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Topic: Can I power four leds in parallel with a single AA battery ? (Read 2172 times) previous topic - next topic

amundsen

Hello,

I want to power four IR LEDs with as few batteries as possible because they would be attached to hand or wrist and I want the weight to be as low as possible.I also want to use AA rechargeable batteries if it's possible because I have plenty of them and the corresponding charger.

The IR LEDs have a max forward voltage of 1.4 V. The forward current is 50 mA. They have a power dissipation of 75 mW.

What circuit could I use ?

guardian of light

#1
Dec 26, 2011, 09:34 pm Last Edit: Dec 26, 2011, 09:38 pm by guardian of light Reason: 1
Might try this: http://evilmadscientist.com/article.php/joulethief and running the multiple LEDs in parallel. Can't say for sure how well it'll work, still looking for some calculations to determine real-world output. With the parts count as low as it is, might be worth a try.

Ninja edit: This link http://www.instructables.com/id/Make-a-Joule-Thief/ they might do better wired serial, since the voltage might be swinging up to 50v.
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amundsen

thanks guardian of light but can I use such circuits with LEDs requesting a maximum voltage of 1.4 V ?

guardian of light

If my thinking is correct yes. The circuit is (appears to be) oscillating between 0v and a higher voltage, making the voltage across the load averaged instead of constantly sitting at the higher voltage. If you have a multimeter, build the circuit without the LEDs, then measure the voltage output where you would hook up the LEDs. Plus, the current should be quite low going into the LEDs. I'd be more worried it won't generat. enough current to light up.

Hopefully someone with some real-world experience will show up in this thread, as inductors are still a bit of a mystery to me  :smiley-sweat:
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amundsen

Joule thieves seem really interesting. I have found a schematics using a 1:1 filtering transformer. It does also add a capacitor to smooth the signal and a diode to prevent backwards currents (please see first attached schematics, visible only if you're logged in - note that the transformer is shown as two coils because there's no drawing for transformers in Fritzing).

However as the signal has a very fast oscillation the voltage between the collector and the emitter can't be measured by a multimeter. An oscilloscope is needed and I don't have one.

So I was wondering if a Zener diode could help to set a specific voltage because I think the voltage I have now is too high for my four LEDs (even connected in series). With this capacitor (which has a value of 2.2 mF actually but I can't ge Fritzing to reflect this - it seems only presets values are possible for capacitances in this software) I get a voltage as high as 7.4 V between the terminals of the capacitor.

Please see second and third schematics. I don't know if either could be a solution (please note that the 4.8V for the Zener was chosen because the LEDs have a typical 1.2 Vf - forward voltage) but I am thinking about using a higher voltage for the Zener so to allow the insertion of a resistor in series with the LEDs.

Any comment over these schematics would be highly appreciated.


Grumpy_Mike

Just ask yourself one question.
What in that circuitry limits the LED current to a safe value?

amundsen

Honestly I dont know. From my understanding the current is so low that it won't be an issue. Sure I could insert a resistor in series with the LEDs but how can I set its value as I can't even get a trustworthy measurement of the voltage between the transistor's emitter and collector.  Any suggestion ?

ematson5897

Yea I don't know about a joule thief. I have one running that has 4 LEDs attached and has been running for over 72 hours straight off of a new AA battery, but these are two series sets of 2 parallel white LEDs. Doing this with IR LEDs could be a challenge

amundsen

A challenge ? Why ? Apart from a lower forward voltage value (1.2 V for the ones I got), what's the difference ?

guardian of light

Current protection is not needed. The maximum output current is going to be in the single digit milliamps, there's not enough wattage going into the LEDs to blow then up. Issue being presented is the forward voltage being dropped by the IR LEDs is about a third that of a blue or white led (which the circuit is designed for) will the combination of increased current due to using a fresh AA plus having a higher relative voltage swing enough to destroy the IR LEDs?

My hunch? I don't think so. I also don't have an oscilloscope (or a logic probe, I hear tell the signal should be around a 20kHz square wave since its transistor driven) so I also can't say for sure.
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Grumpy_Mike

Quote
Current protection is not needed.

Wrong it is always needed.
In this case you say:-
Quote
The maximum output current is going to be in the single digit milliamps,

So you are saying that the current is limited by the limited amount of energy transformed by the coils. But what is this? How are you calculating it and are you in control of it?

If the voltage collapses when 20mA of current is drawn then how do you know if more than one LED will work when you connect them in series?

Quote
there's not enough wattage going into the LEDs to blow then up.

Apart from the fact that you don't have "wattage" going into anything can you say how this figure is derived.

All in all it is a very hairy circuit that only appears to function due to secondary effects that are not under your direct control. It is a very bad design as a result.

Quote
I have one running that has 4 LEDs attached and has been running for over 72 hours straight off

Functionality alone does not a prove a design. Have you measured the light output over that time to see if the life of the LED has been drastically reduced?

amundsen

Grumpy_Mike I understand this design is not the best one but do you have any idea how to set the current and voltage in this kind of circuit especially when using a salvaged transformer ? If it's just a matter of replacing this component with another one having known characteristics I'd be happy to do it. Any idea ?

guardian of light

Being that this is a hack, nothing more is to be expected, other than it working as described. Here is an LTspice simulation http://madscientisthut.com/forum_php/viewtopic.php?f=12&t=3of the original Joule Thief showing that it operates over 200mA (for a single led, 1mS pulses) which is standard for the design. For four 1.4 LEDs in parallel, it runs right at 11mA apiece, and well under 2V, right about perfect. Here's the modified file. http://sharesend.com/z0gby

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amundsen

Thanks guardian of light but I can't open LTSpice files because I am using a Macintosh.  =(

guardian of light

Well, here's some screenshots, at least. Can't play around with the values. http://insapio.files.wordpress.com/2011/12/voltage3.png
http://insapio.files.wordpress.com/2011/12/current3.png
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