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'[EE] Resurrecting my 78xx switcher project -- cap '
2006\04\29@113516 by Picdude

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I'm going to resurrect my 78xx-switcher project.  In this case, 78SW05 -- a small switcher circuit that will be a drop-in for a 7805, and NOT requiring external caps.  So far, I have designed it for 0.5A, since that's well within what I'm using it for.

Although it is functional, I want to find smaller caps to replace the SMD alum electrolytic cans on there now.  But so far there is a significant cost increase for these.  SMD tantalums are expensive and SMD ceramics are ridiculously expensive, especially when we're looking at 25V caps (for the input cap).  So far, I'm seeing $5-ish in 100-pc quantities.

Any other types I can use to replace these?

This is the current version btw...
www.narwani.net/neil/electronics/78SWxx/78SW05_Size_Comparison_2.jpg
http://www.narwani.net/neil/electronics/78SWxx/78SW05_Size_Comparison_3.jpg

Cheers,
-Neil.


'[EE] Resurrecting my 78xx switcher project -- cap '
2006\05\01@115729 by M. Adam Davis
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Without knowing the characteristics that the switcher requires, it's
going to be hard to suggest another capacitor.  Have you looked at
other suitable switchers?  Higher frequency switchers sometimes
require smaller capacitances, not to mention smaller coils, etc.

-Adam

On 4/29/06, Picdude <spam_OUTpicdudeTakeThisOuTspamnarwani.net> wrote:
{Quote hidden}

> -

2006\05\01@120650 by Bob Axtell

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M. Adam Davis wrote:
> Without knowing the characteristics that the switcher requires, it's
> going to be hard to suggest another capacitor.  Have you looked at
> other suitable switchers?  Higher frequency switchers sometimes
> require smaller capacitances, not to mention smaller coils, etc.
>  
Why not tantalum?  they ARE expensive, but have many advantages over
aluminum electrolytics
such as long life and small size. I try never to design in aluminum (wet
electrolyte) types because
eventually they dry out and their capacitance becomes smaller.

--Bob


{Quote hidden}

>> --

2006\05\01@123938 by M. Adam Davis

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Cost is the main issue, I imagine.  However, it looks like the digikey
part 495-1571-1-ND fits the bill and is only $1.05 in hundreds.  I
think he was looking at $5 caps, so this should be an improvement.
It's low ESR as well.

Still expensive given the application.  Perhaps a more diligent search
will yield suitable 50 cent caps in small quantities.

-Adam

On 5/1/06, Bob Axtell <engineerspamKILLspamcotse.net> wrote:
{Quote hidden}

2006\05\01@171638 by PicDude

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On Monday 01 May 2006 10:57, M. Adam Davis wrote:
> Without knowing the characteristics that the switcher requires, it's
> going to be hard to suggest another capacitor.  Have you looked at
> other suitable switchers?  Higher frequency switchers sometimes
> require smaller capacitances, not to mention smaller coils, etc.
>
> -Adam


I was sort of keeping it high-level at this point, trying to determine what
caps can replace the electrolytics for this purpose, at a reasonable price.  
More specifically though, this is an LM2674 switcher chip running at 260khz.

So far, I've found some better tantalums, near the $1-ea. mark (in 100-pc
qtys), which as even lower ESR than the electrolytics I'm currently using.  I
need to experiment with some options to see if I have room to drop the cap
values due to the lower ESR.

Cheers,
-Neil.

2006\05\01@172707 by Peter Todd

picon face
On Mon, May 01, 2006 at 09:06:44AM -0700, Bob Axtell wrote:
> M. Adam Davis wrote:
> > Without knowing the characteristics that the switcher requires, it's
> > going to be hard to suggest another capacitor.  Have you looked at
> > other suitable switchers?  Higher frequency switchers sometimes
> > require smaller capacitances, not to mention smaller coils, etc.
> >  
> Why not tantalum?  they ARE expensive, but have many advantages over
> aluminum electrolytics
> such as long life and small size. I try never to design in aluminum (wet
> electrolyte) types because
> eventually they dry out and their capacitance becomes smaller.

I'm curious, when you say long-life, how long is long? Essentially, do
they still wear out eventually, like electrolytic, or are they more like
ceramic, with an almost indefinit lifetime?

I ask, because I never use electrolytics in my artwork due to the
lifetime issue. I've also been staying away from tantalum, because I
simply don't know. But if tantalums can be relied upon to last decades
if treated properly... Good enough for me!

--
EraseMEpetespam_OUTspamTakeThisOuTpetertodd.ca http://www.petertodd.ca

2006\05\01@173613 by PicDude

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On Monday 01 May 2006 11:06, Bob Axtell wrote:
> M. Adam Davis wrote:
> > Without knowing the characteristics that the switcher requires, it's
> > going to be hard to suggest another capacitor.  Have you looked at
> > other suitable switchers?  Higher frequency switchers sometimes
> > require smaller capacitances, not to mention smaller coils, etc.
>
> Why not tantalum?  they ARE expensive, but have many advantages over
> aluminum electrolytics
> such as long life and small size. I try never to design in aluminum (wet
> electrolyte) types because
> eventually they dry out and their capacitance becomes smaller.
>
> --Bob

Price is a key consideration here, since replacing the linear 7805 will only
be practical if it comes in at a decent price, though I haven't figured out
the exact point at which it will be better to use a switcher since the good
old linear 7805 does the job already, albeit with some heat generated.

In the meanwhile, I am down to some $1 tantalums, which though still
pricier ,than the electrolytics I'm using, do reduce the thickness of the
final product noticeably.

Cheers,
-Neil.

2006\05\01@174313 by PicDude

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face
On Monday 01 May 2006 11:39, M. Adam Davis wrote:
> Cost is the main issue, I imagine.  However, it looks like the digikey
> part 495-1571-1-ND fits the bill and is only $1.05 in hundreds.  I
> think he was looking at $5 caps, so this should be an improvement.
> It's low ESR as well.

I did find some of those, and was thinking that given the lower ESR (than the
electrolytics I'm using), I may be able to get away with less capacitance.  
I'll only be able to determine by testing, but don't have all my equipment
here.


> Still expensive given the application.  Perhaps a more diligent search
> will yield suitable 50 cent caps in small quantities.

Yes, but to be realistic, it will never be completely price-competitive with
the 7805.  There are other benefits, such as not having to heatsink, running
cooler in general, and being a drop-in for some of my existing apps.

Cheers,
-Neil.


2006\05\01@174500 by Paul James E.

picon face

All,

IMHO, even if the switcher assembly costs a little more than a 7805, it's
still better because of the increased effeciency, and reduced heat
generation.   In a lot of cases, this might not make a difference, but in
other cases, it would.

BTW, what is the dropout voltage of the switcher?

                                           
                                                    Regards,

                                                      Jim


{Quote hidden}

> --

2006\05\01@183538 by rwuest

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Peter,

Ta caps have no known degradation mechanisms (Al caps do - they will all fail,
eventually).  If used properly, they should last forever.

Robert


-----{Original Message removed}

2006\05\01@201244 by Bob Axtell

face picon face
Peter Todd wrote:
{Quote hidden}

Hi, Peter. I had a client whose product was always operating hot (110F)
and some electrolytics cooked
out in 18 months. I switched over to tantalums and, frankly, the story
ended. No more problems.

I have a few products designed in 1994 that are still in the field, same
tantalums. Never have heard of
any needing replacement.


Tantalums are YEARS reliable, while electrolytics might be classed as
MONTHS if the temp is elevated.

Didja getcher wirewrap gun yet?
--Bob

2006\05\02@055423 by Michael Rigby-Jones

picon face


>-----Original Message-----
>From: piclist-bouncesspamspam_OUTmit.edu [@spam@piclist-bouncesKILLspamspammit.edu]
>Sent: 02 May 2006 01:12
>To: Microcontroller discussion list - Public.
>Subject: Re: [EE] Resurrecting my 78xx switcher project -- cap types?
>
>
>
>Tantalums are YEARS reliable, while electrolytics might be classed as
>MONTHS if the temp is elevated.
>

Tantalums have some specific problems that electrolytics don't however.  They are VERY voltage sensitive, they have very little over-voltage capacity so in many applications (specificaly supply rail bypassing) they have a higher voltage rating than might be specified for an electrolytic, with the corresponding size/cost penalty.  Manufacturers design notes usualy suggest to apply no more than ~30% of the devices rated voltage in a low impedance circuit.  When tant's do fail, it's very often short circuit and given enough current capacity flames and smoke will often be generated.

Regards

Mike

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2006\05\02@081632 by Alan B. Pearce

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> I'm going to resurrect my 78xx-switcher project.
>In this case, 78SW05 -- a small switcher circuit that
>will be a drop-in for a 7805, and NOT requiring external
>caps.  So far, I have designed it for 0.5A, since
>that's well within what I'm using it for.

Hmm, what chip are you using? I am looking at doing a similar thing using
either an LT1616 or LT3470 from Linear Technology, each capable of doing
about 200-300mA. Other chips they have look like they could do similar to
the full 1A equivalent of the 78xx devices.

The LT3470 is nice in that it will go to 40V input. The circuits in the
datasheet seem to use a couple of ceramic and a couple of tantalum caps,
with an inductor and a handful of resistors.

2006\05\02@110445 by alan smith

picon face
Looked at using OsCon caps? Or look at United Chemi-Con line of caps
               
---------------------------------
New Yahoo! Messenger with Voice. Call regular phones from your PC and save big.

2006\05\02@190035 by Peter Todd

picon face
On Mon, May 01, 2006 at 05:12:18PM -0700, Bob Axtell wrote:
> Hi, Peter. I had a client whose product was always operating hot (110F)
> and some electrolytics cooked
> out in 18 months. I switched over to tantalums and, frankly, the story
> ended. No more problems.
>
> I have a few products designed in 1994 that are still in the field, same
> tantalums. Never have heard of
> any needing replacement.
>
>
> Tantalums are YEARS reliable, while electrolytics might be classed as
> MONTHS if the temp is elevated.

Thanks, that's all very good to hear. Sounds like they are just as good
as ceramics so long as you treat them properly.

The temperature stuff sounds good too since I've got plans for
electronics embedded in polyester resin, and I think the heat of curing
would do a lot of damage to electrolytic caps by boiling them off.

> Didja getcher wirewrap gun yet?

Sure did, and thanks for the unrapping tool.

I'm planning on using it as a point to point wireing tool for PCB's
actually. My idea is to take standard 2 layer PCB's, add some wirewrap
pins, and use the airwires to "add layers" mcuh like I've seen some
extremely compact circuit boards done with. For looks really in projects
with the circuit boards exposed, rats nests of wires can look quite nice
I think.

Similarly I've got plans for building a 64-bit LFSR shift register out
of small reed relays. Anything but wirewrap just wouldn't look as
pretty!

--
KILLspampeteKILLspamspampetertodd.ca http://www.petertodd.ca

2006\05\02@190426 by Peter Todd

picon face
On Tue, May 02, 2006 at 10:53:12AM +0100, Michael Rigby-Jones wrote:
> >Tantalums are YEARS reliable, while electrolytics might be classed as
> >MONTHS if the temp is elevated.
> >
>
> Tantalums have some specific problems that electrolytics don't however.  They are VERY voltage sensitive, they have very little over-voltage capacity so in many applications (specificaly supply rail bypassing) they have a higher voltage rating than might be specified for an electrolytic, with the corresponding size/cost penalty.  Manufacturers design notes usualy suggest to apply no more than ~30% of the devices rated voltage in a low impedance circuit.  When tant's do fail, it's very often short circuit and given enough current capacity flames and smoke will often be generated.

Good rule to follow for sure. I've heard electrolytic caps need a
certain *minimum* voltage or the plates degrade. Is that true for
tantalum too?

My electronics class has blown up a whole lotta electrolytic and
tantalum caps belive me... By the end of the last semester, we had quite
literally blown up every cap in the room, even taken ones out of junked
equipment. Electrolytic make nice booms, but it's the tantalum ones that
flame and release nasty, nasty smoke.

In any case in my designs I always buy caps with 3x more voltage rating
than I plan to put on them, unless it's not something I'm selling and
I'm feeling broke. :)

--
RemoveMEpeteTakeThisOuTspampetertodd.ca http://www.petertodd.ca

2006\05\03@172530 by Diego Sierra

picon face
Hi!

I'm going to resurrect my 78xx-switcher project.  In this case, 78SW05 -- a
> small switcher circuit that will be a drop-in for a 7805, and NOT requiring
> external caps.  So far, I have designed it for


Do you know about this switching 78xx replacements?, it only need the caps.

  http://www.recom-international.com/

The AC/DC converter seems pretty cool too :-)

Cheers,
Diego.

2006\05\03@231917 by Denny Esterline

picon face
> Do you know about this switching 78xx replacements?, it only need the caps.
>
>    http://www.recom-international.com/

I didn't know about those, but I do know about the PT510x from TI.

http://focus.ti.com/docs/prod/folders/print/pt5101.html

-Denny


2006\05\04@004510 by William Chops Westfield

face picon face

On May 3, 2006, at 8:19 PM, Denny Esterline wrote:

>>    http://www.recom-international.com/
>
> I didn't know about those, but I do know about the PT510x from TI.
>
> http://focus.ti.com/docs/prod/folders/print/pt5101.html
>

Also

http://www.dimensionengineering.com/DE-SW0XX.htm


BillW

2006\05\04@122213 by PicDude

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face
There are a few different manufacturers of similar products, but they all
require external caps (at least I have not found any that does not).  My
intention is to use them on boards already designed for a 7805, as a true
drop-in replacement.

Cheers,
-Neil.


On Wednesday 03 May 2006 16:25, Diego Sierra wrote:
{Quote hidden}

2006\05\04@134238 by Paul James E.

picon face

Neil,

Just to be argumentative, if it is in fact a true drop in replacement,
then external caps would be required.  At least when I use as 7805, I
always add a 10uF to 20uF or so cap to the input, and a .01 uF and 10uF
caps to the output for transient response, and an added bit of ripple
reduction.   They may not be truly necessary, but I believe they can't
hurt.


                                             Regards,

                                               Jim







{Quote hidden}

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