Driving single R2 with 4AA + resistors?

[Hack On Wheels]

Hi all,

Just to let you know, this would actually be for a bike light system. I am hoping to make a relatively cost effective system with a spare LED (and optic) that I have, but of course there is no point having it cost effective if it can't still perform... so without further ado, my plan:

-Single R2 (I believe the Vf ranges from 3.5-3.7 depending on current)
-1 ohm and 2 ohm options, to be selected by an On-Off-On toggle switch
-4 x AA NiMH (using a battery holder)

According to my calculations...
If R = 1 then (4.8v- 3.7)/ 1ohm = 1.1A which would be fine for my high setting.
If R = 2 then (4.8v - 3.5v)/ 2 ohm = 0.65A which would work as a bit of a lower setting.

I was planning to use cells with something in the range of 2600mA capacity... high self discharge wouldn't be a big concern as they would be charged frequently. However, I don't know if I need to be concerned about voltage sag or the battery holder knocking down the voltage or anything. Do any of you light/battery wizards have any ideas or suggestions for me?

Also, if any of you guys can tell me how I would wire this up to have a High setting, a Low setting, and an Off using a On-Off-On toggle and two resistors, that would be much appreciated.

Thanks!

 

[Mr Happy]

The actual Vf varies with junction temperature, which varies with run time and how good the heat sinking is. I'd try to take actual measurements with a meter to confirm the calculations. Also be aware that resistances down in the 1 ohm range are hard to get accurate. It's best to wire up the intended configuration in its as-installed form before taking measurements.

 

[rizky_p]

Dont forget that when you calculate you value using 1.2v each battery dont use it directly from the charger as Ni-Mh are 1.4v freshly charged. With 1 ohm resistor that would be around 1.9A. While only for a short time it might kill the LED.

Thanks.

 

[Gunner12]

Drivers aren't that expensive. They will also ensure you get the right current and they will deal with the hot off the charger voltage. What current do you want to drive the LED at?

You could either have a multimode driver(if you chose to use a driver) or a two stage switch.

 

[Hack On Wheels]

Dont forget that when you calculate you value using 1.2v each battery dont use it directly from the charger as Ni-Mh are 1.4v freshly charged. With 1 ohm resistor that would be around 1.9A. While only for a short time it might kill the LED.

Thanks.

Thanks for pointing that out, I didn't know how much higher the voltage would be when the batteries are fresh. I would definitely have to be cognizant of that.

 

[Hack On Wheels]

Drivers aren't that expensive. They will also ensure you get the right current and they will deal with the hot off the charger voltage. What current do you want to drive the LED at?

You could either have a multimode driver(if you chose to use a driver) or a two stage switch.

They aren't, but the point of the light is to use local bits as much as possible and to do something quite different from the fancier + bigger light that I am also making... If I get annoyed with resistors I might end up ordering a driver for it at a later date, but hopefully it works out nicely.

And I am planning to have a On-Off-On switch with options of 1ohm and 2ohm resistors...

Which would also help me with the possible problem that rizky_p pointed out. I could start off with the low beam option, and then switch it up if I need more light after the voltage has dropped a bit. Then again, if the battery holder or anything cuts the voltage down a bit, this might not be a problem and I may want to pick a resistor to compensate for that lower voltage...

Thanks for the help! Any idea on how I would wire this up?

 

[Hack On Wheels]

The actual Vf varies with junction temperature, which varies with run time and how good the heat sinking is. I'd try to take actual measurements with a meter to confirm the calculations. Also be aware that resistances down in the 1 ohm range are hard to get accurate. It's best to wire up the intended configuration in it's as-installed form before taking measurements.

Hmm, I definitely see your point... but I will naturally be buying the bits before I would be able to measure anything, and while I could always make the trip out to the electronics store again to get different resistors or something, I would rather get it right the first time and not have the actually build and all get too complicated...

 

[Mr Happy]

You'll probably want to buy a few extra bits to allow for mistakes and to give room to experiment. Resistors are fairly cheap items.

Resistors are hard to get in the 1 ohm range anyway, so you might need to make up your desired resistances out of combinations. For example two 1 ohm resistors in parallel will give you 0.5 ohms. Another way might be to buy some thin iron wire from the hardware or craft store and make your own resistances to spec. Don't be afraid to be creative.

 

[Hack On Wheels]

You'll probably want to buy a few extra bits to allow for mistakes and to give room to experiment. Resistors are fairly cheap items.

Resistors are hard to get in the 1 ohm range anyway, so you might need to make up your desired resistances out of combinations. For example two 1 ohm resistors in parallel will give you 0.5 ohms. Another way might be to buy some thin iron wire from the hardware or craft store and make your own resistances to spec. Don't be afraid to be creative.

Hmm, I guess I probably will buy some extras so I can have some room to play.

Luckily I do have a source lined up for 1 ohm resistors! I guess I could try a 1 and a 0.47 ohm resistor in series and such, but if the one and two give me appropriate outputs as I hope, then I am set!

Would you mind elaborating about the iron wire? I do know about how to calculate resistance and such in parallel and series, but I don't know any unconventional/outside of the box stuff...

Thanks for the help!

 

[Mr Happy]

Would you mind elaborating about the iron wire? I do know about how to calculate resistance and such in parallel and series, but I don't know any unconventional/outside of the box stuff...

Oh well, it may not be as practical as all that, it was just a thought. This page has some information.

Basically, if you got some fine 30 awg shiny steel wire from the hardware store, it would have a resistance of between 0.5 and 1 ohm per foot of length. You could wind a length of this around a short length of wooden dowel (making sure the turns don't touch), and you would have made yourself a resistor to order. Slip each end of the wire into a notch in the dowel, hold it in place with a dab of glue and wrap it with insulating tape to finish.

By varying the actual length of wire you could tune the resistance to exactly what you want. As a bonus it could handle a 1 amp current without any problem.