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Topic: Voltage regulator needed? Or not? Portable LED PWM (Read 7303 times) previous topic - next topic


Jan 19, 2011, 09:11 am Last Edit: Jan 19, 2011, 11:03 am by SpiffyChee Reason: 1
I need advice for which batteries to use (and if I will then need a voltage regulator or not.) This project is extremely important to me. (Its my first big electronics project ever. I'd really appreciate the assistance.

I'm building an ATmega328 powered LED PWM circuit with 6 PWM channels.

I have to build this device as small as possible, but to run as bright as possible for as close to an hour as I can get (or higher)

I basically have the same setup as http://fritzing.org/projects/arduino-knight-rider-with-8-blue-12v-led-modules/ except I'm only using 6 Leds (or 12 (2 per channel)) and my leds are different. I'm looking at using these leds:http://www.sparkfun.com/categories/173 Which run at 60-75ma) with a forward voltage of 3.0-3.4 volts. Obviously I will not use the 12 source like in the link. I need to figure out my own source.

I need to use rechargeable batteries, so I've narrowed it down to dealextreme.com's 3.7 volt or 3.0 volt CR123A's or the 3.7 18650. (I want it to be as small as possible, but still have near an hour of battery life.)

I'm really desiring a consistent accurate PWM in my circuit, but I don't know what do do about the batteries and voltage regulator...
Should I use 1 CR123A? 2 CR123A's in series? in parallel? 3.0volt? 3.7volt?
Or should I use the 18650 3.7 volt battery? or 2? series/parallel?

then based on my battery choice will I need a voltage regulator? which one?

Also should I be powering 6 leds? or 12 (6 sets of 2 in series)?

Thanks for reading this!
If you need more information I will gladly respond quickly!


Jan 19, 2011, 10:00 am Last Edit: Jan 19, 2011, 10:02 am by retrolefty Reason: 1
Your are also going to have to do a little modification to the circuit from what that fritz project shows. The fritz project is using fixed voltage led modules rated at 12vdc. You apply 12volts to these modules and internal series current limit resistors limit the current flow that the leds are rated for. You are buying more normal bare leds, that can be run at any voltage, but must have calculated series current limiting resistors wired in series with them.

To calculate your resistor value for those sparkfun leds, you will have to figure out what battery voltage you will end up using. Once determined the calc will be:  (batt voltage - led Vf rating) / .060 amps.

Your battery voltage choice should not be based on the led Vf rating, but rather above that rating, as you can't directly power bare leds with a voltage source anyway and you must have a resistor for each led.

I would think you would want at least a 4 series connected AA or AAA rechargeable batteries for a nominal 5vd output voltage. You can power the arduino with this same voltage by wiring it to the +5vdc pin on one of the shield connectors.



Jan 19, 2011, 11:08 am Last Edit: Jan 19, 2011, 11:25 am by SpiffyChee Reason: 1
Thanks for mentioning the resistor, Yes I will take this into account.

I'm needing a lighter weight and smaller size so I don't think AA's will work. And I don't see any advantages of using AAA's over the cells I mentioned? Do you know how I could do this with the cells I mentioned? I believe that my form factor will be best when using these cells? I'm just not sure exactly what combination. (one or two) and (cr123A's or 18650's) and (series or parallel). Do you know how I would address this issue?

also I found this battery. would 1 of these be able to power my system?http://www.dealextreme.com/details.dx/sku.5035#open full view


A lithium cell be a good choice. They have an excellent energy to weight ratio. Charging them safely maybe an issue. Lithium needs to be charged with a constant current/ constant voltage charger. constant current @ 1/3 C rate until the cell voltage reaches 4.2 volts and switch to constant voltage mode of ~ 4.5 volts to top off the cell About  20% of the energy in the topoff. Anything above 4.5volts and the cell may explode.  So, the voltage needs to be controlled.  

I did a white LED drive that used a single lithium cell a few years ago. I used two of these: http:// http://www.maxim-ic.com/datasheet/index.mvp/id/3025 and a buck/boost 5 volt regulator for the power supply. The regulator was programmed to shut off when the cell voltage dropped below 2.5 volts. Lithium doesn't like a deep discharge. PWM of the EN pin on the MAX1916, to control brightness, was done on an AVR.



so do you guys think a 3.7 volt battery is enough to power both the arduino using PWM on 6 pins AND the leds through the mosfets WITHOUT a regulator?
Or should I use 2 batteries and power it with a regulator? should i use the 3.0 or 3.7 volt batteries.


All depends the the cells or batteries that are used. If you use a single cell and the Vf of the LEDs are 3 - 3.4v that's not going to work, you'll need a boost Reg. I would start with the easiest solution, maybe four AA's, resistors and MoFETS,  see what that gets ya. It will give you an idea of what the next step will be.


Jan 20, 2011, 04:33 am Last Edit: Jan 20, 2011, 04:35 am by retrolefty Reason: 1
Fundamentally it comes down to you having to pick the battery you want to use and the leds you want to use.

By choosing blue leds you set a lower voltage limit for the battery voltage you can use. If you instead choose red leds, with their 1.5Vf ratings, then you increase your battery choices.

It's like that old adage, lower cost, quicker schedule, better quality, you are only allowed to choose any two.  ;)


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