[sdiy] MIDI with +3.3V supply

David Brown davebr at earthlink.net
Sun Nov 16 00:18:21 CET 2008

Not a lot of experience but some ...

I designed a MIDI processor that is self-powered from the MIDI 
signal.  The power supply is unregulated but is around 4 volts or 
so.  I dropped the two output resistors to 150R.

In your calculation, remember that the driver doesn't pull the output 
to 0 volts but more like 0.3 volts.  (4 - 0.3 - 1.3) / (150+150+220) = 4.6 mA

It's worked on everything I've used it with.  Not a single problem.


At 02:43 PM 11/15/2008, Seb Francis wrote:
>I'm just designing the MIDI I/O and am having to slightly re-think 
>the way I normally do things as I'm using 3.3V power.
>MIDI is a 5mA current loop.
>Approx 1.3V is lost in the opto-isolator LED.
>With 5V power, that leaves 3.7V which means a resistance of 740R is 
>needed in the loop.
>So allowing for a bit of loss in the MIDI cables, etc. the normal 
>practice seems to be to use 3x 220R resistors, 1 on the input and 1 
>either side of the output.
>But I'm not sure if this is an actual standard that there should be 
>220R on the input side?  All the MIDI spec information I've seen 
>just states that it's a 5mA current loop, but not how the resistance 
>should be divided between input and output.
>Does anyone know the truth of this?
>Assuming that all MIDI inputs do indeed have 220R and ~1.3V 
>opto-isolators, then when driving an output with only 3.3V one 
>should presumably use something like 2x 91R resistors (or a bit 
>less) to get a 5mA current loop.  This would seem to offer less 
>protection if someone plugged an output into an output.  (Actually 
>I've just noticed that the new dsPIC33Fs have an absmax of 4mA per 
>I/O pin - was 25mA in older PICs and dsPICs - so probably necessary 
>to use a buffer on the UART output now.)
>Anyone have any experience doing MIDI with 3.3V?
>I'm assuming the output side of a 6N138 is going to work fine with 
>3.3V - I couldn't see anything in the datasheet which suggested 
>otherwise, although they do talk about 5V in the examples.
>By the way, what do you do with the Vb pin?  I've always connected 
>it to ground via a 1K5 resistor, but the datasheet shows it as unconnected.

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