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'[OT]: Battery power circuit'
2001\03\14@180855 by Kevin Olalde

picon face
Anyone know where I can find general information concerning battery
powering a PIC based circuit?

I'm currently running off a 12V 1A wall wart, through a small 7805
circuit.  I've hooked up a 9V battery in place of the wall wart, seemed
to work, but I'm guessing there's a much better way. I don't know how
much current I'll actually need since I'm still learning/designing.

I know this is a vague, newbie question, I just looking for
considerations for using batteries and support power circuitry that
wastes little but still works with damaging my 16F84.

Any pointers would be appreciated.

Thanks,

Kevin

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2001\03\14@181454 by Bob Blick

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On the test bench I frequently use a pack with 4 nicads. It is very close
to 5 volts, then I don't need a regulator.

For a production device you need to do something more complicated.

Cheers,

Bob

On Wed, 14 Mar 2001, Kevin Olalde wrote:

{Quote hidden}

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2001\03\14@182204 by David VanHorn

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At 06:08 PM 3/14/01 -0500, Kevin Olalde wrote:
>Anyone know where I can find general information concerning battery
>powering a PIC based circuit?
>
>I'm currently running off a 12V 1A wall wart, through a small 7805 circuit.


Not a bad start, but maybe worse than it seems (how's that for encouragement)
12V wall warts come in many flavours.
AC, rectified but unfiltered (the commonest), barely filtered, well
filtered, regulated, and switchers.

You want at least a rectified wall wart, plus a decent sized filter cap.
1000uF/A is typical.

The main thing is that under load, low line etc, the min output voltage
can't be less than about 7V into the reg.

Also, you don't want the voltage any higher than you need, because the reg
will dissipate all of the extra power as heat.  Ex: 12V in at 1A, 5V out at
1A, reg dissipates 7W, project dissipates 5W.


I've used 12V gell cells with a wall-wart charger as a good compromise supply.
You actually run on the batteries, and they are better at supplying pulse
current.

9V transistor batteries are going to make you poor real fast.



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2001\03\14@183551 by Alice Campbell

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Well, start with Microchip's website, specifically the
appnotes, specifically AN606.

Then troll the archives for low-dropout regulators.

The 78xx and 78Lxx series regulators are terrible for
battery-operated equipment, but fine with a wallwart.

There are lots of LDOs, they work great and are simple to
use: for example, desolder 78L05, solder in LDO.

More complex, but interesting, are switching regulators that
boost 1.5 volts to 3 volts, or 3 volts to 5 volts, thus
allowing AA or AAA batteries to be used.  But be careful if
you have circutry that is sensitive to noise, for example, an
ADC you paid >$15 for.  the racket the step-up regulators
make may limit the resolution of a nice ADC to the resolution
you could get with a wet string.

alice



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2001\03\14@231623 by Dan Michaels

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Kevin Olalde wrote:

>>
>>I'm currently running off a 12V 1A wall wart, through a small 7805 circuit.
>>circuit.  I've hooked up a 9V battery in place of the wall wart, seemed
>>to work, but I'm guessing there's a much better way. I don't know how
>>much current I'll actually need since I'm still learning/designing.
>
>>I know this is a vague, newbie question, I just looking for
>>considerations for using batteries and support power circuitry that
>>wastes little but still works with damaging my 16F84.


Kevin,

If you are considering running off 9v battery power some of the time,
then you should look into ways to reduce current draw in the circuit.
Those batteries only have about 500 mAH of juice [IIRC].

Try LDO/low-quiescent current regulators as Alice mentioned, using a
crystal no faster than absolutely necessary for the app, going into
sleep mode between ops, CMOS for opamps and other circuitry.

Changing to a more efficient v.reg is the first step. Also, regarding
the wall wart, you can probably get by with a much smaller variety.
Be aware that a 12v, 1A W/W probably puts out 15-17v unloaded, and
it is unlikely you will need 1A for a while anyway. Even a 6vdc,
200-300 mA W/W will easily run a device eventually slated for
battery ops. For a very low draw datalogger, I have even used 2 3v
lithium batteries in series to drive an LDO v.reg.

The output of a standard DC W/W is usually not regulated, but should
work ok with a v.reg and 10-22 uF electrolytic caps for filtering.
Better to steer clear of AC output W/Ws, as these will require more
complicated power circuitry.

- Dan Michaels
http://www.oricomtech.com
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2001\03\14@233305 by Dan Michaels

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Bob Blick wrote:
>On the test bench I frequently use a pack with 4 nicads. It is very close
>to 5 volts, then I don't need a regulator.
>

Dave Van Horn wrote:
>I've used 12V gell cells with a wall-wart charger as a good compromise supply.
>You actually run on the batteries, and they are better at supplying pulse
>current.


Good, we have 2 battery jockeys here today. This question just came
up yesterday :).

What happens when you operate your PIC off batteries, and the batteries
run down so much that the v.reg output drops down below the normal
operating range? I made some measurements on this [again, yesterday],
and discovered:

  Things tend to function down to a point. The output of the v.reg
  just falls off as the voltage in to it falls. Current draw in the
  ckt continues to drop - the v.reg doesn't seem to go berserk or
  stall/etc.

More importantly, if you allow Vdd to the PIC to drop below the lower
spec limit, I assume the PIC will simply stop running. However, I
wonder whether it will damage the PIC to leave below spec Vdd on
it for a "long" period of time.

Anybody know about this?

- dan michaels
http://www.oricomtech.com
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2001\03\14@233921 by David VanHorn

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>
>What happens when you operate your PIC off batteries, and the batteries
>run down so much that the v.reg output drops down below the normal
>operating range?

Short version: Wierdness.
Don't go there.

You don't even want to run it low enough that the reg stops regulating if
you can avoid it.
Definitely, you should have /MCLR asserted before VCC drops out of spec.


>More importantly, if you allow Vdd to the PIC to drop below the lower
>spec limit, I assume the PIC will simply stop running. However, I
>wonder whether it will damage the PIC to leave below spec Vdd on
>it for a "long" period of time.

Nothing in the micro will be damaged, but it may erase internal EEPROM, or
the outputs may go into states that cause problems in other parts of the
circuit.


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2001\03\14@235353 by Kevin Olalde

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Great, thanks all, this will get me started.  I think a switching
regulator may be what I need.  I'd rather not have to put 4 AAAs in the
project just to use a LDO to get back to 5V (too big/heavy anyway, I'm
going for small).  Are all switching regulators so expensive (>$5
range)?

Thanks,

Kevin

Alice Campbell wrote:
{Quote hidden}

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2001\03\15@003017 by David VanHorn

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At 11:56 PM 3/14/01 -0500, Kevin Olalde wrote:
>Great, thanks all, this will get me started.  I think a switching
>regulator may be what I need.  I'd rather not have to put 4 AAAs in the
>project just to use a LDO to get back to 5V (too big/heavy anyway, I'm
>going for small).  Are all switching regulators so expensive (>$5
>range)?

In small quantities they aren't cheap.
In volume, you buy your inductors in china for pennies, and you might not
even use an SMPS chip, just a couple of transistors.

Low current boost regs are a great way to get started with switchers.
Well behaved, and if anything goes wrong, minimal smoke.

Beware the charlatan with the charge pump capacitor converter though. The
efficiency is terrible.

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2001\03\15@005741 by Dale Botkin

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Just curious - what's wrong with three alkaline AA's?  Planning on usig
lots of LEDs or servos?  I get a month or so out of a set.

On Wed, 14 Mar 2001, Kevin Olalde wrote:

{Quote hidden}

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2001\03\15@012111 by Bill Westfield

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   Beware the charlatan with the charge pump capacitor converter though.
   The efficiency is terrible.

Even compared to a linear regulator?

Most of the "regulated" charge pump capacitors double the battery
voltage and then run that through a LDO linear regulator.  IIRC, the
doubler efficiency is about 85%, and the linear regulator efficiency is
"typical and as required by physics."  On the flip side, 2AA cells
contain quite a bit more energy, for a lower price, than a 9V alkaline,
so if you're specifically after increased battery life, you may still
win...  (and you'll do significantly better if you can do without the
regulator, somehow letting the voltage drop from Vccmax to Vccmin...)

There's an interesting power supply circuit for the (ultra-low power)
MSP430 on TI's web site.  It uses four transistors to drive a capacitive
voltage doubler to run the processor (which requires 1.8 to 3.6V) from a
single 1.5V battery.  It ought to work with higher input voltages as well.

http://www.ti.com/sc/docs/psheets/abstract/apps/slaa105.htm

BillW

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2001\03\15@013849 by David VanHorn

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At 10:21 PM 3/14/01 -0800, William Chops Westfield wrote:
>     Beware the charlatan with the charge pump capacitor converter though.
>     The efficiency is terrible.
>
>Even compared to a linear regulator?

As a switcher.
Linears don't do voltage boost.

Except for size, it's pretty hard to beat a low R coil, schottky diode, and
low RDSon mosfet.


>Most of the "regulated" charge pump capacitors double the battery
>voltage and then run that through a LDO linear regulator.  IIRC, the
>doubler efficiency is about 85%, and the linear regulator efficiency is
>"typical and as required by physics."

The problem is in the switches. The charge pump requires relatively high
currents through the switches, which are never very good switches.

>  On the flip side, 2AA cells
>contain quite a bit more energy, for a lower price, than a 9V alkaline,
>so if you're specifically after increased battery life, you may still
>win...  (and you'll do significantly better if you can do without the
>regulator, somehow letting the voltage drop from Vccmax to Vccmin...)

AA cells are about the best bang for the buck.


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2001\03\15@051719 by Martin Hill

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If you are using a low power circuit, look at the MAX619.  This is
what I use on the AC-22 Performance Meter, that takes about
40mA and will operate for about 30-40 hours from a pair of AA Zinc
Chloride batteries, this is OK for upto about 50mA with input
voltages down to about 2V.

Martin

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2001\03\15@071835 by Kevin Olalde
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One of the compnents in the circuit requires 4.5-5.5V.  Three AA's would
get me to 4.5, but I assumed it'd drop off too quickly and I'd be below
spec.  Part of my concern was how to avoid under powering items (which
seems to be getting discussed).

Can someone eleborate on how to "have /MCLR asserted before VCC drops
out of spec"?  Thanks.

Kevin

Dale Botkin wrote:
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2001\03\15@090027 by Kevin Olalde

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Dan Michaels wrote:
> If you are considering running off 9v battery power some of the time,
> then you should look into ways to reduce current draw in the circuit.
> Those batteries only have about 500 mAH of juice [IIRC].

Yep.  I was just testing with a 9V, but want to run off something
smaller eventually.

> Try LDO/low-quiescent current regulators as Alice mentioned, using a
> crystal no faster than absolutely necessary for the app,

I'm using  4MHz resonator.  Part of the application will be
sending/receiving serial data over RF links, I'd like to get 9600 buad
out of it, so I'm guessing on my 16F84A-20 I won't want to go to a much
slower crystal.  (yes?)

> going into
> sleep mode between ops, CMOS for opamps and other circuitry.

I definately like this path too.

> Changing to a more efficient v.reg is the first step. Also, regarding
> the wall wart, you can probably get by with a much smaller variety.
> Be aware that a 12v, 1A W/W probably puts out 15-17v unloaded, and
> it is unlikely you will need 1A for a while anyway.

I should have been more clear.  It's actually a rated for DC 9V 1A, I'm
driving very little, so I was assuming it was putting out 12V.......I
just measured it, about 12.8V before the regulator.

> Even a 6vdc,
> 200-300 mA W/W will easily run a device eventually slated for
> battery ops. For a very low draw datalogger, I have even used 2 3v
> lithium batteries in series to drive an LDO v.reg.
>
> The output of a standard DC W/W is usually not regulated, but should
> work ok with a v.reg and 10-22 uF electrolytic caps for filtering.
> Better to steer clear of AC output W/Ws, as these will require more
> complicated power circuitry.

Yes, great advice.  Lucikly for me I'm following some samples that I got
for Peter Anderson's BS2 starter kit.  47uF caps electrolytic caps
before and after the regulator and a diode across the 7805 for reverse
voltage protection.  So the WW and power supply is intended to be useful
for small 12V motor experiments too.

If I'm reading things correctly (which is a pretty big IF) the PIC will
draw less than 2mA (unless in sleep mode).  The RF transmitter's
datasheet (TWS-434) says it has a peek current rating of 9mA.  I'm going
to keep away from things like LED/LCDs.  The received data (over a
RWS-434) will be serially sent to a pda (have to lookup the current draw
there).  The pda will also provide input to the PIC which it will send
through the transmitter.  I'm not sure of the receiver's current draw,
I'm guessing it'd be less than that of the transmitter (although I don't
know why I'd guess that, just sounds right).

Thanks for all the input, great group!

Kevin

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2001\03\15@114307 by Dan Michaels

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Kevin Olalde wrote:

>
>> Try LDO/low-quiescent current regulators as Alice mentioned, using a
>> crystal no faster than absolutely necessary for the app,
>
>I'm using  4MHz resonator.  Part of the application will be
>sending/receiving serial data over RF links, I'd like to get 9600 buad
>out of it, so I'm guessing on my 16F84A-20 I won't want to go to a much
>slower crystal.  (yes?)
>

Actually, with finely-tuned code, you should be able to get upwards
to 115,200 baud when using an xtal in the 4Mhz range. This would
be where the instructions that sample the RxD line and build up
received bytes exactly equal the time to process one character,
with no additional delays inserted in the code.
===========


>
>Yes, great advice.  Lucikly for me I'm following some samples that I got
>for Peter Anderson's BS2 starter kit.  47uF caps electrolytic caps
>before and after the regulator and a diode across the 7805 for reverse
>voltage protection.  So the WW and power supply is intended to be useful
>for small 12V motor experiments too.
>

IIRC, the reverse diode across the 7805 doesn't actually provide for
reverse voltage protection, but prevents the cap on the output of the
v.reg chip from discharging back into the chip when power is removed.
You would actually need a diode in series in the line from the
battery to protect from reverse voltages.
============


{Quote hidden}

Also, if interfacing with a PDA, you will have to check on relevant
voltage levels - PDA<->RWS-434 and possibly also PDA<->PIC. If you
are going to use RS-232 comms, then your PIC needs a level translator
like MAX232 chip or something similar. Unfortunately the MAX's draw
several mA continuously, so they will also drain your battery. You
might look at some of the BJT RS-232 circuits, such as that used on
the Basic Stamps, which draw zero quiescent current. Can be found
somewhere on piclist/techref, and I also have an appnote about this:

AN-ECD02 - A Simple Inexpensive Network-Compatible RS-232 Interface

http://www.oricomtech.com/appnotes.htm

- dan michaels
==============

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2001\03\15@120841 by David VanHorn

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>
>IIRC, the reverse diode across the 7805 doesn't actually provide for
>reverse voltage protection, but prevents the cap on the output of the
>v.reg chip from discharging back into the chip when power is removed.
>You would actually need a diode in series in the line from the
>battery to protect from reverse voltages.
>============

Almost right.
Large caps on the output of the reg can destroy the reg, if the reg input
is shorted to ground.
The diode provides a safe place for that current to go.

It's documented sporadically, but is true for all 78XX, 340T-XX and similar
regs.


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2001\03\15@124203 by Alice Campbell

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Ok, heres a booster ive been messing with, it delivers about
15mA in this incarnation. Im trying for about 20 mA and 4 ot
5 volts.


                in 2-3 V
                  |
                  |
  ----------------|---------|
  <               |         |
  > R1         ---K  pnp    < R2
  <            |   \        >  D1
  |            |    \       o-->I-o-o---out ~4-5V
  |---o-------K npn  |      |     | |
  |   |        |     |      |     Z = C2
  |   -----------||--o-RR--K npn  | |
  |            |  C1   R3   |     | |
  |-------------------------------| |
               |            |       |
              vss          vss     vss

R1=1M
R2=220-450r
R3=1K or more
C1=0.1u
C2=>50u
Z=zener, your choice, 5.1 maybe
D1=small diode maybe shottkey

I would welcome improvements, this is my first attempt.

alice

 Are all switching regulators so expensive (>$5
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2001\03\15@130319 by David VanHorn

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At 09:38 AM 3/15/01 -0800, Alice Campbell wrote:
>Ok, heres a booster ive been messing with, it delivers about
>15mA in this incarnation. Im trying for about 20 mA and 4 ot
>5 volts.


What is R2 doing for you?

I'd put a current sense R in the emitter lead (much smaller) and use that
developed voltage to turn on an NPN that sucks the base drive from the
final NPN.  The more gain in that final NPN the better.

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2001\03\15@162547 by Andy Jancura

picon face
Hello Kevin,

I use in my NiCd powered application only MAX88x family as it can adjust the
output voltage in wide range and what is much more important for NiCd, when
they drop to 0.9V/cell it automaticaly disconnect the whole circuit and
takes only about 10 - 15uA from bateries. I have of course external reset
circuit like MCP130 too, to assert the /MCLR pin correctly before the
definitive voltage shut down.

Andrej

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2001\03\15@170102 by Alice Campbell

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Thanks Dave,

> At 09:38 AM 3/15/01 -0800, Alice Campbell wrote:
> >Ok, heres a booster ive been messing with, it delivers about
> >15mA in this incarnation. Im trying for about 20 mA and 4 ot
> >5 volts.
>
>
> What is R2 doing for you?

err, well, last night it was keeping the magic smoke inside
of T3.

>
> I'd put a current sense R in the emitter lead (much smaller)
of the final?

and use that
> developed voltage
in the collector?
to turn on an NPN that sucks the base drive from the
> final NPN.  The more gain in that final NPN the better.
>
maybe 1M?

> --
alice

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2001\03\15@171857 by David VanHorn

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>
>err, well, last night it was keeping the magic smoke inside
>of T3.

If you arrainge it so the emitter resistor has enough drop to turn on
another transistor, a bit before the inductor saturates, then you are there.



> > final NPN.  The more gain in that final NPN the better.
> >
>maybe 1M?

??? 20-50k is the best I've ever seen, in a slowlington.
I'm just thinking of the wasted base current when my Turner-Offer
transistor is on.

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2001\03\15@174153 by David Duffy

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Kevin wrote:
>One of the compnents in the circuit requires 4.5-5.5V.  Three AA's would
>get me to 4.5, but I assumed it'd drop off too quickly and I'd be below
>spec.  Part of my concern was how to avoid under powering items (which
>seems to be getting discussed).
>
>Can someone eleborate on how to "have /MCLR asserted before VCC drops
>out of spec"?  Thanks.

This means using a voltage supervisor of some sort. We use the Seiko S8054
series and Microchip MCP120 series in our designs all the time. It's a 3 pin
device that pulls its output low when the supply drops below a certain level.
This way, the PIC is kept in reset until the power is up to spec. This prevents
the PIC from running when the voltage is too low and doing "weird" stuff !  :-)
Regards...

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2001\03\16@081139 by Roman Black

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Alice Campbell wrote:
{Quote hidden}

Hi Alice, if your buck inductor is a toroid type,
maybe you could wrap an extra few turns of wire
and use this winding to give neg feedback. This lets
you use only one transistor (cost savings) and
also gives some simple regulation as the load affects
the voltage on the feedback winding.

I see these in Akai VCRs, they don't use an inductor
but instead a tiny RF can type coil with two windings
driven by a small 500mA transistor. One coil, one tran
and a couple of caps. Cheap as it gets! :o)
-Roman

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2001\03\16@155050 by Peter L. Peres

picon face
>Most of the "regulated" charge pump capacitors double the battery
>voltage and then run that through a LDO linear regulator.  IIRC, the
>doubler efficiency is about 85%, and the linear regulator efficiency is
>"typical and as required by physics."  On the flip side, 2AA cells

Check out S8230

Peter

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2001\03\16@160136 by Peter L. Peres

picon face
> If you arrainge it so the emitter resistor has enough drop to turn on
> another transistor, a bit before the inductor saturates, then you are
> there.

Sorry to disagree but unless one uses an opamp or an amplified shunt that
won't do. At the low operating voltage the shunt rise to 0.6V is not
affordable. There is a simple <grin> scheme that does this using the
voltage drop on the saturated transistor proper to sense current ...

Peter

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2001\03\16@160153 by Peter L. Peres

picon face
>More importantly, if you allow Vdd to the PIC to drop below the lower
>spec limit, I assume the PIC will simply stop running. However, I
>wonder whether it will damage the PIC to leave below spec Vdd on
>it for a "long" period of time.

I have had PICs in battery powered devices running with the battery down
to 2V and below. Sometimes they do not do what they are meant to do
anymore.

If this is important you can use the brownout protection circuit to cause
a reset when the battery goes low. One interesting problem is that
brownout circuits without a hysterezis or 'latching' will cause the
circuit to operate intermittently for many thousands of times near the end
of life. What happens is:

Circuit works -> Battery droops slowly -> brownout trips -> circuit in
reset -> battery not loaded, increases slowly -> brownout releases reset
and circuit works some more etc.

I found it important to find out how long these periods can be and to
build an initial delay into my code that prevents it from doing anything
at all for up to 10 times as long. This means a 1-3 sec delay after reset
and before setting up any TRIS registers in my code. This is still not
enough if you try to do something at each turn-on (like increment a
counter stored in EEPROM or such).

hope this helps,

Peter

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2001\03\16@163929 by Peter L. Peres

picon face
>On the test bench I frequently use a pack with 4 nicads. It is very close
>to 5 volts, then I don't need a regulator.

But you need a fuse ;-). I also used NiCds for this until what waited to
happen happened. Since then I use 4 cheap AA batteries ($1 for 4) in a
holder for that. I wear them out using a bulb first, until the voltage
drops some, then run the circuit through one or two 1N4007 diodes (each
drops ~0.5V at that current). The weak batteries are relatively short
circuit proof as opposed to the NiCds and I can change the voltage for
testing by clipping a crocodile clamp after one or two diodes.

Of course the benchtop PSU is much better for this unless all its outputs
are already used up...

Peter

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2001\03\17@065413 by Peter L. Peres

picon face
The circuit I have sent in another thread (CCFL backlight) some time
before uses one transistor and a transformer to achieve CCFL drive or
boost (unregulated). The usual way to regulate for my circuit is a zener
arrangement after the rectifier.

In my experience it is not realistic to try to achieve more than 3:1 boost
from a single coil booster under reasonable load. 2:1 is a more
conservative figure that is more likely to work for beginners. To obtain
more another coil is required (in series with the driven coil).

Peter

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