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'[EE]: 7805 replacement'
2001\06\02@141051 by David Dunn

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there seems to be a plethora of devices that work better than a 7805 for a auto-powered application, and i can't
seem to nail down what would be best.

i've got a device that will pull between ZERO and 125 mA depending on it's mode of operation (sleep modes and such)

it will be powered from a car battery

i already have my PCB laid out for a TO-220 device so i can't change that

i've noticed that with a 13 volt input, a 7805 is still dumping 10 mA of current all by itself (my device in sleep
mode)

it's a 16F877 based device, with an 20x4 LCD (backlit)

anybody have suggestion on what i should be looking at ?


(one i'm considering is a LM2940CT)


thanks for any suggestions !


dld

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2001\06\02@142730 by Bob Blick

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Hi David,

If you are in a car, personally I don't worry about 10mA of continuous
draw. The 7805 is tougher and cheaper than almost anything out there.

Just my .02

Cheers,

Bob

At 01:10 PM 6/2/2001 -0500, you wrote:
>there seems to be a plethora of devices that work better than a 7805 for a
auto-powered application, and i can't
>seem to nail down what would be best.
>
>i've got a device that will pull between ZERO and 125 mA depending on it's
mode of operation (sleep modes and such)
>
>it will be powered from a car battery
>
>i already have my PCB laid out for a TO-220 device so i can't change that
>
>i've noticed that with a 13 volt input, a 7805 is still dumping 10 mA of
current all by itself (my device in sleep
{Quote hidden}

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2001\06\02@143339 by David Dunn

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hm. good point, i didn't realize that durability was an issue on some of the other LDO devices vs a 7805.

thanks for your comments !

dld


On Sat, 2 Jun 2001 11:23:59 -0700, Bob Blick wrote:

{Quote hidden}

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2001\06\02@150022 by Jeff DeMaagd

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----- Original Message -----
From: Bob Blick <spam_OUTbblickTakeThisOuTspamSABER.NET>

> If you are in a car, personally I don't worry about 10mA of continuous
> draw. The 7805 is tougher and cheaper than almost anything out there.

The 7805 seems to be a solid device, but my question is if the device is
connected to constant-on or connected to key-on-power?

Every little electronic gadget attached to constant power seems to draw
some, if you let the car sit for three months, the collective power draw
from lots of gadgets drain the battery to where it's hard to start.  It
doesn't matter when the car is running, there's lots of power being
generated.

Just a consideration that I've run into a few times, particularly in cars
with lots of built-in gadgets, somehow their batteries get drained a lot
quicker than those without them.

Jeff

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2001\06\02@150434 by Chris Carr

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> hm. good point, i didn't realize that durability was an issue on some of
the other LDO devices vs a 7805.
>
> thanks for your comments !
>
It's also an issue amongst 7805's all are not created equal. Whatever
you use my advise would be, as it is an automotive application,
put in protection against voltage spikes and load dumps.

Chris Carr

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2001\06\02@153216 by Olin Lathrop

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> i've got a device that will pull between ZERO and 125 mA depending on it's
mode of operation (sleep modes and such)
>
> it will be powered from a car battery
>
> i already have my PCB laid out for a TO-220 device so i can't change that
>
> i've noticed that with a 13 volt input, a 7805 is still dumping 10 mA of
current all by itself (my device in sleep
> mode)

10mA sounds like a lot until you put it in car battery perspective.  10mA
continuous comes out to 1.68AH per week, which is not much for even a small
car battery rated at 45AH.  Unless you go on vacation for over a month, this
won't make any difference.  You might consider a mechanical OFF switch for
those situations.  There are other regulators out there that take much less
quiescent current, but the 7805 is the brick outhouse of regulators.  This
can save your butt in the hostile car environment.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, .....olinKILLspamspam@spam@embedinc.com, http://www.embedinc.com

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2001\06\02@154857 by Spehro Pefhany

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At 03:26 PM 6/2/01 -0400, you wrote:

>
>10mA sounds like a lot until you put it in car battery perspective.  10mA
>continuous comes out to 1.68AH per week, which is not much for even a small
>car battery rated at 45AH.  Unless you go on vacation for over a month, this
>won't make any difference.  You might consider a mechanical OFF switch for
>those situations.  There are other regulators out there that take much less
>quiescent current, but the 7805 is the brick outhouse of regulators.  This
>can save your butt in the hostile car environment.

Also, the 7805 typically only draws about 4-5mA, so your circuit is also
drawing some juice.

Actually, though, there are regulators that are better rated for the auto
environment than the 7805, supposedly requiring little protection against
load dump and overvoltage to meet auto specs. At the least you should
protect the 7805 against overvoltage (series resistor and reasonably
hefty zener) and reversed input/load dump (series 1N4005 etc.).
7805s can and do fail from overvoltage and from discharging the
output filter capacitors through the internal circuit (high negative
dV/dt at the input). With a normal transformer/rectifier neither
of these is a concern unless the filter cap gets shorted or you are
operating close to the abs. maximum input voltage.

Best regards,
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speffspamKILLspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
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2001\06\02@200859 by Bill Westfield

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I've wondered recently how much protection you can gain by "chaining"
78xx regulators.  For instance, for the "typical" 5V project from a 12V
car supply, you might use a 7509 regulator to get from 12 to 9V, and a 7805
to get from 9 to 5V, or something similar.  Also has the advantage of
spreading out your power dissipation.  Similar tricks could be played with
multiple lm317 type regulators...

Thoughts?
Bill W

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2001\06\02@212910 by Dan Michaels

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BillW wrote:
>I've wondered recently how much protection you can gain by "chaining"
>78xx regulators.  For instance, for the "typical" 5V project from a 12V
>car supply, you might use a 7509 regulator to get from 12 to 9V, and a 7805
>to get from 9 to 5V, or something similar.  Also has the advantage of
>spreading out your power dissipation.  Similar tricks could be played with
>multiple lm317 type regulators...
>
>Thoughts?


You can also use a 3-4V, 5W zener inline instead of another 78xx to
help spread the dissipation.

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2001\06\02@220455 by David VanHorn

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At 05:07 PM 6/2/01 -0700, William Chops Westfield wrote:
>I've wondered recently how much protection you can gain by "chaining"
>78xx regulators.  For instance, for the "typical" 5V project from a 12V
>car supply, you might use a 7509 regulator to get from 12 to 9V, and a 7805
>to get from 9 to 5V, or something similar.  Also has the advantage of
>spreading out your power dissipation.  Similar tricks could be played with
>multiple lm317 type regulators...

Dosen't help your input side at all, and adds additional parts to fail.

It does spread dissipation, but maybe a switcher would be better? :)

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2001\06\02@221313 by Spehro Pefhany

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At 05:07 PM 6/2/01 PDT, you wrote:

(sorry for the 2nd copy, Bill)
>I've wondered recently how much protection you can gain by "chaining"
>78xx regulators.  For instance, for the "typical" 5V project from a 12V
>car supply, you might use a 7509 regulator to get from 12 to 9V, and a 7805
>to get from 9 to 5V, or something similar.  Also has the advantage of
>spreading out your power dissipation.  Similar tricks could be played with
>multiple lm317 type regulators...
>
>Thoughts?

Not much, the input voltage rating is the same for the whole series
except for the 7824, which is a bit higher. THe 78xxx is designed to fail
'off' even with overvoltage. With two in series, the likelihood of that
happening at reasonable overvoltages is probably very high.

Of course it depends on what the stuff on the other side is worth..  if
it's a lot, a TVS rather than a zener, with some series impedance, might
be a much better investment, they are available at quite reasonable
prices.

Best regards,
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Spehro Pefhany --"it's the network..."            "The Journey is the reward"
.....speffKILLspamspam.....interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

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2001\06\03@090725 by Olin Lathrop

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> I've wondered recently how much protection you can gain by "chaining"
> 78xx regulators.  For instance, for the "typical" 5V project from a 12V
> car supply, you might use a 7509 regulator to get from 12 to 9V, and a
7805
> to get from 9 to 5V, or something similar.  Also has the advantage of
> spreading out your power dissipation.  Similar tricks could be played with
> multiple lm317 type regulators...

I'm not sure how much extra "protection" this affords, although it would
spread the power dissipation.  A 50 or 100 volt spike will still fry
something.  Also, this scheme will increase the quiescent current, although
that's not an issue if the device is off when the car is off.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, EraseMEolinspam_OUTspamTakeThisOuTembedinc.com, http://www.embedinc.com

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2001\06\03@170614 by Mike Hardwick

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>It's also an issue amongst 7805's all are not created equal. Whatever
>you use my advise would be, as it is an automotive application,
>put in protection against voltage spikes and load dumps.

National's LM2940 is a good choice in automotive applications. Much of the
necessary protection is built-in...

Mike Hardwick
Decade Engineering
http://www.decadenet.com

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