Re: Tech: WPC Driver Board Switch Matrix Over-Current


"Jim Knight" <rottendogNOSPAM@xxxxxxxxxxx> wrote in message
news:d5qdncgGEJKD1uvenZ2dnUVZ_sGdnZ2d@xxxxxxxxxxxxxx
> Thanks for the input.
>
> The LM323 us out of the design. I am using a low-dropout linear regulator
> that only needs 0.5V of headroom as opposed to the 1.5V that the LM323
> needs. I think this should fix most of the reset issues when AC voltages
> are low.

LDO's are good but they generate a lot of heat - as does any other linear
such as that old LM323K. I also prefer oversized heatsinks to take care of
the heat and keep it as cool as possible. Discrete rectifier diodes with
lower forward voltage drop and low thermal resistance and smaller but lower
ESR caps should help as well.

>
> I tried to use a switching regulator on a mock up board and ended up with
> quite a bit of noise being generated on the ground and other AC supplies.
> One issue I immediately saw was false triggering on the zero cross.

This is a sign of incorrectly sized filter caps or too high of ESR.
Sometime's a sign of a bad ground or power run as well. The traces need to
be sufficient (Switchers NEED, MUST HAVE, GOTTA HAVE ground and power planes
because of noise). Better filter caps along with ceramics help take care of
this. And go with a FET swticher instead of an NPN switcher - the FET ones
are more efficient. I'd look into that first as you can't beat switchers.
Also go with extra low forward votlage drop on the catch diode and heat sink
him as well.

If I were you, I'd revisit the switching regulator approach. Nothing stays
cool like a good switching design. The National Semiconductor 'Simple
Switcher' (as recommended by Martin) is a superb part - I use lots of them.
But, I recommend using the 5 Amp part instead. I regularly use the
LM2679's as there isn't much price difference between the 3 amp part and the
5 amp part. The LM2679's have built in Soft Start plus over current
protection. Plus the 5 Amp part would open up room for people to use the
LED DMD displays. The LED DMD displays chug lots of current from the 5V
line. With a linear supply, I doubt you could provide enough. A switcher
reduces the incoming current due to high efficiency and -may- have enough
head room left for the LED DMD's.

Honestly, I hate the LED DMD's though. I had the prototype LED DMD from
Dale (Vishay) a few months back. Direct plug in replacement for the
existing DMD's and seemed to work well. BUT the reaction time of the LED
DMD's is wayyyyy too fast. The resulting flicker during any animation was
unbearable!

Another thing I recommend is don't cut nickel and dime parts out of the
design to reduce price. Users want a reliable design even if it costs a bit
more.

Good luck,
-- Ed


>
> Thanks for the input.
>
> "martin" <martin.reynolds@xxxxxxxxx> wrote in message
> news:1131825627.515831.292630@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>> If you have a slow-blow fuse in there, the TIPs could be damaged by a
>> short. I see that if there is a short on the lamp matrix, it is MUCH
>> easier to find if you only have the bad column out, rather than having
>> to troubleshoot the whole matrix. Also, one bad column means that the
>> game is still playable.
>>
>> So I think that you need to leave it in.
>>
>>
>> And I would really like to see one of thee for the 5V supply:
>> http://cache.national.com/ds/LM/LM2576.pdf
>>
>> Smaller caps (only needs 100uF), no more hot and failing bridges. Got
>> to be cheaper overall than all those parts for the LM323.
>>
>
>


.

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