At 11:17 PM 10/19/2005, Jason wrote:
>>I can't help but notice that the suggestions to throw in a 7805 don't
>>match well with the fact the current supply is spec'ed at 2 amps :-)
>
>Yeah I noticed that too.  I hadn't looked up the specs of the 7805's yet 
>but I figured at least I was looking at a TO-220 with a hefty heatsink.  A 
>few volts a a couple of amps is a big amount to turn into heat.

This is one of the things that doesn't make sense in your story.

No one would drive 2 A through 200 LED's controlled by pass transistors 
through a linear regulator unless absolute brightness control were 
necessary, and even then they'd wire the transistors as current regulators 
instead. Otherwise, the LEDs and transistors don't need highly regulated 
power. The PIC does. A 78L05 on the PIC power supply should be more than 
sufficient. Running all the current through a 78xx regulator would just 
waste 30% of the power that could have been used to light LEDs instead.

Driving a PIC beyond its absolute maximum voltage will cause breakdown and 
shorting within the chip, which could be why the voltage drops to 6.2 V 
from 7.3 V, and the LEDs (which are through resistors) don't light. It 
would also, of course, fry the PIC.

Once you've determined you are violating a key endurance specification, why 
not fix that problem and then see what happens? Why is any more discussion 
necessary?

================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: ral@lcfltd.com
Least Cost Formulations, Ltd.            URL: http://lcfltd.com/
824 Timberlake Drive                     Tel: 757-467-0954
Virginia Beach, VA 23464-3239            Fax: 757-467-2947

"Vere scire est per causas scire"
================================================================

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