[sdiy] MIDI phantom power...over 5 pin MIDI connector ?

[eidorian at aladan.net]

Thanks Brian, that's helpful to know.  I figured I must be missing 
something there!

Cheers,
A.


On 2015-09-10 17:48, rsdio at audiobanshee.com wrote:
> Hi A.
>
> 5 V into 660 ohms is not 7.5 mA, because the opto-isolator LED
> accounts for an additional voltage drop. The 6N138 has a typical
> forward voltage of 1.3 V, but it can be as high as 1.7 V or 1.75 V
> depending upon temperature or current. Thus, 5 V into 660 ohms with a
> 6N138 in the current loop works out to be 5.6 mA typically, or 
> exactly
> 5 mA at high temperatures. They probably planned it that way so that 
> 5
> mA would be the minimum, but it would only go only about 10% above
> that.
>
> 3.3 V wouldn't really produce 5 mA, but more like 3 mA (due to the
> LED drop) or even as little as 2.4 mA.
>
> Those 20 mA ratings are Absolute Maximum, not recommended for normal
> use. Usually, a data sheet will recommend typical values after the
> absolute maximums. In the specific example of the 6N138, the device
> operates on as little as 0.5 mA, so there's not really much need to
> exceed the standard MIDI value of 5 mA. MIDI is already designed to
> send 10 times the minimum current required for operation.
>
> But, you do have a point, A. We need to ask JP what he really wants
> to solve here. There might be a way to solve the problem without 
> "more
> power."
>
> Brian Willoughby
> Sound Consulting
>
>
> On Sep 9, 2015, at 11:40 PM, eidorian at aladan.net wrote:
>> I just re-read the original post, and it might be that JP isn't 
>> asking how he can get power from a MIDI cable, he might actually be 
>> asking a subtly different question: how can he *send* more power over 
>> the two MIDI pins (presumably without affecting the ability of those 
>> pins to transmit MIDI to older devices at the same time).
>>
>> If it really is the case (that you have control over the MIDI-out) 
>> then maybe you could run the current loop at higher than 5mA.  I 
>> looked at data sheets for PC900, 6N137, 6N138 and 6N139 and they all 
>> have various maximum rated If(avg) of 20mA or greater, so something 
>> like 15mA should be safe.  Much more than that and you run the risk of 
>> burning out MIDI-in opto-couplers on older gear.
>>
>> http://www.midi.org/images/midihw.gif
>>
>> 15mA from a 5V supply would require replacing each of the MIDI-out 
>> 220 ohm resistors with 56 ohm.
>>
>> Interestingly, 5V into 660 ohms is already 7.5mA.  But 3.3V into 660 
>> ohms is 5mA!
>>
>> But yeah, there's no fun to be had doing this.  Any device that 
>> requires your "high powered" MIDI-out ports for power is going to 
>> damage your "normal" gear if you ever plug it in accidentally :-(
>>
>> Cheers,
>> A.
>>
>>
>> On 2015-09-10 15:11, rsdio at audiobanshee.com wrote:
>>> Hi JP,
>>>
>>> I would strongly recommend against designing any product that pulls
>>> power from the MIDI cable. MIDI simply wasn't designed for that, 
>>> and
>>> if you try to take advantage of the vague power that's there, 
>>> you'll
>>> still run into many devices which don't work. The link that A 
>>> provided
>>> below has long lists of incompatible MIDI devices (because MIDI
>>> Solutions is deviating from the standard by pulling power where 
>>> it's
>>> not supposed to be used).
>>>
>>> That said, the best clues (if you're going to do this anyway) come
>>> from the MIDI Specification.
>>>
>>> You can see that pin 4 of all MIDI outputs is supposed to be tied 
>>> to
>>> 5 V through a 220 ohm resistor. I believe that A is wrong in saying
>>> that you can get only 5 mA, because the 5 mA limit is the amount of
>>> current that is supposed to flow when a MIDI bit is being 
>>> transmitted.
>>> That 5 mA flows out of pin 4 of the MIDI Out, in pin 4 of the MIDI 
>>> In,
>>> through the optoisolator, out pin 5 of the MIDI In, and in pin 5 of
>>> the MIDI Out. That makes the total circuit.
>>>
>>> However, if you make a different circuit path for current to flow,
>>> then you can potentially pull much more than 5 mA from the +5 V 
>>> supply
>>> of the MIDI Out device. This current would flow out of pin 4 of the
>>> MIDI Out, in pin 4 of your MIDI In, through your charge pump, and 
>>> then
>>> out pin 2 of your MIDI In, before finally flowing in pin 2 of the 
>>> MIDI
>>> Out. Of course, the more current you pull, the lower the voltage - 
>>> but
>>> that shouldn't be a problem for your charge pump. At 5 mA, the 
>>> voltage
>>> you see will be 3.9 V, but if you double the current to 10 mA then
>>> you'll still have 2.8 V. I've used the MAX1595 charge pump (not for
>>> MIDI, but for USB-MIDI) in products, and it works with as little as
>>> 1.8 V on input. That would allow you to pull 14.5 mA, but there 
>>> would
>>> only be 9.5 mA available when the MIDI signal is using its 5 mA.
>>>
>>> Note that you might cause problems for the MIDI Out device by 
>>> pulling
>>> more than 5 mA, because their 5 V supply may have been designed to
>>> only provide 5 mA and no more. If there isn't any headroom for 
>>> extra
>>> current, then your device could overheat the MIDI Out circuits and
>>> cause them to fail. This is the biggest reason why I argue against
>>> designing anything like this.
>>>
>>> MIDI Solutions suggests that customers who make their own power
>>> supplies connect ground to pin 2 and power to both pins 4 and 5. 
>>> This
>>> suggests that MIDI Solutions is also pulling current from pin 5. 
>>> Note
>>> that this will only work if the buffer circuits in the MIDI out are
>>> not open collector. MIDI Solutions is probably assuming a typical 
>>> pair
>>> of inverting gates which drive both high and low logic levels. This
>>> would indeed provide some power to your charge pump. However, any 
>>> MIDI
>>> device that uses an open collector output will only be driving 
>>> logic
>>> low levels, and not logic high levels. This is perfectly valid for
>>> MIDI, but not so helpful when abusing MIDI for free power. I assume
>>> that most of the power is coming through the resistor at pin 4 
>>> because
>>> it's always available, whereas the Voltage on pin 5 depends upon 
>>> the
>>> logic level of the data and the type of driver.
>>>
>>> I think you'd need a couple of diodes to combine the power from 
>>> both
>>> pins 4 and 5 to your charge pump, but those diodes would rob more
>>> voltage. You can select LDO (low-dropout) diodes to minimize the
>>> losses there. Or, you can just ignore the power that may or may not 
>>> be
>>> coming in on pin 5.
>>>
>>> Finally, you're depending upon the MIDI Out cable shield for 
>>> ground.
>>> Personally, I think it's a bad choice to assume that the shield 
>>> will
>>> have a low resistance to ground. There are many ways to shield a
>>> cable, especially when you are aiming for low noise. A good 
>>> designer
>>> of MIDI audio gear might choose to put a 1 Mohm resistor in 
>>> parallel
>>> with a 1 uF capacitor, and only connect the MIDI Out shield to 
>>> ground
>>> through that filter network. This will make their MIDI audio device
>>> much less prone to picking up noise from the cable shield, and it's
>>> perfectly legal in terms of normal MIDI functionality. If you pay
>>> careful attention to the MIDI Specification, a different GND symbol 
>>> is
>>> used for the opto-isolator versus the symbol for cable shield
>>> connections. This means that it's perfectly legal if they do not
>>> connect directly. By the way, this means that MIDI Solutions is 
>>> wrong
>>> when they claim that MIDI devices that aren't compatible with their
>>> power supply "don't conform to the MIDI specification" - the truth 
>>> of
>>> the matter is that MIDI Solutions has not studied the original MIDI
>>> Specifications carefully enough.
>>>
>>> There you go. I think that's plenty of clues.
>>>
>>>
>>> On Sep 9, 2015, at 5:57 PM, eidorian at aladan.net wrote:
>>>>
>>>> Also see http://www.midisolutions.com/faqs.htm
>>>>
>>>> Maybe contact those guys and see if they're willing to send you a 
>>>> service manual?
>>>>
>>>> Cheers,
>>>> A.
>>>>
>>>> On 2015-09-10 09:43, jpdesroc wrote:
>>>>> Does anybody have schematics of an easy way to send
>>>>> power using the 2 MIDI pins in a 5 pins cable ?
>>>>> I don't want to use any of the 3 remaining wires of the cable
>>>>> for universal use on ANY MIDI cables..
>>>>> I know this company uses that technology:
>>>>>
>>>>> http://www.midisolutions.com/prodmrg.htm
>>>>>
>>>>> but I can't find any circuit ideas that uses it..
>>>>> I know I'll need some kind of charge pump design to get something 
>>>>> like 5 volts
>>>>> out of the MIDI (idle or in use) signal.
>>>>>
>>>>> Any clues ?
>>>>>
>>>>> JP