[dorkbotpdx-blabber] ARRGGHH - Please check this circuit

[Hans Lindauer]

Yes, R3 is really .56 ohms.  I'll try checking the rail voltage as I 
increase the PWM - that's a good idea.  I have a friend who's going to 
let me borrow a LabView DAQ so I can try to figure out better what's 
going on, but the DVM should give me some idea.

One of the reasons I wanted to explore using 12V, was so that I could 
place a 7805 regulator between 12V and the logic portion of the board, 
with the idea that it would help to isolate it further while providing 
lots more overhead.

It seems like the power sags more when I strobe the LEDs to full power 
than when I leave them on, so maybe the PSU has too slow of a response?  
I can definitely see the Dorkboards' power LEDs go dim when the LEDs 
flash.  To address Mykle's concern, I am pretty much right at the upper 
limit of current draw for this PSU.


Greg Grunest wrote:
>
> Hey Hans,
>
>  
>
> Is the value of R3 really .56ohms?
>
>  
>
> So, here are a couple of thoughts.
>
>  
>
> Put a Volt Meter across the power supply leads and then slowly 
> increase the PWM to fully on and watch the volt meter.  If you see the 
> voltage drop as the PWM values increase to maximum then that is 
> probably your problem.  Probably...
>
>  
>
> If it is your problem, you can sometimes squeeze a bit more tolerance 
> out of your processor by going to a faster chip.  You may find that 
> putting a faster processor (or a more voltage tolerant one) in the 
> Dorkboard increases the drop out voltage.  I don't know what chip you 
> have or what chip comes in the Dorkboard.  I keep meaning to buy one 
> and play with it but I am lazy and busy and I suck!
>
>  
>
> Chip replacement might be the easiest way to do it if you have 36 of 
> these circuits already built and only a couple of Dorkboards running it.
>
>  
>
> The next question is of course... how are you powering it now?  You 
> said 5V but is that a lab power supply, a really cool 5v battery, a 
> wall wort and a 7805 regulator, what?  You may just be exceeding what 
> the 5V power supply you now have can handle and it is shutting down.
>
>  
>
> One solution might be to separate your power supplies and run your LED 
> Driver circuits off of an additional 5V supply and run the Dorkboard 
> off of your existing 5V supply.  Just be sure to tie the grounds 
> together so they have a common reference.  Otherwise, you can / will 
> blow up something. (typically, the item to blow up will be the most 
> expensive or the most difficult and time consuming to replace)
>
>  
>
> greg
>
>

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