[sdiy] MIDI input resistor

[Harry Bissell]

sort of..

the resistor at pin 7 does decrease the turn off time, at the expense of lower current gain in the phototransistor. This
reduces the amount of collector current (in saturation) so you need to make the pull-up resistor larger as a side effect
(less current output is available.

The trade off is decided by the designer according to their needs... more speed or more drive.

Optocouplers have a pretty large variation in CTR (current transfer ratio), so much so that they are often 'binned' by this parameter
and you can buy higher gain devices (usually at higher prices). So from manufacturing lot to lot, you might tweak those values...

H^) harry


----- Original Message -----
From: Ullrich Peter <Peter.Ullrich at kapsch.net>
To: Tom Wiltshire <tom at electricdruid.net>, synthdiy diy <synth-diy at dropmix.xs4all.nl>
Sent: Tue, 20 Mar 2012 07:36:22 -0400 (EDT)
Subject: Re: [sdiy] MIDI input resistor

Hi Tom!

The purpose of this resistor is to free the base pin of the output transistor from electrons when switching off with the 
effects that it switches much faster. I first saw this resistor in the maplin midi thru box schematics and made some tests - and it 
works perfectly with these resistors added! 

I built some midi interfaces and midi patchbays over the years and had problems without this resistor on some units. Especially if you 
build a midi chain of lets say 3 or 4 devices as the midi serial pulses get more and more distorted due to slow switching optocouplers.
It is normally no problem if you don't build a chain, but I also add it here.

A midi patchbay/matrix of course needs this resistor for fast switching as adding a patchbay is already a midi chain with 2 units.

And it goes somehow hand in hand with the resistor on Pin6. With the resistor on Pin7 added you can have higher ohm values on the 
output resistor on pin6 (and saving current and maybe reducing negative EMC effects).

Ciao
Peter

http://www.ullrich.at.tt
http://go.to/datadial (Synth addon)


-----Ursprüngliche Nachricht-----
Von: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-bounces at dropmix.xs4all.nl] Im Auftrag von Tom Wiltshire
Gesendet: Dienstag, 20. März 2012 12:26
An: synthdiy diy
Betreff: [sdiy] MIDI input resistor

What's the purpose of the resistor from Pin 7 to ground seen in some 2N138/139 MIDI In optoisolator circuits?

Some schematics have it:

	http://www.ucapps.de/midimon/midimon.pdf

And others don't:

	http://www.sparkfun.com/datasheets/DevTools/Arduino/MIDI_Shield-v13.pdf

We've discussed the various values of the resistor between the output pin 6 and Vcc here before, so I won't go back over that. The summary IIRC was that different manufacturer's versions of the 2N138/139 seem to require different values, so pick something that works between 270R and about 1K8.

Thanks,
Tom



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