LEDs and resistors

[Lex]

Hello,
Hopefully I'm posting this in the right sub-forum
I'm making my first LED flashlight, I want to make it last as long as possible - it's kind of an emergency light. I'll use two D sized alkaline cells and just one LED. Now, I don't know if I should use a resistor or not. Assuming the LED is 3.4-3.6 volts and two batteries output 3 volts (which decreases further as energy is drained), do I even need to use one?
Another question - doesn't a resistor waste energy as heat? I think it does, and I'd like as few energy-wasting components in my flashlight as possible

 

[RetroTechie]

Welcome here, Lex! :wave:

That will work well... if your LED is a 5 mm. red one, for example (Vf around 2V). Plus series resistor. Otherwise...

Assuming the LED is 3.4-3.6 volts and two batteries output 3 volts (which decreases further as energy is drained)

Theory says you shouldn't expect any (significant) amount of light from that, if it lights at all. In the reverse case (battery voltage more than enough for what the LED needs), how good the batteries are, decides how fast the LED blows. In short: yes, you need a series resistor. Always (well there are exceptions to this rule, but that's beyond what you're trying to do).

Btw: assumption is the root of all evil. And two D cells in series won't output 3V when you need 'em.

Another question - doesn't a resistor waste energy as heat?

Yes. So does the LED. Or more exactly: EVERYTHING in the circuit, including conductors and on/off switches. It's only a matter of how much, and where.

Perhaps a "Joule thief" is more up your alley. :thinking: For some background, you might find this thread interesting.

 

[Lex]

Hey, thanks for such a thorough reply!!! Also, the Joule thief concept is exciting, this is the first time I've heard of it :thumbsup:

 

[DIWdiver]

What LED are you planning on using?

This driver should power an XR-E or XP-G pretty well from 2 alkalines, and run for 12+ hours on low. http://dx.com/p/3-mode-led-driver-circuit-board-for-flashlight-dc-0-8-4-2v-106484

When you say 'as long as possible' what do you have in mind? The combination above would give you more light than a standard Mag 2-D light. If you want to run it for longer, give us some idea of what you are looking for, and we can make better suggestions.

 

[alpg88]

no you don't need resistor for your set up, you have shortage of voltage as it is. your led will be dimmer than it would with correct voltage and current.
resistors do turn energy into heat.

 

[Lex]

I intend to use a LED that would be barely bright enough for reading. I want it to last as long as possible, brightness is not of greatest importance. A 3.4V 20mA LED should last for hundreds of hours on two D sized alkaline batteries. But that may be a bit to dim. So I'll probably end up using 3.4V 50mA or 100mA LED. Or maybe I'll make it a 3-cell light, get up to 4.5 volts that way, and use a resistor to even things out. I'm still at square one, as you can see

 

[DIWdiver]

Ah! Now we're getting some useful information!

Even 3 alkalines won't drive a white led very well through a resistor. Throughout their life, the cells will drop from 1.5V to around 0.9V. If you use a resistor, the output will drop something like 70% by the time the batteries are halfway discharged.

I kind of doubt you're going to be able to buy a regulator that will give you what you want, due to the low current, unless you can find an adjustable one you can turn way down. Are you up to building one, with proper recommendations?

 

[HarryN]

My first home made light was a bunch of parts mounted on a pine 2x4. It really helped to be able to test different setups without the need to first force the shape into a flash light.

Alkaline D cells look more powerful and reliable than they really are. An AA energizer lithium cell has a much longer shelf life, will run under colder conditions, and holds more charge than a D size alkaline. There are adapters around which can hold 3 AA cells in the space of a D cell if that is interesting.

As a practical matter, a good reason to build your own flashlight is for fun and learning. Most people spend a lot more building a flashlight than they would just buying a commercial one. That is not a good reason to not do it, just an observation.

 

[vinsanity286]

I have mocked up a hi cri xpg with 2 aa alkalines. The set up pulled over 400ma direct drive from fairly fresh batteries. That is quite a bit of light from a modern led. Of course alkalines fall off quickly in voltage, however I think if you use a led with a similar vf you can make a very long running light with a resistor in series. For reference, I have made another 2 stage keychain light like this one but with the same hi cri xpg. On low mode, while drawing less than 20ma, it is MORE than enough to read at night. http://www.candlepowerforums.com/vb...Modded-solid-brass-coin-cell-light-from-Lowes

I hope this helps,
Vin

 

[RetroTechie]

If you pick a LED with low enough Vf (for example the XP-G2's datasheet shows ~2.6V @ 50 mA), it could work with 2x 1.5V alkalines. But they'd quickly dim & as the light becomes unusable, still have a lot of their energy left. With NiMH's you'd need a 3rd battery for sure, and that makes sense with alkalines too.

Enter the 3x AAA + switch + resistor + LED light, to be had starting from $2 or so when ordering from the far east... Easy to diy using AAA or AA cells. No clue why you'd want to use C or D cells in that config, if it's just for a few lumens reading light.

Beyond that, you can either:

• Add more batteries + increase resistor value. Each added cell wouldn't help much for runtime, just improve regulation. A very low drop linear voltage regular might do the same.
• Put a switching regulator in between (buck, boost, or buck-boost). Both efficiency and regulation would be good, and you can choose battery config as you like. But at the cost of a more complex (and expensive?) circuit. If you have to ask about how to drive a LED with a few batteries, such a design is probably more than you can chew. Btw: there's ready made flashlight driver boards out there.

Is this (mostly) a 'problem' to be solved, or a learning exercise? :thinking:

 

[Lex]

Hey, thanks for all the feedback! I'm surprised there's a forum with users this helpful and polite to a newbie like me

DIWdiver, battery regulating seems to be a bit too complicated, I'd like to make this project as simple as possible. I was hoping I could successfully build this using just a LED, batteries and a resistor Once that is done, I'll consider making a new, similar flashlight, but with Joule-thief/battery balancing improvements. Currently it's out of my reach
HarryN, I chose D cells because, at least in my country, I get the best energy-for-money ratio buying these. So it would be best for me to stockpile on D-cells, money-wise. AA cells are second in line though. I admire the performance of lithium cells in cold temperatures, that is actually relevant for me because I live in North-east Europe. But lithium cells are horribly expensive over here.
I agree that building a flashlight is a fun thing to do and provides valuable knowledge. Another reason to make one, is that you can build a flashlight to suit your specific needs. I've never seen a flashlight sporting D-cells and a tiny LED, which would last for hundreds of hours. That's something I have to build myself
vinsanity286, you make some good-looking flashlights, mine will probably be encased in a fat layer of duct tape
RetroTechie, 2.6V 50 mA LED powered by two 1.5V cells sounds right. Would be bright enough and use just 0.13W. I'll take a look for it at the shops. I just hope the 2.6V LED is white, not blue or something?
Like I said before, battery regulation/joule thief concepts seem very interesting and I'll be happy to explore it once I have the time, and, more importantly, once I increase my knowledge at least a little bit. I'll only have December 21st-22nd for messing with this project of mine, so I'd rather make it as simple as possible
What I want to build is a flashlight that could reliably output light just bright enough for reading, for as long as possible. So it's not just for fun or knowledge reasons, I'd actually like to use the flashlight I build - consider it an emergency measure. For example, I'd use it in case of electricity shortage. But yeah, it's also my first step into making more reliable flashlights - I really like the concept of using all the capacity of the batteries. But I'll leave it for 2014

 

[Revolvr]

SparkFun sells a little boost regulator circuit board using the NCP1402 chip. It outputs a fixed 3.3V with an input as low as 1V. It can source up to 200mA but that depends on the input voltage.

You might still need a small resistor depending on the Vf and If. This and one D cell and you can't get much simpler.

I used this to power some white LEDs on a magnifying head visor, and it worked quite well.

 

[Sway]

4XAA alkaline cells with a 1K ohm resistor will run a XM-L for well over a month non-stop and still make more than enough light for reading.