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Topic: resistors for midi out (Read 338 times) previous topic - next topic

rrrrrrr

hello

i am planning to build a midi out circuit using a teensy 3.5 following this:
https://www.pjrc.com/teensy/td_libs_MIDI.html
i have a potentially silly question: the 2 resistors are labelled "47" on the schematic, but the ones i see on the photo of the board there do not look like 47 ohm resistors. any clue whet 47 means, then? or are they actually 47 ohm ones?

thanks so much

Grumpy_Mike

There are two diagrams with different resistors depending on what Teensy you have. But only one photograph showing the physical layout. So how do you know what diagram corresponds to what photograph.

Anyway there is a Teensy forum that the Teensy creator regularly contributes to so you are best asking there.

Paul is a member here also but is not such a frequent visitor.

slipstick

47 Ohms for a 3.3V system. 220 Ohms for a 5V system. That's because MIDI is not voltage but current driven and has to take account of the LED voltage drop in the MIDI Input opto-isolator that it will be driving.

47 Ohms is actually a bit high, the official MIDI spec says 33 and 10 but I guess 47 and 47 will work in most cases.

Steve

Grumpy_Mike

Yes but the resistors in the photograph behind the MIDI out looked awfully like 22R.

slipstick

It's true they do look that way but in that case the resistors on the input are also completely wrong (22/27 - should be 220/270). So maybe the black is really a dark brown?

And isn't that exactly why we prefer (and trust) properly notated circuit diagrams rather than photos or Fritzings?

Steve

Grumpy_Mike

And isn't that exactly why we prefer (and trust) properly notated circuit diagrams rather than photos or Fritzings?
Exactly, my money is on the value in the schematic.

JohnLincoln

#6
Sep 01, 2018, 10:16 pm Last Edit: Sep 01, 2018, 10:22 pm by JohnLincoln
If you refer to the MIDI Manufacturers Association Specifications,
you will see that the original specification (for 5V operation) had two 220Ω resistors in the output circuit.
The MIDI input circuit had one 220Ω resistor in series with the LED in an opto-isolator.
Thus when a MIDI output is connected to a MIDI input, there is a total of three 220Ω resistors in series, which allow a current of 5mA to flow through the opto-isolator LED.



When the MIDI specification was updated in 2014 to cater for 3.3V circuits, the 220Ω resistors in the output circuit were reduced to 33Ω and 10Ω for 3.3V operation.
The 220Ω in the input circuit remained unchanged. (So that it will work correctly from both 5V and 3.3V outputs)



The two 22Ω resistors shown in the photo, are close in value to the 33Ω + 10Ω in the specification.
If two 47Ω resistors are used as per the schematic, then the current will be reduced by about 15% (remember that there is another 220Ω in series), but is still likely to work.

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