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RE: Power optimization and some (stupid?) LED questions

From: Bruce Cannon   bcbrucecannon.org
Date: Wed, 18 Aug 2004 12:23:03 -0700

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You need a good basic electricity reference on your desk.  Buy Art of
Electronics and try and plow through it.  Also look at Radio Shack for their
battery handbook and books on power supply design.

1. Dark-activated switch could be v divider made from a CdS cell and a pot,
feeding a schmitt trigger inverter or a micro input etc. driving a big
transistor switch.  You'd have to take into consideration the power loss in
the switch too.

2. Yes you could leave out a current-limiting resistor if your supply
voltage is matched well-enough to your LED voltage, I've seen designs which
do it.  But in your case your chosen battery chemistry causes voltage
decline as the battery capacity diminishes so LEDs which don't burn up when
the battery is fresh will stop illuminating as soon as the battery voltage
starts to droop.  Much easier to use a current-limiting resistor value cal'd
for highest voltage.

3. For max batt life you might do better with a Vreg set up as a
current-limiter, but I haven't run the numbers to see if it's worth the
trouble.

4. Batts in series add their voltages but not their capacities, in parallel
add their capacities but not their voltages.

5. LEDs in series add their voltage drops but not their current, in parallel
all share the same voltage drop but divide their current.  In other words:
	Case A: two LEDs each rated at 20mA continuous safe current and 2v drop, in
series each drop 2v so they won't light at all from a 3v source.  And need a
ohm current-limiting resistor for a 4.5v source (because E/I=R, and so 0.5v
(4.5v supply minus 2+2v LED drop) divided by 20mA (.02A) equals 25 ohms, or
27 closest std value up.
	Case B: two same LEDs in parallel drop 2v total, so 2.5v (4.5v supply minus
2v LED drop) / .04A (.02 for each LED) = 62.5 ohms needed, or 68 closest
standard value.

6. To figure out how long you can run on a battery, you need to look at the
mfr's discharge curve and try and estimate time to cutoff voltage.
According to the Digikey catalog each alkaline AA will provide 170hrs at 100
ohm cont load to a 0.9v cutoff.  It would be the same for three if you ran
them in series, and the 0.9v cutoff per cell gives 2.7v cutoff for three so
the LEDs are still working at end of life.  That means you could run 1 LED
at 15mA for a week (24x7=168), or 3 LEDs if you use a dark cutoff and it's
only light in your space for 8hrs a day.

7. High-brightness LEDs cost more but can easily give the same light output
at 1.5mA as a cheap LED at 15mA, so again look to the digikey catalog and
study specs carefully and you can increse the number of LEDs for a given
battery capacity by an order of magnitude.

Once you think all this through, and do some reading, you'll be able to
figure out how to apply all this info to your own situation.  But I think
you either have a lot of studying to do or hire someone to do it for you.

Good luck,

--Bruce




> -----Original Message-----
> From: woner-arg-list@xxxxxxxxxxxxxxx
> [mailto:woner-arg-list@xxxxxxxxxxxxxxx]On Behalf Of Demetrius Anger
> Sent: Wednesday, August 18, 2004 9:47 AM
> To: arg-list@xxxxxxxxxxxxxxx
> Subject: ARG: Power optimization and some (stupid?) LED questions
>
>
> A! Sent via the Art & Robotics Group mailing list:
> arg-list@xxxxxxxxxxxxxxx
> R! Use your "Reply All" to  reply to the list, "Reply" for
> private response
> G!
>
>
> I don't know if these questions are way off topic or not - it's not really
> about robotics.  It is just the beginning of learning for me though so I'm
> trying to start simple.  Let me know if it is totally off-topic.
>
> I have some questions about battery power.  I'm making lighting for a
> scultpure that uses LEDs.  I'm going to (try to) make a dark-activated
> switch so it only runs at night, but it needs to run for about a week.  I
> can always go switch out batteries but I'd rather just make something that
> doesn't need any attention so I'm trying to figure out if there are
> different ways to do things to optimize the power use.
>
> I got a couple 6 volt lantern batteries that I was going to use.
>
>
> Okay stupid question(?) first - if the battery is 6V and the LEDs have a 2
> volt drop and want to run on 20mA do I need to put a resistor in
> there if I
> use three LEDs in series?  This is totally confusing me.  Do I
> calculate the
> resistance as if the LEDs weren't there?  Like the ideal LED is without
> resistance right, but then there's a voltage drop so then it's got to be
> doing something right?  I am feeling stupid about this because I think
> someone has explained this to me before.
> Aaa!
>
> Also, does it make a difference if I set the batteries up in series or
> parallel in terms of power use?  Or does it make a difference in power how
> much the LEDs are in parallel or series?
>
> I want to run about 20 of the LEDs.  I'm just trying to get a general idea
> of whether or not it matters how I set it up.  Like 2 batteries in series
> with parallel sets of 6 LEDs in series.  Or 2 batteries in parallel with
> parallel sets of 3 LEDs.  Or should I put every single LED in parallel for
> some reason?
>
>
> Anyone done battery powered stuff?  Should I get more batteries
> or are my 2
> 6V batteries going to last the week?  I can't find any
> information about how
> to calculate battery life.
>
> The only thing I can find is that battery life is longer at lower rates of
> discharge.  I went to some of the battery manufacturer sites and
> can't find
> anything about amp/hours (which is what someone told me how battery
> discharge was measured) or battery life at all.
>
> Thanks for any help!  If these are too many questions or whatever anyone
> have a source I can read up on?
>
> -aaron
>
>
> A!
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