Re: TI-H: SPinTerface build question
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Re: TI-H: SPinTerface build question
Tony Do wrote:
> Hi,
>
> I'm in the process of building a SPinTerface for my calc and am testing it
> before I cut and glue it into the case...
>
> My only doubt so far is that I'm only getting ~4.4 volts through +5V and
> GND (last two rows on the port.) Is this normal? I am using the 7805.
Aww man.... here we go again...
I have been wondering in the back of my mind since all of the
SPinTerface
threads started what regulator Rich was using. At the risk of being a
snob,
I have to report that Rich didn't quite do all of his homework.
There are two problems, the first minor and the second very significant.
I should probably take another look at the schematic, however, a 7805
can handle about an amp on a heatsink and is probably overkill for a
SPinTerface... if the idea is small size, why not use a (harder to find
but much cheaper) 78L05 which comes in a TO-92 package? But even
a 78L05 will not cut it...
The real problem comes with dropout
voltages. If you already know what these are, just skip to the bottom of
my message... but it seems that a lot of you are very curious, so I'll
explain my humble understanding of voltage regulators (Mel, correct
me if I miss anything ;-) Every semiconductor has a "dropout" voltage...
because they are semiconductors, they can't pass out, say, exactly
5V if they only get 5V in.
The problem is that the spec sheet for most 7805s states that the part
has a 2.0-2.5 volt "dropout." This means that the part will not operate
correctly on anything less than 7-8 volts -- This is required overhead.
Now, if you are powering the 7805 with 6 volts... which would work,
in theory, you are essentially encouraging sporadic operation. I haven't
gotten a clean 5 volts out of a normal 7805 with less than about 9
volts.
Yes, it will put out ABOUT 5 volts with less, but then it will operate
sporadically and probably not regulate the voltage... which I think is
what you're seeing. If you have old batteries, it gets even worse...
you're only feeding a regulator that's supposed to put out 5 volts 4.8
volts if you use old AAs!
(This is one of the reasons the 7805 is such a crappy regulator... it
dissipates almost half the power that it gets!)
The good news is that for a long time humanity has had the technology
to produce low dropout regulators that can regulate 5 volts with a
0.5 to 1 volt dropout or less. The bad news is that Radio Shack doesn't
sell any of these regulators (just another reason to avoid Radio Shack
like a cow avoids a butcher) However, if you flip through the
DigiKey or Mouser catalog you'll find no shortage of very low dropout
5V TO-92 package regulators (an SMT version of one of these is probably
what the calculator itself uses)
Hopefully this will clear up some confusion and problems...
P.S. Mel Tsai informed me via IRC that he will be gone for about a week,
so if you send him E-Mail you probably won't get a response for a while.
Regards,
Bryan Rittmeyer
bryanr@flash.net
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