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'[EE] Single AA battery for my board'
2008\06\25@214434 by Tomás Ó hÉilidhe

picon face

I have bi-colour LED's on my board that have about 2.2 volts across them
when there's 25 mA flowing through them.

I'm thinking of using a single 1.5 volt AA battery to power my board.

I've got two questions:

1) Can I go straight from the 1.5 V battery into the LED's without a
current-limiting resistor?
2) What kind of regulator should I use to provide my uC with 5 volts?

Very little current will be drawn from my uC's output pins because all
the output pins go straight into driver chips, so this can be taken into
account when choosing a regulator. Something small and cheap to step 1.5
volts up to 5 volts.

My uC is the 887.

2008\06\25@220023 by M. Adam Davis

face picon face
The LEDs will not work if the voltage is lower than their forward
conducting voltage (Vf).

Your LEDs are likely to have a 2.2 volt Vf (measured by you), and
thuse won't light at 1.5V from a single AA.

You'll need a step up regulator to power them from 1.5V.

-Adam

On Wed, Jun 25, 2008 at 9:43 PM, Tomás Ó hÉilidhe <spam_OUTtoeTakeThisOuTspamlavabit.com> wrote:
{Quote hidden}

>

2008\06\25@221451 by James Nick Sears

flavicon
face
If you want quick and easy for a one-off I've heard good things about this:
http://www.sparkfun.com/commerce/product_info.php?products_id=8248

There's also a 2xAA version.

-n.



On Wed, Jun 25, 2008 at 9:43 PM, Tomás Ó hÉilidhe <.....toeKILLspamspam@spam@lavabit.com> wrote:
{Quote hidden}

>

2008\06\26@072013 by Alan B. Pearce

face picon face
>2) What kind of regulator should I use to provide my uC with 5 volts?

You need a 'boost' switching regulator. These are not trivial to deal with
like linear regulators that reduce the voltage.

For your use I would strongly suggest that you use two cells, i.e. 3V
nominal input - for a very good reason, batteries tend to be packed in pairs
and not singularly. If size is a problem use two AAA cells instead of AA
cells. This also has the advantage that you will achieve twice the mWh than
a single cell will give.

For a suitable step up regulator I suggest you try this chip.
http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1042,P1958 and
use the S8 package as this is the larger SO-8 package of the two options,
and reasonably easy to handle. Pay careful attention to laying out the
circuit board, the data sheet gives a good example of how to do it. By
paying attention to detail the example circuit on the first page can be
adapted to give 5V output with sufficient current capacity for your purpose.

If you want to investigate further the last page of the datasheet gives
alternative possible devices from the same manufacturer. Just be careful
which devices you wish to select, as some of them only come in packages
designed for use in things like cell phones, and are not hobbyist friendly.


2008\06\26@073317 by olin piclist

face picon face
Tomás Ó hÉilidhe wrote:
> I have bi-colour LED's on my board that have about 2.2 volts across
> them when there's 25 mA flowing through them.
>
> ...
>
> 1) Can I go straight from the 1.5 V battery into the LED's without a
> current-limiting resistor?

Think about it.  You have already measure the LED voltage to be 2.2V, at
least at 25mA.  Will the LED draw more or less current at 1.5V?  You could
use your measuring setup to decrease the current until the voltage drops to
1.5V and see what happens.  Can you even see any light from the LED at 1.5V
in normal office illumination?

In fact, here is a good exercise.  Measure the LED voltage at various
currents, like every 5mA going down from 25mA.  (are you sure you LED is
really rated for 25mA, not 20mA?)  Also include 2mA and 1mA.  Then plot
current as a function of voltage and show us the result.  For extra credit
make some rough notes about how readily visible the light was in office
illumination, daylight, and in a dark room.

Seriously, you should really do this.  You will learn a lot about LEDs and
get some intution that is nearly impossible to get from any datasheet.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.

2008\06\26@081159 by Bob Ammerman

picon face
Tomás:

> I have bi-colour LED's on my board that have about 2.2 volts across them
> when there's 25 mA flowing through them.
>
> I'm thinking of using a single 1.5 volt AA battery to power my board.
>
> I've got two questions:
>
> 1) Can I go straight from the 1.5 V battery into the LED's without a
> current-limiting resistor?

No, and for two reasons:

1) Basically an LED conducts no current until a certain threshold voltage is
reached, which for your LEDs is about 2.2 volts. Thus, a 1.5 V supply will
not provide enough voltage to start the LED conducting at all.

2) If your voltage source was great enough to get the LED to conduct (say 2
x 1.5V AA = 3V) then excessive current would flow, becuase the LED would try
to load the source down until the voltage across the LED was reduced to its
'magic' 2.2V forward drop. Since the only resistance in the circuit would be
that in the battery, wiring and driver chip way too much current would go
through the LED.

> 2) What kind of regulator should I use to provide my uC with 5 volts?

You would need a step-up switching regulator in this case.

HOWEVER:

There is no reason to provide an exact 5V to the PIC. It will work fine on
quite a bit less, and does not even need to be regulated.

A SHORT TUTORIAL ON LED's (and power sources)::

An LED (or any 'forward-biased' regular diode) has a parameter called Vf, or
'Voltage, forward biased'. This value varies from one type of LED/diode to
another, and is determined by the particular type of semiconductor in the
LED/diode. Basically, no matter what current is flowing through the diode,
the same (actually nearly the same) voltage will appear across it. This is
in contrast to a resistor, where the voltage across the device is
proportional to the current through it. As mentioned above, this has two
consequences:

1) The LED won't conduct at all until you reach the threshold.
2) The LED will (try to) draw whatever current is needed to get the source
voltage to drop to its threshold voltage.

Now a voltage source (like a power supply or battery) has its own internal
resistance which determines its 'stiffness'. For example a tiny button cell
battery has a rather large internal resistance, so when you try to draw
significant current from it much of its voltage is 'dropped' across the
internal resistance (good old Ohm's law) and the voltage seen at the battery
terminals tends to drop quite quickly as the current drawn increases. In
fact, I have a cheap little keychain LED flashlight which depends on the
internal resistance of two button cells to limit the LED current (no
explicit resistor in the circuit). On the other hand a car battery, for
example, has a very low internal resistance. You would have to draw a very
large current from it for much of its voltage to be dropped internally.

So, when you hook up your LEDs, your circuit is effectively the following
(all in series):

Voltage source (battery or power supply) with its internal resistance, call
this Vsupply and Rsupply
Resistance of wiring (Rwiring)
LED with its forward voltage and its own internal resistance (pretty low)
(Vf.led and Rled)
Explicit resistance in circuit (Rdrop)
Resistance and forward voltage drop of driver transistor/chip (Vf.driver and
Rdriver)

Ohm's law says the current in your circuit will be:

(Vsupply - Vf.lef - Vf.driver) / ( Rsupply + Rwiring + Rled + Rdrop +
Rdriver )

Notice the only term in the denominator that you have explicit control over
is 'Rdrop'. Normally you would design your circuit so that Rdrop dominates
the other resistances in the cicuit in order to result in a predictable
current.

-- Bob Ammerman
RAm Systems





2008\06\26@082207 by Bob Ammerman

picon face
> I'm thinking of using a single 1.5 volt AA battery to power my board.

One thing to remember: TANSTAAFL (there ain't no such thing as a free
lunch).

Let's assume you have an AA battery rated at 2000maH.

First, This specification does not, by the way, necessarily mean that you
can draw 1000ma for two hours, or 2000ma for one hour, or 100ma for twenty
hours. It is measured at a given discharge rate [see the datasheet :-)] and
at higher rates the available power will be less. This is (at least partly)
due to the energy dissapated in the battery's internal resistance, which
goes up as the current goes up.

Second, with a 1.5V source you are going to need some sort of step-up
converter. Conservation of energy says that the current into the converter
will always be (at least) proportionally greater than the current out. For
example, given a 100% efficicient converter from 1.5V to 5V (which of course
doesn't exist), drawing 500ma out would require 500ma*5V/1.5V = 1666ma input
current (TANSTAAFL!).

-- Bob Ammerman
RAm Systems

2008\06\26@083420 by Jinx

face picon face
> I have bi-colour LED's on my board that have about 2.2 volts across
> them when there's 25 mA flowing through them

Tomas, if you look at the spec sheet for the bi-colour you'll see that
each colour has a different forward voltage, Vf. Red is lowest (~1.7V)
and typically Vf rises as you go up the rainbow through yellow, orange,
green, blue, to white. Green also typically uses more power than red
for the same perceived brightness. Before getting very specific about
other components, you really should sort out what colour LEDs and
what style (standard, diffused, water-clear, superbright, ultrabright etc)
are going to be in the final product. To make that decision you are
going to have to get stuck into LED data sheets

> I'm thinking of using a single 1.5 volt AA battery to power my
> board

> 1) Can I go straight from the 1.5 V battery into the LED's without
> a current-limiting resistor?

You can, but if Vf is higher than Vbatt, and it is, the LED will not light

> 2) What kind of regulator should I use to provide my uC with
> 5 volts?

If you're converting an AA to 5V, then you'll need a boost circuit,
and obviously the higher the conversion efficiency the better as
the AA is a limited reserve of power

2008\06\26@084658 by Jinx

face picon face
> given a 100% efficicient converter from 1.5V to 5V (which of
> course doesn't exist), drawing 500ma out would require 500ma*
> 5V/1.5V = 1666ma input current (TANSTAAFL!).

ISTR Tomas reckoned 800mW for the circuit, so the conversion
process might explain the "Why not millwatt hours ?" thread

800mW out will be (800mW/conversion efficiency) in, more or less.
533mA @ 1.5V would be drawn from the AA to make 160mA @
5V, giving a realistic lifetime of 3-4 hours at a reasonable conversion.
Less battery current when fresh than when it's running out of puff. I
think battery capacity rating is down to 1V terminal voltage but you'd
have to check that

2008\06\26@084831 by Jinx

face picon face
Bob Blick, Russell and Roman Black have made small discrete 1.5V
boost circuits, Google

2008\06\26@092611 by James Nick Sears

flavicon
face
How about a Li-Ion battery with a charging IC from Maxim?  Good
density and easy recharging (the chip and a couple of caps are all you
need), along with an easily usable voltage (3-3.7V for almost the
entire life).  For the good, but not cheap (but it's rechargable
remember), route, Mouser has a number of different options.  This one
is 1.7Ah for $13.46:
http://www.ultralifebatteries.com/documents/techsheets/UBI-1014_UBP103450.pdf

For the cheap, but maybe not good route, AllElectronics (or maybe it
was Electronics Goldmine) had surplus batteries awhile back that were
OK.

The MAX1551/1555 Li-Ion charging ICs are ridiculously easy to use
(http://www.maxim-ic.com/quick_view2.cfm/qv_pk/4002).  You could
either learn to deadbug wire the SOT23/5 package or buy a little
breakout board to solder it to
(http://www.sparkfun.com/commerce/product_info.php?products_id=717) or
get your own PCB done.  Or you could just buy this:
www.sparkfun.com/commerce/product_info.php?products_id=726
Maxim will give you samples of the chip for free though if you go one
of the other routes.

-n.


On Fri, Jun 27, 2008 at 3:46 AM, Jinx <joecolquittspamKILLspamclear.net.nz> wrote:
{Quote hidden}

> -

2008\06\30@205248 by Charles Rogers

picon face
Sent: Thursday, June 26, 2008 6:35 AM
Subject: Re: [EE] Single AA battery for my board


Tomás Ó hÉilidhe wrote:
> I have bi-colour LED's on my board that have about 2.2 volts across
> them when there's 25 mA flowing through them.
>

As someone on this list said, "The journey is the reward".

CR

In fact, here is a good exercise.  Measure the LED voltage at various
currents, like every 5mA going down from 25mA.  (are you sure you LED is
really rated for 25mA, not 20mA?)  Also include 2mA and 1mA.  Then plot
current as a function of voltage and show us the result.  For extra credit
make some rough notes about how readily visible the light was in office
illumination, daylight, and in a dark room.

Seriously, you should really do this.  You will learn a lot about LEDs and
get some intution that is nearly impossible to get from any datasheet.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.

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