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'[PICLIST] !Re: [EE]: +12V and +5V in automoble'
2001\01\24@093429 by Thomas C. Sefranek

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Richard Sloan wrote:

> +12V @ 2.5A and +5V @ 1A, what is the best solution can have +/-10% on them.
>
> I have looked at switching regs but they seem to be not the best for
> availability and require expensive inductors etc for use. I want to build a
> very cost sensitive device and am looking for other suggestions.

Well you CAN'T have your cake and eat it too!
You have temperature issues, you probably are looking at a switching topology
solution.

> Yes heat is an issue that needs to be addresses also as a 7805 was attempted
> and man does it get HOT!

Ohms law:  12V - 5V = 7 volts at 1 amp, = 7 WATTS of heat.
You can waste some of that heat in a 5 ohm resistor (5-10 watts) in series with
the +12
to the 7805.

> Thanks.
> Richard.
>
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 *
 |  __O    Thomas C. Sefranek  spam_OUTtcsTakeThisOuTspamcmcorp.com
 |_-\<,_   Amateur Radio Operator: WA1RHP
 (*)/ (*)  Bicycle mobile on 145.41, 448.625 MHz

ARRL Instructor, Technical Specialist, VE Contact.
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http://www.harvardrepeater.org

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2001\01\24@113100 by Dan Michaels

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face
Thomas C. Sefranek wrote:
>Richard Sloan wrote:

>> Yes heat is an issue that needs to be addresses also as a 7805 was attempted
>> and man does it get HOT!
>
>Ohms law:  12V - 5V = 7 volts at 1 amp, = 7 WATTS of heat.
>You can waste some of that heat in a 5 ohm resistor (5-10 watts) in series with
>the +12
>to the 7805.
>

Not the most elegant or cheapest solution, but you might also
consider small-V 5W zeners instead of a series R. I have one
item where I do this, but is Vz = 4.3v @ .3A, so only about
1.5W.

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2001\01\24@134654 by Barry Gershenfeld

picon face
>Thomas C. Sefranek wrote:
>>Richard Sloan wrote:
>
>>> Yes heat is an issue that needs to be addresses also as a 7805 was attempted
>>> and man does it get HOT!
>>
>>Ohms law:  12V - 5V = 7 volts at 1 amp, = 7 WATTS of heat.
>>You can waste some of that heat in a 5 ohm resistor (5-10 watts) in series
with
>>the +12
>>to the 7805.
>>
>
>Not the most elegant or cheapest solution, but you might also
>consider small-V 5W zeners instead of a series R. I have one
>item where I do this, but is Vz = 4.3v @ .3A, so only about
>1.5W.
>

No matter how you do it, if you aren't switching then you are
dissipating it.  And the combination of resistors or zeners
or pass transistors all add up to that 7 watts of heat and
you will have to get rid of it somehow.  And heat sinks
aren't free, either.

And a mobile unit that's too hot to touch is rather embarrassing
(don't ask how I know this:)

Barry

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2001\01\24@140735 by Dan Michaels

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face
Barry Gershenfeld wrote:

>>Not the most elegant or cheapest solution, but you might also
>>consider small-V 5W zeners instead of a series R. I have one
>>item where I do this, but is Vz = 4.3v @ .3A, so only about
>>1.5W.
>>
>
>No matter how you do it, if you aren't switching then you are
>dissipating it.  And the combination of resistors or zeners
>or pass transistors all add up to that 7 watts of heat and
>you will have to get rid of it somehow.  And heat sinks
>aren't free, either.
>
>And a mobile unit that's too hot to touch is rather embarrassing
>(don't ask how I know this:)
>

Yes, my scheme was simply a way to distribute the heat,
so the 7805 wasn't dissipating all of it. In my system,
[15-5v]*.3A = 3W in 7805 w/o distribution. Removing 1W+
to the zener helped cool everything down "enough" to apply
Roman's test --> take a baby's finger and hold it on the
hottest part of the ckt for 30 seconds; measure sound output
at 3 feet using Radio Shack sound meter; if >=105 dB, turn off
power. [jus kidding about the baby, folks].

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2001\01\24@143442 by Arthur Brown

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face
Thank god Roman was joking about the baby,
me thinking *do i have to go throgh that again?*

regards Art

{Original Message removed}

2001\01\24@145726 by M. Adam Davis

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face
Dan Michaels wrote:
> Roman's test --> take a baby's finger and hold it on the
> hottest part of the ckt for 30 seconds; measure sound output
> at 3 feet using Radio Shack sound meter; if >=105 dB, turn off
> power. [jus kidding about the baby, folks].

Actually the hard part is that your test equipment (baby) needs constance
maintenance, and you lose a significant amount of development time keeping
the equipment up.  Of course as the baby-meter gets older it becomes more
intelligent and disallows such usage - keeping a supply of babies around
gets expensive, not to mention that replacement parts are non existant,
nevermind the cost.

At this point it is less expensive in both time and cost to let someone
buy you a laser temperature meter for your next major holiday.

-Adam
(who keeps his baby as far away from his work bench as physically
possible...)

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2001\01\24@153742 by Dale Botkin

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On Wed, 24 Jan 2001, M. Adam Davis wrote:

> Dan Michaels wrote:
> > Roman's test --> take a baby's finger and hold it on the
> > hottest part of the ckt for 30 seconds; measure sound output
> > at 3 feet using Radio Shack sound meter; if >=105 dB, turn off
> > power. [jus kidding about the baby, folks].
>
> Actually the hard part is that your test equipment (baby) needs constance
> maintenance, and you lose a significant amount of development time keeping
> the equipment up.  Of course as the baby-meter gets older it becomes more
> intelligent and disallows such usage - keeping a supply of babies around
> gets expensive, not to mention that replacement parts are non existant,
> nevermind the cost.
>
> At this point it is less expensive in both time and cost to let someone
> buy you a laser temperature meter for your next major holiday.

Agreed.  Far less cost and trouble.  Much less fun to make though.

Dale (5 kids, 2 in college)
---
The most exciting phrase to hear in science, the one that heralds new
discoveries, is not "Eureka!" (I found it!) but "That's funny ..."
               -- Isaac Asimov

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2001\01\24@160918 by Dan Michaels

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Dale Botkin wrote:

>>
>> At this point it is less expensive in both time and cost to let someone
>> buy you a laser temperature meter for your next major holiday.
>
>Agreed.  Far less cost and trouble.  Much less fun to make though.
>
>Dale (5 kids, 2 in college)


Neighbor's cat works good, too.

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2001\01\24@173735 by Dale Botkin

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On Wed, 24 Jan 2001, Dan Michaels wrote:

> Dale Botkin wrote:
>
> >>
> >> At this point it is less expensive in both time and cost to let someone
> >> buy you a laser temperature meter for your next major holiday.
> >
> >Agreed.  Far less cost and trouble.  Much less fun to make though.
> >
> >Dale (5 kids, 2 in college)
>
>
> Neighbor's cat works good, too.

Naah.  That's a waste of perfectly good food.  <evil gin>

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