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TLDR; how to make a MIDI output withstand shorting it to \$\pm12\$V?

I'm designing a MIDI output in a eurorack modular synth module on a TRS jack (this is a semi-standard non-standard way to save space), where it's quite possible for the user to accidentally plug in anything between \$\pm12V\$ to the output. I obviously need to protect the output, while keeping the circuit maximally compatible with other MIDI devices.

The standard MIDI output circuit adapted to the TRS connector:

schematic

simulate this circuit – Schematic created using CircuitLab

where tip, ring and sleeve are of course the pins of the TRS output connector. The buffer is a 74AHCT1G125 with a 5V supply, which also acts as a level translator.

The problems with this circuit are:

  1. if someone connects \$-12V\$ to the tip, the current through R2 is \$77\$mA, and power is \$1.3\$W, so R2 has to be a rather big resistor, and would anyway get quite hot.
  2. same problem for R1 when UART TX is high and ring is connected to \$-12V\$, and further the buffer has an absolute maximum output current of \$\pm 20\$mA, so even if I'd use high power resistors, the buffer wouldn't survive.
  3. The buffers current limit is also exceeded when ring is at \$+12\$V.

So some slightly more clever protection than just bigger resistors is needed. Here's my current best attempt:

schematic

simulate this circuit

Here the R1 and R2 are 3/4W resistors (so still pretty big). M3 restricts the current if the tip falls below about 2V, limiting the output current to between \$-12\$mA to \$32\$mA, which the resistor can take. M1 does the same for the ring, and M2 limits the current to about \$16\$mA when the ring exceeds \$+5\$V. The voltage divider R3 - R4 is there to bring the threshold of M1 down a bit, so that cathode of D2 is pulled closer to ground. The specific MOSFETs mentioned here are just examples, I'm yet to decide on the exact part numbers.

Now, according to my LTSpice simulations, this should work. However, there's a number of reasons why I'm not perfectly happy with my solution:

  1. I feel like I'm using a lot of parts for something very simple.
  2. As a specific example of the above, I'm tempted to leave out the buffer and just use the MOSFETs to drive the signal. However, I can't seem to figure out how to simultaneously drive the signal, do the level translation from 3.3V, and protect the components, without introducing even more transistors.
  3. The voltage divider generating the gate voltage for M1 is a bit annoying: to bring the -12V and it's ground return to this digital part of the circuit would break my star grounding (as the star point is on the other side of a connector I can't change at this point). I could leave out the voltage divider (i.e. connect the gate of M1 to ground), but in that case the ring doesn't get pulled quite all the way to 0V, and the loop current (assuming a forward voltage of 1.7V for the optocoupler on the receiving end) is only about \$4\$mA, which is just a tad below spec.

So, I'm looking for thoughts on simplifying the circuit, while still keeping the protection and staying comfortably in the MIDI spec?

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The purpose of the 220 Ω resistors is to limit the current to about 5 mA when connected to a receiver that complies with the MIDI specification.

If you replace these resistors with a BJT-based current source/sink, you get the same current limit even if connected to wildly different voltages, and the voltage drop over Q1+R1 and Q3+R3 is smaller so that you can afford to insert diodes to protect against reverse voltages:

MIDI output current and reverse voltage protection

(There are devices that use a MIDI output as power supply and try to draw more than 5 mA; consider making R3 smaller.)

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  • \$\begingroup\$ Did a quick simulation in LTspice, seems to work well for various abusive inputs from -24 to +24 volts on tip and ring. The only thing getting moderately hot are Q1 and Q3 but not more than 100 mW if you keep it under ±12V. Awesome! \$\endgroup\$ – pipe Feb 19 at 15:01
  • \$\begingroup\$ Also did the LTspice sim, and seems great! Now, since I have non-inverting buffers on my BOM anyway, I moved the signal input to the ground side of R4, that shouldn't bring any problems? At least the simulation suggests that everything still works \$\endgroup\$ – Timo Feb 20 at 9:42
  • \$\begingroup\$ Not every MIDI receiver uses the circuit from the specification; many cheap USB/MIDI adapters have no optocoupler and measure the voltage at pin 5, so you should use low-side switching, if possible. (The inverter doesn't need to be fast; it could be implemented with a PNP and two resistors.) \$\endgroup\$ – CL. Feb 20 at 10:00
  • \$\begingroup\$ @CL. thanks, good point! I wrote a similar question concerning the input side, check out electronics.stackexchange.com/questions/423360/… if you're interested in giving it a go! \$\endgroup\$ – Timo Feb 20 at 10:41
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Do you have a clamp on the 5V supply? Most regulators will not sink current, so you may have a problem there.

If you use a voltage source in your simulation that problem won't show up until you build it.

Consider a couple of MOSFETs such as BSS139 in series for the buffer output protection.

schematic

simulate this circuit – Schematic created using CircuitLab

They're about 60 ohms max (25 ohms typical). That's just a concept, you need to do the sums.

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  • \$\begingroup\$ good point about the sinking, I hadn't thought of that! Concerning the MOSFETs for protection: the effective Ron is highly variable, right? That's going to be a problem since the MIDI output is a current loop, so a difference of 35 ohms can make a pretty big difference... \$\endgroup\$ – Timo Feb 19 at 13:12
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    \$\begingroup\$ @Timo Old question but I just revisited it for a project and read this comment. If you implement a real current loop, then it doesn't matter much if it's 25 or 60 Ohms - the same current will flow anyway and the receiver will see exactly the same signal. You just lose a few hundred mV of headroom for 5 mA of current. \$\endgroup\$ – pipe Sep 1 at 23:11
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    \$\begingroup\$ @pipe yes, you mean what CL's answer suggested? That's what I ended up doing :) \$\endgroup\$ – Timo Sep 3 at 6:42

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