TI-H: About the Regulator

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TI-H: About the Regulator



>Date: Fri, 15 Aug 1997 09:22:14 -0500
>From: Tony Do <tonyd@execpc.com>
>To: ti-hardware@lists.ticalc.org
>Subject: Re: TI-H: SPinTerface build question
>
>Bryan Rittmeyer wrote:
>
>
>> 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...
>
>I noticed that.  This thing is HUGE!  35V input voltage?  1A output
>current?  I don't think this is needed.  I'm not big in electronics
>(yet) but this sure doesn't seem like the smallest thing we can use.
>
>> 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.
>
>That sounds right.  But then how come Rich can somehow expect us all to
>get 5 volts in the troubleshooting section of his page?  Did he verify
>it on his calc?
>
>> 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)
>
>So can I just swap regulators and that will solve the problem?  Or is
>there additional wiring involved?
>
>THanks for clearing this up for me.
>Tony


I'm sorry for this problem. I tested my calc and it came out to 5v, so I 
assumed it was OK. You see, I was told by someone that you could use the 
higher power part cause the devices would only draw what they needed. I 
don't know if this is true, but I couldn't find the 78L05 till recently. 
I asked if the larger one could be used and I was told it would work. I 
have never had problems with it before, except that the speaker cart 
tries to draw more power than it should. I am going to change my voltage 
regulator later, but I'm going to test if with several devises, to make 
sure they work with it. If they do, then I'll feel OK about leaving it. 
If it doesn't work then I will send the replacement part to the one guy 
whose calc I did with the higher power 7805. I can't send parts to those 
who did it themselves, but I haven't had any major problems. I knew the 
speaker cart was drawing too much power, but i didn't realize that it 
was the regulator. I would recomend replacing the regulators. There are 
no changes in wiring. YOU JUST SUBSTITUTE. If the regulator was 
purchased recently, and you have the packaging, most Radio Shacks will 
accept a return. I'm going to get a bulk amount of the regulators, so if 
you can't get it yourself, I'll send you one for $2. all new calcs that 
I work with will use the lower power version.

Sorry for my mistake.



Richard Piotter



e-mail: richfiles@hotmail.com



home page: <A 
HREF="http://www.geocities.com/SiliconValley/Lakes/5081/Richfiles.html">The 
Richfiles</A>

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