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LED for headlamp
By Tim C, Section Homebrewed Electricity Posted on Sun Dec 21st, 2003 at 04:58:18 PM MST
I have an inexpensive headlamp which takes a screw in incandesant bulb.

I think it is 4.8 v 700 m. I have look everywhere and have not found a replacement. I found two white LED's at The Electronic Gold Mine. One is a 5 mm and has a minimum output of 7,000 mcd@4.0V DC. The other is 10mm white LED operates on 3.6V to 4.2VDC @ 30ma.
My light uses 4 AA batteries. Can I simply wire this in or will I need a resistor. If so can anyone recommend one? I realize if operated at over the rated voltage, the LED may not last as long. This is OK since the current bulbs do not last long anyway. Thanks for any and all help.
Tim C.

LED for headlamp | 17 comments (17 topical, 0 editorial)

Re: LED for headlamp (none / 0) (#1)
by Norm (peppysue@suite224.net) on Sun Dec 21st, 2003 at 05:41:34 PM MST
(User Info)

I just got a Dorcy solid state (white led) from QVC about 2 months ago the batteries (4-AA) last a lonnnngggg time and the bulbs probably lifetime. The only drawback...about half the length of a regular flashlight...so I keep misplacing them. BTW They would make a nice Christmas Present...(batteries last a lonngg time)
( :>) Norm.
( :>) Norm

Re: LED for headlamp (none / 0) (#2)
by wooferhound (tim((NoSpamAt))wooferhound.com) on Sun Dec 21st, 2003 at 05:45:36 PM MST
(User Info) http://wooferhound.com

An LED without the resistor is just a diode. so with no resistor you have almost a Dead Short...

}=- W o o f -={

Re: LED for headlamp (none / 0) (#3)
by Tim C on Sun Dec 21st, 2003 at 05:56:41 PM MST
(User Info)

Thanks for the help. So what would I need to make this LED operate? I have read the information on the board on the use of LEDs. They can work without the resistors, but may last a shorter time if you exceed their voltage.

[ Parent ]

Re: LED for headlamp (none / 0) (#4)
by Gordy on Sun Dec 21st, 2003 at 08:39:18 PM MST
(User Info)

If your worried a about over voltage, could'nt you just replace one of the batteries with a peice of wood dowl with a hole drilled down the center and some wire in the hole and a tack on each end. To pass the current through from the other batteries.
Just a thought,
Gordy

[ Parent ]

resistor built in with the led? (none / 0) (#7)
by Norm (peppysue@suite224.net) on Mon Dec 22nd, 2003 at 07:38:35 AM MST
(User Info)

The 'Dorcy' looks almost like it has a regular flashlight bulb...except you can see the diode instead of a filament maybe they have a built in resistor? Norm
( :>) Norm
[ Parent ]

Re: LED for headlamp (none / 0) (#5)
by visionarynz (dynamics_feedback (at) hotmail (dot) com) on Mon Dec 22nd, 2003 at 01:12:34 AM MST
(User Info) http://www.dynamics.orcon.net.nz

Going by the formula I use for the LED lights I make, you would want a 150 ohm resistor wired in series with each LED.
Running from 4x 1.5volt cells, this should keep the current through each LED down to around 20mA, and at this current level the LEDs will work well, and likely last longer than you do. Also your batteries will last a very long time, even if you put 4 or 5 of them in there.

There are a few special cases where LEDs can be connected straight to batteries, but this isn't one of them, they will likely die in a matter of seconds without a resistor.

vis_nz

Re: LED for headlamp (none / 0) (#6)
by dburt on Mon Dec 22nd, 2003 at 07:22:40 AM MST
(User Info)

Rechargable batteries (NiCD or NiMH) have a nominal voltage of 1.2 volts, so if you use them, you'd need a smaller resistance value to get the same brightness from your LED... Or, if you get the 4.6V LED, you could get away with running without a resistor (as long as you dont forget and put regular 1.5v batteries in!)

Dave in PA

[ Parent ]

Re: LED for headlamp (none / 0) (#8)
by RobD on Mon Dec 22nd, 2003 at 07:50:03 AM MST
(User Info) http://www.dsgnspec.com

Yes, that's right. LED's are CURRENT devices. That means the voltage is relatively unimportant. What you want to control is the current.
I'm not sure how bright your LED needs to be but the 20 ma. ones won't give you much light.
I have several LED flashlights that I built using high power LEDs but they require a current limiting circuit.
If you are not concerned with brightness and want just a cheap indicator light you can figure your current with ohms law (E=IR) this way:
First you will need to know the voltage and current specs of the LED for example, let's say your LED is rated at 20 ma(.020 amps) and 1.7 volts.

8 - 1.7 = 3.1 volts.
1 / .020 = 155 ohms.

The closest standard resistor to 155 is 150 ohms and this will be fine.
.020 x.020 x 150 = .060 or 60 mw (milliwatts) so you can use a 1/4 or even a 1/8 watt resistor.

What is happening here is the resistor is dropping the excess voltage (3.1) and allowing the 1.7 volts to go to your LED. The 1.7 volts will not overdrive the LED so the 20ma will not be exceeded.
You would put this in series with your LED(battery-----///---->|---ground). Make sure the LED is in the circuit in the right direction (The flat spot (collector) is the ground end and goes to the neg post on your battery).
Hope this helps,
RobD

Re: LED for headlamp (none / 0) (#9)
by jubalearly on Mon Dec 22nd, 2003 at 08:18:18 AM MST
(User Info)

I think it is a good idea to always use some resistance to prevent thermal runaway. That having been said, I often power LEDs from NiCads or NiMH batteries at 3.6v (3 batteries or in a minim@g, 2 batteries @2.4-3.2v) with no resistor. However, if there is room, even a 10 ohm resistor will usually prevent thermal runaway.
Note that the rating of 3.6v or 3.6-4.2v is the SAME thing. The one that gives you the range is telling you the typical variation in Vf (forward voltage). Higher quality LEDs tend to vary less.
I would recommend the 3 batteries + a dummy as was mentioned above. With 4 batteries you waste most of the 4th battery (70%?) in heating the resistor. Also, I very rarely have any LEDs fail at 50ma, even when they are rated for 20-30ma max. So I would use something around 50 ohm for your 4 battery setup or 10-20 ohm for the 3 battery setup assuming alkaline batteries.
If you use NiMH rechargeables, the voltage will be much flatter (about 1.25v/battery with a 50ma load) and you can use a smaller resistor (22 or 27 ohm with 4 batteries). Note that the actual value of the resistor varies enough that exact values aren't critical. So just use whatever is closest to your calculations.

Thermal runaway (none / 0) (#10)
by Norm (peppysue@suite224.net) on Mon Dec 22nd, 2003 at 09:38:45 AM MST
(User Info)

Isn't this what would happen in fluroscent lights you have to have something to limit the current and whatever happened to the big heavy ballast that they used to have in the old fluroscent lights? To my understanding a ballast is a 'choke coil' they used to use them quite often in the old tube radios. Norm.
( :>) Norm
[ Parent ]

Re: LED for headlamp (none / 0) (#16)
by Tim C on Wed Dec 24th, 2003 at 05:40:48 AM MST
(User Info)

So could I simply replace one of the 4 AA batteries with "spacer" wired with a 10 to 20 ohm resistor? The light still needs to be practical but I do not want it to burn out right away.

[ Parent ]

Re: LED for headlamp (none / 0) (#11)
by RobD on Mon Dec 22nd, 2003 at 05:34:17 PM MST
(User Info) http://www.dsgnspec.com

Basically a ballast 'absorbs' the initial high inrush current of cool low resistance filaments. Now we don't need them because we have very good current limiting circuitry that never allows the inrush current to go above a specific point. In the old days the military used to keep their tubes on constantly because the found they lasted longer. Apparently they did because we changed very few of them.
An important point about LEDs in flashlights is to keep the voltage of the source close to the working value of the LED to limit losses from heat through resistors. This is why I use PWM (pulse with modulation) drive circuits through MOSFETS with low internal resistance to cut my losses.
If you are dropping half your current in a resistor your batts will burn down in half the time.
RobD

Re: LED for headlamp (none / 0) (#12)
by Gordy on Mon Dec 22nd, 2003 at 07:56:22 PM MST
(User Info)

While we're on the subject of led's, has anyone thought of using them on christmas light strings and what would be involved in doing so. A 50 light string at Wal-Mart is $1.49 a 50 led light string would be $??????
Just tierd of changing bulbs, and thinking of how much electricity the "old lady" insists on wasting with 1000+ of these little buggers running 12 to 18 hours a day.

Let me know if I'm just wishfull thinking
Gordy

[ Parent ]

Re: LED for headlamp (none / 0) (#13)
by hydrosun on Mon Dec 22nd, 2003 at 09:23:47 PM MST
(User Info)

We bought a bunch of 120 volt Led Christmas lights last year. Only one hardware store in town carried them. We bought one string for $15 but later they were discounted to $7.50. They plug in like regular strings and can be connected in series and parrallel and use 1/8th the power of regular bulbs. also check ebay or a search of the web to find who is carrying them this year. We really liked the blue almost glowing strings.
I've converted many regular bulb christmas strings into 12 volt led strings. I had to cut and splice and add resistors . I'd put 5 in each group for red, green or yellow, and 3 in each group of white or blue (I'd calculate 2 volts each for the first group and 4 volts each for the whites and blues) usually I''d use one resister for the whole string. The trickiest part is keeping the polarity straight with the direction of the leds.
I've found the best deals for Leds at Superbrightleds.com I
ve order from them 3 times. I used 50 in a string of 12 volt christmas lights that are on in the living room all year. Very bright and chearry. I especially like the 12 volt cluster of 24 white for $15. I use it for my bike light. I've used 12 volt 12 led clusters for a couple bright headlamps. I use 12 volt nmh battery packs to run these led clusters. One pack was two cell phone batteries I rescued from the recycling bin . One other battery was from a camcorder. I ususally take my multi meter when I take in my recycleables and rummage through the old batteries. Many still have lots of life for low power applications. they have great desriptions and outputs of each individual led.

[ Parent ]

Re: LED for headlamp (none / 0) (#17)
by desertratjack on Wed Dec 24th, 2003 at 10:52:44 PM MST
(User Info)

Backwoods Solar sold LED light strings (12 vdc) but I found that by buying the WalMart colored light strings and rewiring the 120vac strings to 12 vdc that I could make the strings alot dimmer so that they are more pleasing to the eye and they also run on a lower voltage with longer life. If I remember right the bulbs I had were 5 in series for 12 vdc. Be aware that there is great variability in bulbs so different strings from different manufacturers have different resistances so it becomes a big puzzle (worse than just finding a bad bulb)if you mix different bulbs :-)

[ Parent ]

Re: LED for headlamp (none / 0) (#14)
by Bach On on Tue Dec 23rd, 2003 at 05:13:04 AM MST
(User Info) change AT: bach_on AT hotmail.com

I bought a couple of LED "bulbs" designed to retrofit to a standard flashlight. They were something like $8.00 each from some guy on eBay. These were not screw-in, but the bayonet type. Each bulb was actually three LEDs. The resistors were internal and already wired in there. I used these to replace bulbs on a bike light. Worked pretty well. Just a bit of bluish tint to them.

Bach On.
- - I'm not superman, but I am very dense! -

Re: LED for headlamp (none / 0) (#15)
by RobD on Tue Dec 23rd, 2003 at 07:18:24 AM MST
(User Info) http://www.dsgnspec.com

Try the Luxeon Stars. They are up to 5 watts and the light is white.
RobD

LED for headlamp | 17 comments (17 topical, 0 editorial)

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