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PICList Thread
'Supercap for backup power'
2000\01\29@204501 by Jason Harper

picon face
Does anyone have experience with using a 1 Farad or so supercap as backup
power for a PIC design?  I'm building a clock, and would like to have a few
minutes of reserve power to handle brief power outages, and to allow the
clock to be moved to a different outlet without losing the time setting.  I
assume I can't just connect the supercap across the 5V power supply, as
this would result in the /MCLR rise time being too slow to reliably reset
the PIC.  (Or would it?  There seems to be an inherent limit to the
charging current a supercap will accept.)  Do I have to worry about the
supercap discharging through the output of the voltage regulator?

It seems like any reliable design might have to have a diode drop or two
between the regulator and the PIC, but I'm concerned that this might cause
other problems.  The circuit will also include some TTL chips, powered
separately so that the supercap won't try to maintain them: having the
PIC's power supply be at a different voltage might cause logic level
problems.
       Jason Harper

2000\01\29@211649 by Rob

picon face
Well, we used these supercaps at one time, and they don't work to well. They
have all the negative aspects of both a battery and a cap, and not many of
the benefits of the two.. good luch on this one.
Use a large lithium AA battery or something.
{Original Message removed}

2000\01\30@071723 by paulb

flavicon
face
Jason Harper wrote:

> Does anyone have experience with using a 1 Farad or so supercap as
> backup power for a PIC design?

 Nope, I note the other chap's unfavourable experience though.

>  I'm building a clock, and would like to have a few minutes of reserve
> power to handle brief power outages, and to allow the clock to be
> moved to a different outlet without losing the time setting.

 You'll need a power fail circuit to tell the PIC to shut off all
outputs - pull down to zero is probably best as it avoids "floating"
pins.

>  I assume I can't just connect the supercap across the 5V power
> supply, as this would result in the /MCLR rise time being too slow to
> reliably reset the PIC.  (Or would it?  There seems to be an inherent
> limit to the charging current a supercap will accept.)

 So it appears.  If you put a 47 ohm resistor (no diodes, at the low
currents in question, the resistor will drop far fewer millivolts while
powering the PIC) in series with the supercap, directly across the
(bypassed) PIC supply, you guarantee no more than 106 mA draw anyway.

 But you'll want a brownout circuit too.

>  Do I have to worry about the supercap discharging through the output
> of the voltage regulator?

 Yes.  Not because the reference draws significant current at any
voltage below 5V, but because the regulator may try to conduct current
back to the input side and those other devices connected there.  Careful
examination of specs may find a suitable regulator, I can't recall
offhand.

> It seems like any reliable design might have to have a diode drop or
> two between the regulator and the PIC,

 One diode in series with regulator output (in which case, can be the
same regulator as powers the LED drivers anyway) can be balanced by a
diode in series with the reference leg, or just use an LM317.

> but I'm concerned that this might cause other problems.  The circuit
> will also include some TTL chips,

 You do mean HCMOS?

> having the PIC's power supply be at a different voltage might cause
> logic level problems.

 Half a volt shouldn't matter.  But think in terms of an LM317 set to
5.6V with a 1N4004 from output to each part of the circuit.
--
 Cheers,
       Paul B.

2000\01\30@094210 by Reginald Neale

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<x-flowed>>Does anyone have experience with using a 1 Farad or so supercap as backup
>power for a PIC design?  I'm building a clock, and would like to have a few
>minutes of reserve power to handle brief power outages, and to allow the
>clock to be moved to a different outlet without losing the time setting.

  Certainly is doable. Bob Blick's Propeller Clock used just such a
  circuit. Might still be on his web site.

  Reg Neale

</x-flowed>

2000\01\30@112752 by Donald L Burdette

picon face
The first question I would ask myself is whether to use supercap or a
battery.  The answer depends (IMHO) on several things - over its life,
how much time will the device spend on backup power, and how much current
draw is there in backup mode, and can you make a battery replaceable.

The higher the current draw, the less total lifetime backup power a
battery can provide.  A supercap won't maintain backup power for extended
periods, but it will recover once primary power is established.

There are several ways to switch between primary and backup power.  The
most obvious is to use diodes.  One from primary and one from backup to
Vcc will work.  If you use a supercap, a diode and resistor from primary
power to the cap will charge it to slightly less than the primary power
voltage.  You CAN use schottkys in some cases.  They get leaky when hot
and with high reverse bias, but if kept to 30 deg C and 5V reverse bias,
a 40V diode will have leakage probably 100 times less than the max spec.

There are also a number of chips made specifically for switching from
primary to backup power.  The '691/693 chips made by Analog Devices,
Maxim, and others are an example.  These use FET swtiching to minimize
voltage drops.  I've used MAX691's and '693's, and they work well, but
are very sensitive to static, even after the product is assembled and in
the field.  Be sure to check supply current specs, as they vary a lot
from one mfr to another.

The last caveat is to be very careful when powering only part of your
circuit on backup power.  There are two things that will get you.  One is
if something that's on backup power tries to drive a logic high to
something that's not powered up.  The input protection diodes will dump
current from the unpowered input pin to the unpowered rail.  The other
thing is if the unpowered portion of the circuit doesn't rapidly go to 1
volt or less.  That could hold PIC input pins in the transition region
where it causes the PIC supply current to go up dramatically.  Just one
input at 2 volts can increase the supply current by 10 mA or more, even
in sleep mode.  I'd recommend powering only the PIC itself on backup, and
making sure both its input and output pins are driven low during backup
mode.  This means the PIC needs to know when it's on backup.  Most
supply-switching supervisor chips have an output to tell you this.

Don

2000\01\30@161054 by Harold M Hallikainen

picon face
       I have a prototype of a PIC circuit running using a 1F capacitor to back
128 kbytes of static RAM. The PIC itself is not backed. The power and
chip select for the RAM are run through a Dallas/Maxim chip (I think it's
a 12... something, I'm at home now, not at work). The chip handles power
switching and supports two backup batteries. By gating the chip select,
it prevents false writes and also prevents reads during a power failure
where the memory might be trying to put something on the databus and the
powered down pic clamp diodes are conducting (the RAM would be attempting
to power the system). I used a diode and series resistor to charge the 1F
capacitor from the 5V supply.
       Seems to work fine thus far!

Harold

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2000\01\30@204009 by William K. Borsum

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Watch out for leakage, too.  I cannot use Tantalum caps on any of my
designs because the leakage (1-5 uA) severely impacts battery life on a
device that draws an average of 32-65 uA total.  Digikey now carries large
value (to 10uF at 6.3V) caps in 1206 surface mount ceramic (X7R and
X5R)--be sure to avoid the Z5U  temperature tolerance devices like the
plague!  Most of the data sheets on the Z5U imply +/-20%--but its really
+20 -80%--that -80% is a killer!

Kelly



At 08:42 PM 1/29/00 -0500, you wrote:
{Quote hidden}

William K. Borsum, P.E. -- OEM Dataloggers and Instrumentation Systems
<spam_OUTborsumTakeThisOuTspamdascor.com> & <http://www.dascor.com>San Diego, California, USA

2000\01\30@205051 by Graeme Zimmer

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Hi,

> >Does anyone have experience with using a 1 Farad or so supercap as backup
> >power for a PIC design?

It's a long time since I played with SuperCaps, but I vaguely remember that
I eventually ruled them out due to their very high internal leakage....

e.g., they don't stay charged for very long!

regards ............. Zim

2000\01\31@071739 by mark

flavicon
face
<x-rich><color><param>0000,0000,FF00</param>> >Does anyone have experience with using a 1 Farad or so supercap as backup

> >power for a PIC design?  I'm building a clock, and would like to have a few

> >minutes of reserve power to handle brief power outages, and to allow the

> >clock to be moved to a different outlet without losing the time setting.

</color>>


Why use those tiny 1 Farad caps ???


Why not something BIGGER, like this :


http://www.powercache.com/



<nofill>
Marcelo Puhl
.....markKILLspamspam@spam@plug-in.com.br
-------------------------------------------
Get paid to surf the WEB !
Ganhe dinheiro enquanto surfa na Internet !
http://alladvantage.com/go.asp?refid=DTJ608
-------------------------------------------

</x-rich>

2000\01\31@084515 by Terry A. Steen

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face
Yes, I have... it was in a project that used a basic stamp. The only thing
it was there for wat the BS-II. I fed it with a low forward voltage diode
from the main supply so it would not feed back to the rest of the devices.
I ran a test on it... the thing ran for 10-1/2 hours before it dumped out!

Terry


At 08:42 PM 1/29/2000 -0500, Jason Harper wrote:
{Quote hidden}

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2000\01\31@120348 by Richard Poland

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part 0 1488 bytes content-type:text/x-vcard; charset=us-ascii; (decoded 7bit)

Richard Poland

Jason Harper wrote:

{Quote hidden}

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2000\01\31@124757 by carbonbased

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----- Original Message -----
From: "Graeme Zimmer" <.....gzimmerKILLspamspam.....BIGPOND.COM>
To: <EraseMEPICLISTspam_OUTspamTakeThisOuTMITVMA.MIT.EDU>
Sent: Sunday, January 30, 2000 8:49 PM
Subject: Re: Supercap for backup power


> Hi,
>
> > >Does anyone have experience with using a 1 Farad or so supercap as
backup
> > >power for a PIC design?
>
> It's a long time since I played with SuperCaps, but I vaguely remember
that
> I eventually ruled them out due to their very high internal leakage....
>
> e.g., they don't stay charged for very long!
>
> regards ............. Zim
>

Soon AVX Corp. will be releasing a 5.5 volt supercap with less than 10
microamps of leakage current. They have already announced the product and
are currently sending out samples.

See AVXCorp.com for more details.

2000\01\31@132326 by Quitt, Walter

flavicon
face
Do you hook it to the battery input?
Just as would be done as shown in the DALLAS
app sheet about the 13xx clocks?

-Walt...

{Original Message removed}

2000\01\31@140610 by Richard Poland

flavicon
face
part 0 1056 bytes content-type:text/x-vcard; charset=us-ascii; (decoded 7bit)

read from the RTC on power cycles.

Richard

"Quitt, Walter" wrote:

> Do you hook it to the battery input?
> Just as would be done as shown in the DALLAS
> app sheet about the 13xx clocks?
>
> -Walt...
>
> {Original Message removed}

2000\01\31@144617 by Quitt, Walter

flavicon
face
I have some of the 8F Maxwell Ultracaps and will try them.
They are about 4mm thick, 23mm wide and 29mm long.
How big are your .47F caps?

-Walt...

{Original Message removed}


'Supercap for backup power'
2000\02\01@032550 by Donald L Burdette
picon face
>Watch out for leakage, too.  I cannot use Tantalum caps on any of my
>designs because the leakage (1-5 uA) severely impacts battery life on a
>device that draws an average of 32-65 uA total.  Digikey now carries
large
>value (to 10uF at 6.3V) caps in 1206 surface mount ceramic (X7R and
>X5R)--be sure to avoid the Z5U  temperature tolerance devices like the
>plague!  Most of the data sheets on the Z5U imply +/-20%--but its really
>+20 -80%--that -80% is a killer!


What?  I have about 5000 copies of a product in the field, which draws an
average of 4uA - and that is in pulses of about 1.5 mA at a time.  My
meter with 1uA resolution doesn't even register any current between
pulses.  This device has 3 tantalum caps on it, ranging from 1uF to 33uF.
Maybe at high temperature or high voltage they get leakier, but neither
would be encountered in a clock.

By the way, for low power applications, I just switched from 1uF ceramic
to 1uF tantalum, because the previous version of my product had such a
high failure rate on those 1uF, 1206 surface mount ceramics.  They
occasionally get VERY leaky.  I had a 0.1 uF yesterday that pulled a 10K
resistor to within 0.2V of ground.   The worst part is that if you use
them across the power supply, you never know you have a problem until
your battery runs out - very rapidly.  That single cap probably accounted
for 3-5% of my failures.  That's unacceptable for a single capacitor on a
complex board.  And in case you're wondering, no it wasn't a bad batch.
They were at least 20 batches from 2 different vendors.

You're right, you have to be very careful of tolerances on ceramic caps.
Philips specs some of their Z5U caps at +-20%, others at +80 -20%.
That's initial (room temperature) tolerance.  Then there's the +0 -50%
over temperature.  Y5V can be +-20% initially and +20 -70% over temp, or
+80 -20 initially and +30 -80% over temp.  What a mess!  Of course there
are Z4V, N750, X5R, X7R, NPO, COG, Y5P....

It's enough to make your head spin.


Don

2000\02\02@200617 by Donald L Burdette

picon face
>>What?  I have about 5000 copies of a product in the field, which draws
an
>>average of 4uA - and that is in pulses of about 1.5 mA at a time.  My
>>meter with 1uA resolution doesn't even register any current between
>>pulses.  This device has 3 tantalum caps on it, ranging from 1uF to
33uF.
>> Maybe at high temperature or high voltage they get leakier, but
neither
>>would be encountered in a clock.

William K. Borsum wrote:

>Absolutely fascinating!
>
>What brands???  What power rail, and cap voltages?  Everything I've
tried
>so far leaks like a sieve--so to speak--or takes a long time to
stabilize.
>However, I can also reduce power consumption 10-50% by washing the board
>thoroughly.

If you can reduce power consumption by washing the board, then your
problem isn't the caps (at least part of it isn't).  Are you sure you're
getting the boards really clean even when you wash them 'thoroughly"?
One dot of certain fluxes under an 0805 surface mount component can
really do a number on you.  I've had boards come out of our
solvent-filled ultrasonic cleaner with goop still under the components.

I think most of the caps are 10V, the rail is 3.0V, but it doesn't change
much if you raise it to 5.0V.  Even at 7.8V (defective boost switcher),
it wasn't too bad.  Miraculously, everything survived that voltage, which
was applied for quite some time!

>You mention a clock--what sort?

The question that started this thread was about an alarm clock project
someone was building.  That's what he wanted backup power for.

> I need a stable interrupt generator that gives me 10 KHz, 1 Khz,
>and one second pulses.  Using a pic 509 at the
>moment, and the best I can do is about 8-10 uA at 3.3 volts.

That's pretty good.  My product puts the PIC to sleep, and wakes up every
18/72 mS on watchdog timeout.  I toggle the watchdog scaler from 1:1 to
4:1 or back every time it wakes up, since I wanted asymmetrical pulses
out.  The PIC wakes up only long enough to toggle an output, change the
prescaler, and go back to sleep.  Other activities are initiated by
wakeup on pin change.

Of course, the WDT period is something like +-50%, which I can tolerate,
but it sounds like you can't.

>> ... capacitor failures...

>Yeah, I've had a couple of failures straight out of fabrication--but
they
>are rather obvious and fixed before the units got out of initial
testing.
>No field failures so far.   Same rate of problems with tantalums as
well.
>Are you hand assembling or using automated systems (Pic'n'place, etc)?
Mine
>has mostly been hand work to date, but all future runs will be fully
hands
>off.  Also, which vendors?  We are moving to Vishay since that is what
>Digikey is selling.  Can't afford reels of 1 and 4.7 uF caps yet--

Most of our leaky cap failures were after the product was deployed.
They're pretty obvious, but unfortunately they are intermittant.  The
come and go randomly, or with temperature, or with physical stress
(flexing) on the PC board.  I hope you have better luck than we did.  We
were using automatic insertion, and parts from AVX and I think NIC.
Haven't built enough by hand to know the reliability.

To be fair, almost all our leaky cap failures were with 1uF 1206 surface
mount caps, which I think is really pushing the technology of ceramic
caps.  With 0.1uF and 0.01uF 0805's from the same manufacturers, we've
had extremely good success.

Don

2000\02\03@014722 by William K. Borsum

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At 07:56 PM 2/2/00 -0500, you wrote:
Big Snip

>If you can reduce power consumption by washing the board, then your
>problem isn't the caps (at least part of it isn't).  Are you sure you're
>getting the boards really clean even when you wash them 'thoroughly"?
>One dot of certain fluxes under an 0805 surface mount component can
>really do a number on you.  I've had boards come out of our
>solvent-filled ultrasonic cleaner with goop still under the components.

Use only water soluble fluxes and low melting point solders on Getek
boards.  Wash with very hot water and 409 or some similar cleaner, blow
dry, then do it again with denatured alcohol.  Final bake at 50 degC for a
couple of hours, pot in RTV  in a NEMA-4 box, then add desiccant and close
it up (with the battery installed of course).

Ultrasonic cleaner, eh?  What effect does it have on crystals?  Watch and
regular?  Always been a bit scared of busting something with the high
frequency (audio range anyway :_)).  Might go back to it, but we have been
having good luck with the present process.  I've seen some of the boards
our new fab house does and they are truly spotless--even under the parts.

{Quote hidden}

I use one of the pulse outputs to feed the counter in a '74--which keeps
going while the main chip is in sleep mode.  Wakes up on count down to
zero.  Gives me very accurate time intervals--well as accurate as the watch
crystal anyway.

>We
>were using automatic insertion, and parts from AVX and I think NIC.
>Haven't built enough by hand to know the reliability.

First batch we ran of the 1uFs was from Mouser, and who knows who made them.
Latest batches are Vishay from Digikey.  I've never known Vishay to make
junk of any sort.

>To be fair, almost all our leaky cap failures were with 1uF 1206 surface
>mount caps, which I think is really pushing the technology of ceramic
>caps.  With 0.1uF and 0.01uF 0805's from the same manufacturers, we've
>had extremely good success.

Never had a problem with anything smaller than 1uF.  I suspect the size
contributes to the failures through thermal expansion during soldering.
The 4.7's a just a tad taller, but not much.  Failures when they do occur
are usually obvious, fortunately.

Thanks for the feedback.
Kelly

William K. Borsum, P.E. -- OEM Dataloggers and Instrumentation Systems
<borsumspamspam_OUTdascor.com> & <http://www.dascor.com>San Diego, California, USA

2000\02\03@032038 by Michael Rigby-Jones

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part 0 2074 bytes
<P><FONT COLOR="#0000FF" SIZE=2 FACE="Arial">Donald L Burdette wrote:</FONT>
</P>
<UL>
<P><FONT SIZE=2 FACE="Arial">Most of our leaky cap failures were after the product was deployed.</FONT>
<BR><FONT SIZE=2 FACE="Arial">They're pretty obvious, but unfortunately they are intermittant.&nbsp; The</FONT>
<BR><FONT SIZE=2 FACE="Arial">come and go randomly, or with temperature, or with physical stress</FONT>
<BR><FONT SIZE=2 FACE="Arial">(flexing) on the PC board.&nbsp; I hope you have better luck than we did.&nbsp; We</FONT>
<BR><FONT SIZE=2 FACE="Arial">were using automatic insertion, and parts from AVX and I think NIC.</FONT>
<BR><FONT SIZE=2 FACE="Arial">Haven't built enough by hand to know the reliability.</FONT>
</P>

<P><FONT SIZE=2 FACE="Arial">To be fair, almost all our leaky cap failures were with 1uF 1206 surface</FONT>
<BR><FONT SIZE=2 FACE="Arial">mount caps, which I think is really pushing the technology of ceramic</FONT>
<BR><FONT SIZE=2 FACE="Arial">caps.&nbsp; With 0.1uF and 0.01uF 0805's from the same manufacturers, we've</FONT>
<BR><FONT SIZE=2 FACE="Arial">had extremely good success.</FONT>
</P>
</UL>
<P><FONT COLOR="#0000FF" SIZE=2 FACE="Arial">I would have thought 1uF in a 1206 would be pretty sorted by now.&nbsp; You can get 1uF in an 0805, and that's using X7R dielectric!&nbsp; You don't want to see the price though.&nbsp; Hmm..I work right next to an AVX factory making tantalum caps...I hate to think what chemicals are floating around in the air.</FONT></P>

<P><FONT COLOR="#0000FF" SIZE=2 FACE="Arial">Mike</FONT>
</P>

</BODY>
</HTML>
</x-html>

2000\02\03@180939 by William K. Borsum

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<x-rich>At 08:17 AM 2/3/00 -0000, you wrote:

>>>>

<excerpt>RE: Supercap for backup power


Donald L Burdette wrote:<color><param>0000,0000,ffff</param>


Most of our leaky cap failures were after the product was deployed.

They're pretty obvious, but unfortunately they are intermittant.  The

come and go randomly, or with temperature, or with physical stress

(flexing) on the PC board.  I hope you have better luck than we did.  We

were using automatic insertion, and parts from AVX and I think NIC.

Haven't built enough by hand to know the reliability.


To be fair, almost all our leaky cap failures were with 1uF 1206 surface

mount caps, which I think is really pushing the technology of ceramic

caps.  With 0.1uF and 0.01uF 0805's from the same manufacturers, we've

had extremely good success.



I would have thought 1uF in a 1206 would be pretty sorted by now.  You
can get 1uF in an 0805, and that's using X7R dielectric!  You don't want
to see the price though.  Hmm..I work right next to an AVX factory making
tantalum caps...I hate to think what chemicals are floating around in the
air.


Mike

</color></excerpt><color><param>0000,0000,ffff</param>

Just talked to an RF manufacturer--says 26-52 week delivery on tantalums
these days--being sucked up by the telcom industry.


</color>

William K. Borsum, P.E. -- OEM Dataloggers and Instrumentation Systems

<<@spam@borsumKILLspamspamdascor.com> & <<http://www.dascor.com>San Diego, California, USA

</x-rich>

2000\02\04@013250 by Dennis Gearon

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I will follow William K. Borsum's advice below when I get to this point.
I wonder if there is anything cheaper than RTV? Also I have had actual
experiences with water soluble fluxes causing asthma and pneumonia.
And......water soluble fluxes conduct a LOT better than rosin core
fluxes, (but are easier to clean off and probably therefore don't
collect 'conductibles' later on in product life)


<big snip>
<Willaim K. Borsum's> said:

>Use only water soluble fluxes and low melting point solders on Getek
>boards.  Wash with very hot water and 409 or some similar cleaner, blow

>dry, then do it again with denatured alcohol.  Final bake at 50 degC
for
>a couple of hours, pot in RTV  in a NEMA-4 box, then add desiccant and
>close it up (with the battery installed of course).


--
-----------------------------------------------------------------
Time Spent With Your Dog(Kid) is Time Well Spent
-----------------------------------------------------------------
   Sincerely, Dennis Gearon

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