I understand that the bad thing about overdriving LEDs is that they heat up and die. I don't really need 100,000 hours of LED life, so I would be willing to sacrifice some LED lifetime for light.
My question is that is it better to overdrive the LED using voltage or current, while maintaining the same power output? For example, you can stack some button batteries together to get about 6 volts @ 50-60mA, or you could use some AA's and run it at 3V at 100mA. Both draw 300mW. I know some LEDs require 4 volts min, but I am curious about which method will be brighter, and possibly safer for the LED.
My only experience is with the 8000mcd 8 degree Toshiba LEDs, where the 3V @ 100mA is brighter than 5.5V 50mA (the stacked CR2016's weren't brand new).
Er, bit complicated.
Basically LEDs are current-demanding. It`s an excess of current that kills them, but current is also related to the voltage that is put accross them- to overdrive them you need more volts, and so innherantly more current flows too. It`s not really like a resistor (V=IR) cos they`re semiconductors and non linear, and mathematically more than I can even pretend to understand really.
A Toshiba 8000mcd one (red, yellow, orange) will pull 20-30mA at about 2 volts. A Nichia white, blue, emerald-green, turquoise, will pull 20-30mA at about 3.6 volts. Probably what you saw with the Toshibas is the led wanting to pull waaay too much current from those 6 volts worth of batteries so their voltage fell drastically to under 4v and they only gave 50mA. With a big laboratory grade constant-voltage (ie- voltage will not drop regardless of current drawn) power supply an led connected at 6 volts may pull over an amp and melt, puff out loads of stinky white smoke and die within seconds, but a couple of teeny CR2016`s won`t give nearly that much current so the led won`t get fried.
I havn`t actually experimented with my beast of a power supply and Nichia`s finest products, but that`s cos I can`t bring myself to blow up a $6 led in the name of science (anyone from Nichia watching....free samples???? [img]images/icons/grin.gif[/img] [img]images/icons/grin.gif[/img] please??) but basically small batteries can`t give enough current to immediately blow a led so their voltage will fall and a lower current will flow that they can cope with.
Next time you hook a Toshiba led to 3v of batteries, (or a Nichia white, etc, to 6v of batteries), hook a volt meter up to the led`s pins as well and see what voltage it`s getting. It`ll almost certainly be less than 3v (4.5v with nichias) according to what my Fluke bench meter has been telling me in the past.
Basically to overdrive a led you need more current. To get more current you need to put more voltage accross it so the current that it draws is higher, but you have to make sure your batteries can give that amount of current in the first place.
Does any of that make sense....?!! Told you it was complicated [img]images/icons/tongue.gif[/img] !
When I had it hooked up to the 3V source, 2 AAAs (not AA's like I previously stated), I was measuring 2.7-2.8V across the leads.
I realize that it is complicated, as you can get a higher voltage, but be current limited by the battery type. But even with the voltage fall, they still can put out more voltage at less amps than with a pair of AAAs. Seems counter-intuitive if we assume that the LED resistance is the same. That's kinda how the button batteries work, like in a photon w/o a resistor right? While the N battery mag light conversions need resistors for the same voltage.
When you destroy an LED, will it at least get hot first, so you can know when to stop giving it power? [img]images/icons/smile.gif[/img]
Just trying to get as many candelas without the LEDs melting in my hand.
When you destroy an LED, will it at least get hot first, so you can know when to stop giving it power?
Not always, some will just flash briefly and quit. Others may crack and puff smoke at you but it`s rare thankfully especially on battery level current. Depends on the current. Put a 2 volt led on 6 volts of D cells and it may blow. Smaller cells may burn bright and die after a short while. A good sign is that if you can increase the current with a regulated power supply or something similar, watch for exessive heat like you said, but also a noteable change in colour and brightness. White leds will go very blue/purple, green or yellow turn orange/red. Also they will dim noticeably. Back off the power and hopefully they`ll live to shine again but watch the Nichias (white, blue, etc) they are more prone to damage than red/yellow/orange leds.
Best thing for you to do is as others have suggested- go to radio shack or look online/in a component catalogue, and get yourself a 1k multiturn pot (variable resistor). Connect in series with your batteries on a bench test setup, then turn away until you have a brightness you like and an led that isn`t getting very hot or showing signs of quitting life...then without adjusting the pot disconnect it, measure its resistance with a meter and replace it in circuit with the led/batts with a fixed value resistor of the same value. Can`t get the right fixed resistor value? Use smaller value ones and connect all in series (just add the resistances up) and put inside your flashlight of choice. This way you can set a brightness you are happy with for a particular led/batteries setup and don`t run the risk of killing those expensive leds.
it's my, albiet very limited, understanding of power that amperage is drawn, not given. for instance the outlets in your (northern american at least) house is 120v@20a, and while everything you plug into the wall is taking 120v not everything is drawing 20 amps.
so in order to change amperage you have to change voltage (assuming the same LED with no resistor), yeah?
the amperage differences you describe above sound more like battery resistance differences from using different battery types (lithium/alkaline).
<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by papasan: it's my, albiet very limited, understanding of power that amperage is drawn, not given. for instance the outlets in your (northern american at least) house is 120v@20a, and while everything you plug into the wall is taking 120v not everything is drawing 20 amps.<HR></BLOCKQUOTE>
Power is a fickle thing.
When you plug a suicide cord in and short the ends, for the brief instant before your mains fuse or breaker pops, that outlet is feeding hundreds or even thousands of amperes into your cord.
Bypass all of the breakers or fuses in your house, and your house wiring will either melt, the pole pig outside will explode, or (more likely) the big expensive fuse protecting that pole pig will blow, blacking out every home served by that stretch of line.
So the 20A rating is solely determined by the size of the fuse or breaker protecting that outlet.
<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>
so in order to change amperage you have to change voltage (assuming the same LED with no resistor), yeah?<HR></BLOCKQUOTE>
Since LEDs need a certain voltage across them before they'll even turn on, nothing happens to the circuit until that voltage is reached. THEN, and only then, as the voltage is increased further, the current passing through the LED also increases.
Limiting factors include the LED's own internal resistance (can be between 4 and 50 ohms depending on LED type), internal battery resistance (lithium & alkaline have a much lower internal resistance than zinc carbon, for example and can supply greater peak currents) and to a much lesser degree, the size and length of the wiring going from the battery to the LED.
Overall, it's a formula that I am not qualified to solve. To do that I'd need degrees in mathemetics and electrical engineering. [img]images/icons/blush.gif[/img]
I need to find a battery or batteries to supply about 50 mA at 3 - 3.5V, but I don't have a lot of space (AA is way too large). They need to operate for many seconds, which I assume is the same as forever (vs. instantaneous).
A total lifetime of 1 hour would be fine to get started, so I need something like 1 hour * 50 mA = 50 mAh. This seems readliy attainable.
I see button batteries described, like on Wikipedia, http://en.wikipedia.org/wiki/CR2016, but am not sure how meaningful the current limits are. Can anybody suggest one or more good options for such batteries?
I need to find a battery or batteries to supply about 50 mA at 3 - 3.5V, ....
This is unfortunately a poor thread to resurrect, it wasn't particularly accurate.
To the first order, an LED doesn't conduct below its Vf and at Vf it will conduct any current the circuit can provide. While that is a simple way to think about it, it isn't really accurate. The LED's V-I curve will slightly conduct below Vf and isn't a perfect conductor above. (Mathmatically the curve has a slope other than 0 and 1.) The equations and math to predict accurately get a little complicated.
Typically a easy circuit will use a resister to limit the current. An initial resister value will be predicted using the first order approximation above. Then the circuit will be prototyped and measured. Any adjustments being made then.
I.e: R = (Vbatt - Vf) / I
Using your current and some numbers for example:
R = (6v - 3.7v) / 0.05A = 2.3 / 0.05 = 46 ohms
For a 6v battery I'd try the circuit with a 40-50 ohm resister, measure it, and maybe adjust the value of R. (I'd expect variation to come from Vf other than nominal and from resistance outside the resistor, e.g. in the batteries.)
A Vbatt of 6v would be nominal for a pair of 2016 lithiums. Other options you might consider are a 9V alkaline or a 12v A23 alkaline. (The A23's are used in the old GloToobs and, I'm told, garage door openers.) Just remember to adjust and recalculate R for your batteries.
I'm not sure what you're trying to accomplish, but have you considered getting a "fauxton" from Lighthound, Battery Junction, (DX, if you're not in a hurry), or elsewhere? They've typically matched a cheap white LED with some button cells and packaged it. They get reasonable life, though I'm sure the batteries are killing the LED and vice versa.)
Last edited by LED_astray; 12-01-2008 at 01:17 AM.
Reason: Add Fauxton notee.
As LED_astray has said, this thread may not be the best place to gather information until some corrections are made....
One technique to testing for what R value you need, is to use a variable resistor (rheostat). Hook up the LED in series with the rheostat, and in series with a current meter (most DMMs can do this), and of course, in series with the power source, start with the rheostat set to maximum resistance, and slowly dial it down until the LED starts to light up, then carefully adjust the rheostat until the current reaches the level desired. Allow it to run there for a little while to ensure the current stays reasonably stable. Shut it all down and use the DMM to measure the resistance of the rheostat at that final position that produced the current desired. Use a resistor as close to that as possible, rounding up and not down, as in the final configuration, there will probably be less resistance in the circuit with the DMM removed.
In response to the original concept laid forth in this thread, overdriving LEDs doesn't deliver much useful gain in output. LEDs drop in efficiency as the power is increased. There isn't much to gain above the maximum rated current for most LEDs.