RE: RE: Power optimization and some (stupid?) LED questions
From: Demetrius Anger danger
waste.orgDate: Wed, 18 Aug 2004 13:32:41 -0700
RE: RE: Power optimization and some (stupid?) LED questions
From: Demetrius Anger dangerwaste.org
Date: Wed, 18 Aug 2004 13:32:41 -0700
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! Wow thanks so much for all the feedback everyone! I think I have enough now to at least get a grip on what to do. I've got until next weekend to get it done so the dark-switch might not work and I'll have to switch it on and off manually. It looks like (if I understand) the batteries I've got are rated at 13 AH so in parallel that would be 26 AH and with what jim ruxton said: > This means you use about 10 x .020 = .2 amps . > Batteries are rated in Amp Hours. For example if your battery had a 5 > amp hour capacity your battery would last approx. 5/.2 or 25 hours. Do > some research and find the amp hour capacity of your 6 volt batteries 26 / .2 = 130 hours. Close enough to 168 hours. As far as I understand too the amp-hours listing is based on a super low discharge so that makes the amp-hours look bigger so 130 hours is conservative. I'm going to do my best to make a dark-activated switch or somehow not have it run 24/7. Hopefully that will make it last out the week. We'll see. Jessica thanks for the resistor bar idea I will look for that, it sounds like it will save some time. Bob - I think I will let it run at 100%. If I understand what you are saying you are switching it on and off super fast? I will look into this. I probably have my work cut out for me with making a dark-activated switch anyway... I'll check out the books mentioned too. Thanks again for all the help. I looked all over the web and found lots of stuff for LEDs, but nothing really more specific about batteries or how things change when they run in parallel and also there was no real answer to what is happening current-wise when the LED voltage drop matches the source exactly (I still don't understand that). I think I now get why you want a resistor in there and have the total voltage drop across the LEDs to be a bit less than the battery because then the battery can go down and as it does the current will go down but it won't get cutoff by the LEDs right away. I'll post a link to some pictures of the results when it's all over. later! -aaron > -----Original Message----- > From: The ARG Mailing List On Behalf Of Bruce Cannon > Sent: Wednesday, August 18, 2004 1:28 PM > To: Arg-List@Interaccess. Org > Subject: ARG: RE: 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! > > 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! > > R! messages saved at http://www.interaccess.org/arg/arg-list.html > > G! unsubscribe/help requests to mailto:Majordomo@xxxxxxxxxxxxxxx > > > > > > > > > > A! > R! messages saved at http://www.interaccess.org/arg/arg-list.html > G! unsubscribe/help requests to mailto:Majordomo@xxxxxxxxxxxxxxx A! R! messages saved at http://www.interaccess.org/arg/arg-list.html G! unsubscribe/help requests to mailto:Majordomo@xxxxxxxxxxxxxxx