an EP standard that puts +supply on the tip and control on the ring & a mono jack can't damage that.
Also, with +supply on ring, the EP pot can be damaged if tip-ring is low resistance while the plug is inserted (tip gets +v & ring gets ground).

[ElectricDruid]

I was aware that not all expression pedals adopt the same wiring, but from my researches it *looked* like ground on sleeve, +v on ring, signal on top was commonest, even if it's not the most sensible.
Do have any idea on the relative frequency of these different wirings, Jim? Maybe I shouldn't worry and just adopt the "best practice" wiring. That seems to be the one you suggest, although the other is a logical enough extension from the usual layout, even that doesn't make for the best result.

Tom

[anotherjim]

Answering for 3rd time (due to forum move?)..
Yamaha and Zoom use Tip+ but I've no idea how prevalent that standard is overall. It seems many keyboard players prefer the Yamaha FC-7 for cost/performance and either mod them to Ring+ or use an adapter cable when needed.
M-audio fit a DPDT in the pedal so it can do either, but that seems to be beyond the rest.