CV/Expression Jack wiring question

[spectraljulian]

Googled and searched the forum, found similar conversations but not one that answers my specific question: 

There are some pedals that allow for Expression or 0-5V CV. 

Generally for expression if it's voltage based you have a Vref on the ring of the jack.  But let's say a user wants to plug in a TS jack from a 0-5CV source.  Now the sleeve is connecting to the ring and sleeve thus grounding out the Vref.  How do you prevent this from causing power issues?  What if I have two expression jacks, each would need its own Vref then right? 

[Ripthorn]

I don't know how it's usually done, but you could put in a current limiting diode so that you never pull more than X mA. I've used them in the far distant past and they worked well.

As for two jacks, the Vref line will always see the EXP resistance to ground, it's the wiper that will see the variable resistance to ground, and thus the returned EXP CV will vary. I don't think you would need two separate Vref sources, you should be able to have them each off the same +5V line. Then again, I've not played with expression inputs much, though my current project does use it, so I'm in the process of learning.

[Govmnt_Lacky]

The Lovetone Meatball has 2 expression pedal jacks. Maybe look into that and it will get you the info you need 

[ElectricDruid]

Googled and searched the forum, found similar conversations but not one that answers my specific question: 

There are some pedals that allow for Expression or 0-5V CV.

There are, and it is often the case that those pedals are based on a microprocessor. The ADC inputs on a processor don't care whether they get 0-5V from a pot wired across Gnd and 5V rails, or whether it comes in directly.

Generally for expression if it's voltage based you have a Vref on the ring of the jack.  But let's say a user wants to plug in a TS jack from a 0-5CV source.  Now the sleeve is connecting to the ring and sleeve thus grounding out the Vref.  How do you prevent this from causing power issues?  What if I have two expression jacks, each would need its own Vref then right?

I did this "expression pedal or CV input" thing on the FilterFX pedal I designed:

R24/220R limits the current that's drawn when the 5V supply gets shorted directly to ground when a TS jack is inserted. 220R on the top of 10K doesn't limit the upper end of the range in a significant way when there's nothing plugged in (pot still goes to 97.8%).

R25/C10 provide a bit of filtering and further current protection for the processor when someone sticks a CV in.

If you had two expression jacks, each jack would have its own "R24" resistor from the power rail to the Ring connection, so yes "its own Vref", kind of. Since if both inputs had TS plugs put into them, the resistors would be in parallel (and therefore the current would double) you might want to reconsider the required R value in such a case.

HTH

[spectraljulian]

The Lovetone Meatball has 2 expression pedal jacks. Maybe look into that and it will get you the info you need 

Based on the McMeat schem it looks like there's a resistor between the jack and Vref/ground.  So I'd want something like this (note the part I'm using appears as a mono switched jack in Eagle, but the actual part is a TRS jack, with R being at the switch connection)

[ElectricDruid]

Based on the McMeat schem it looks like there's a resistor between the jack and Vref/ground.  So I'd want something like this (note the part I'm using appears as a mono switched jack in Eagle, but the actual part is a TRS jack, with R being at the switch connection)

Yep, that's the same basic idea as the 220R I used. 1K from 5V to Ring reduces the current that flows when a TS plug is inserted with CV. And depending on the value of the expression pedal, 1K top and bottom doesn't really hit the range much. Putting resistors top and bottom keeps it symmetrical, I suppose. Depending what the pedal is, it might not matter at all - many don't cover the full track of the pot anyway.

[spectraljulian]

It occurred to me that the sleeve's going to ground to the enclosure anyways, so the ground resistor wouldn't do anything. 

[ElectricDruid]

It occurred to me that the sleeve's going to ground to the enclosure anyways, so the ground resistor wouldn't do anything.

The sleeve *doesn't have to* ground to the enclosure though, does it? If you use a plastic jack for the expression/CV input, you can use the "top and bottom" style resistors like you've shown, and then the range of the pot gets affected only slightly at both ends, instead of all off one end like I did it.