[sdiy] LEDs and noise

[Tim Ressel]

Hi,

In my experience, for example designing the Livewire Chaos Computer, you need to go from a positive rail to ground, use a separate 'dirty' ground just for the display stuff, and make sure you have enough local capacitance to handle the glitch current. In fact it would be best to use a separate power trace as well. Keep the high current glitches separate all the way back to the power section. 

More on that last bit: the idea is capacitors act like little batteries that can supply during the moment when everything switches. That way the glitch current doesn't come from the rails. You need to use different size capacitors so that both low and high frequency glitch current is taken care of. On the Chaos I went nutz and used a 1000uF cap at the power connector, then 10uF and 100nF at each CD4050 driving the display. In retrospect, 1000uF may have been a bit large. Maybe a 100uF or 47uF would suffice.


Tim Ressel
Circuit Abbey
503-750-9331
timr at circuitabbey.com




________________________________

From: David Moylan <dave at westphila.net>
To: 'synth-diy List' <synth-diy at dropmix.xs4all.nl> 
Sent: Thursday, May 7, 2015 4:32 PM
Subject: [sdiy] LEDs and noise


If you're dealing with a lot of "blinkenlights" what's the best way to 
handle them?  Is it better to power them between the + and - rails or 
from one rail to ground?  Is it worth using a "dirty ground" scheme, or 
is the noise induced on the supply rails itself?

Thanks.

Dave
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