[sdiy] Why dual grounds? (LEDs)

[Oren Leavitt]

David G. Dixon wrote:
>>> Since many of the modular synth standard power supply connectors
>>> (MOTM, Oakley, Blacet etc) have two ground wires, is one of these
>>> used for a "clean" ground and the other "dirty ?
>> No. Usually, both ground pins are shorted. This is true for all
>> MOTM, Tellun, Blacet, Patchell and Ruberto boards I've seen.
>>
>> On Oakley boards, all signal and power ground traces go to the
>> ground pin that's on the +15 V side. The ground pin on the -15 V
>> side is only used to ground the sleeve contacts of the jacks. It's
>> not clean vs. dirty or signal vs. power, it's frame ground vs.
>> everything-else-ground.
> 
> Why?  Ground loops?
>  
>> More modules should use the 6-pin power connector and power LEDs
>> etc. off the +5 V line. If nothing else, it would mitigate the
>> derating of the power supply caused by asymmetrical loads (most
>> modules draw more current from the positive rail).
> 
> Most schematics I've seen specify current-limiting resistors for LEDs which
> are too small.  I typically use at least 2.2k for an LED powered off of a
> 15V rail, while most schematics specify 1k.  My LEDs are plenty bright, and
> draw less than half the power.  Of course, I tend to use blinking lights
> fairly sparingly...
> 

True - I think that is a holdover from a time when LEDs were not as 
efficient as they are are now. Modern red/green LEDs are quite bright at 
even 5 mA of current.

- Oren