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I'm building something that would benefit from a midi-clock input, but also a analog clock input. In order to save space, i'd like to use a mini-jack with an adapter for the midi cable (as seen in the Arturia Beatstep pro for example) and have it double as a analog clock / gate input.

Seeing some synths have gate signals that go up to +15v, I'd need some protection in order not to fry the opto-coupler mandated by the midi input. A simple resistor in front of the ir-led won't do the trick: the forward current will either be too low in midi-mode, or too high in +15v gate mode.

I guess the capacitance of a zener prevents me from using that as voltage protection, because it will be too slow for the midi signal. Maybe there is some other thing I overlooked?

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    \$\begingroup\$ How about a normal diode in series with a Zener? \$\endgroup\$
    – supercat
    May 8, 2017 at 20:29
  • \$\begingroup\$ My guess is a single ESD-protection diode has a low enough capacitance and high enough recovery speed to act as a voltage protection. \$\endgroup\$ May 8, 2017 at 20:45
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    \$\begingroup\$ You can get special low capacitance TVS diodes. \$\endgroup\$
    – pjc50
    May 8, 2017 at 21:01
  • \$\begingroup\$ Tip tangent: those MIDI DIN-to-3.5mm adapters don't appear to all be the same pinout.... \$\endgroup\$
    – Daniel
    May 8, 2017 at 21:45

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There are special low-capacitance TVS diodes, but they are designed for transients, not for continuous current.

HP's Optoelectronics Application Manual suggests to limit the current with a constant-current source in front of the optocoupler (section 3.6.1.2):

optocoupler with constant-current source

However, with MIDI, there is not enough voltage for this to work. (The 220 Ω resistor in the standard circuit drops about 1.1 V.) So what you have to do instead is to shunt the surplus current:

optocoupler with active voltage clamp

This is the equivalent of a Zener; you still need another resistor to limit the overall current.

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  • \$\begingroup\$ Thanks, this seems like it should work. I'm not too good with analog stuff, but I can simulate this to find out what parts and values would work nicely. \$\endgroup\$ May 9, 2017 at 16:37
  • \$\begingroup\$ actually, coming back to this, I first simulated and then built the current source solution and it works like a charm! \$\endgroup\$ Jul 4, 2019 at 1:08

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