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Old 11-02-2012, 09:37 PM   #1
Raduque
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Default LEDs and voltage (electrical gurus help plz)

If I have a string of 4.5v AA battery powered LEDs, what will happen if I give them 13-14.5v DC? Will I let the magic smoke out?
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Old 11-02-2012, 09:58 PM   #2
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Probably. You could maybe counter this by adding a lower ohm resistor but it's not really an efficient way of going about it.
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Old 11-03-2012, 12:10 AM   #3
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Quote:
Originally Posted by Red Squirrel View Post
Probably. You could maybe counter this by adding a lower ohm resistor but it's not really an efficient way of going about it.
You would need a higher resistance series resistor, and one rated to dissipate all of the extra power.

@OP: yes, it's possible to let the magic smoke out this way, if your power supply can supply enough current. If you're trying to get higher brightness, then you can get some but assuming a normal circuit design you won't get that much more before you just kill the LEDs. It is possible to simply add an extra resistor in series with the new power supply and the old circuit (so that the LEDs will "see" the same current), although picking the value requires you to figure out a little bit about your existing circuit.

I had a longer explanation, but deleted it because I'm not sure that it's really necessary or helpful.
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Old 11-03-2012, 01:40 AM   #4
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I always thought all LEDs used 4.5v but not so, my garden ones use a single 1.2v AA battery. All the flashlights I have used use 4.5v (3 A class batteries).
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Old 11-03-2012, 02:12 AM   #5
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Quote:
Originally Posted by Staples View Post
I always thought all LEDs used 4.5v but not so, my garden ones use a single 1.2v AA battery. All the flashlights I have used use 4.5v (3 A class batteries).
They have a very small boost converter inside.

Red and green LEDs tend to have the lowest Vfd requirements(1.8 - 2.2V), while blue and white the most(3-4.5V).
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Old 11-03-2012, 02:16 AM   #6
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Quote:
Originally Posted by Staples View Post
I always thought all LEDs used 4.5v but not so, my garden ones use a single 1.2v AA battery. All the flashlights I have used use 4.5v (3 A class batteries).
It really does depend. A few tenths of a volt can mean a lot to an LED. One, because it's brightness is not a linear function of voltage. With an incandescent bulb, if you give it half of the voltage, it will typically go down half in terms of light output. That isn't the case for LEDs. Second, they can be very picky about voltage and most of the time overvoltage is what kills an LED prematurely. But yeah, 13 V is most likely going to kill the LEDs.
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Old 11-03-2012, 03:23 AM   #7
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What eli said

I presume your '4.5V' LEDs have internal resistors, suitable for ~4.5V. (or are relying on internal resistance of teh battery).

If you exceed that by a lot, you will need additional external resistance.
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Old 11-03-2012, 03:39 AM   #8
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Quote:
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It really does depend. A few tenths of a volt can mean a lot to an LED. One, because it's brightness is not a linear function of voltage. With an incandescent bulb, if you give it half of the voltage, it will typically go down half in terms of light output. That isn't the case for LEDs. Second, they can be very picky about voltage and most of the time overvoltage is what kills an LED prematurely. But yeah, 13 V is most likely going to kill the LEDs.
Yes, and the forward voltages can vary widely.

Green and blue dies tend to use similar construction, and also have similar forward voltage ranges. (White LEDs use a blue die, with a phosphor layer over it.) Those are generally 2.5-3.5V, though I've seen some go all the way up to 3.99V.
Then there are the red and amber dies, which usually share similar properties. They tend to have a lower Vf, but they too have a wide range. I've seen them vary from 1.80V up to 3.5V, with 2.00V being quite common, at least in my experience.

But yes, they are quite sensitive to the voltage. A small increase in voltage can drive a lot more current through the LED. Example, on pg16 (PDF). Note the scaling on the X-axis, particularly on the top graph. That only covers 1V, but you can go from 100mA up to over 1000mA in a big hurry. And to make things even more interesting, you've got the thermal runaway problem: As the die heats up, the forward voltage drops, meaning that the same amount of voltage across the LED will now end up pushing even more current through the die. So the die heats up more, and the forward voltage droops even more. You can quickly end up with a roasted LED.


I suggested Supertex's CL2 to Oldsmoboat awhile back for an application he had.
Connection diagram example.
It's a constant current regulator.
It acts as a smart resistor, dynamically changing its own voltage drop to ensure that 20mA gets through to the load. It does this by dumping out the unused power as heat, so it's going to get warm. In this case, the more LEDs in series you put on the end of it, the cooler it will be, because the voltage will get dropped across the LEDs, leaving less for the CL2 to dissipate as heat.

They've also got the CL220N5 now. Just to try it, I ran a CL220 + bridge rectifier straight off of 120VAC to power a single LED, though that's not necessarily efficient or safe.
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Old 11-03-2012, 02:59 PM   #9
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This is pretty handy.
http://led.linear1.org/led.wiz
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Old 11-03-2012, 04:07 PM   #10
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Quote:
Originally Posted by Aluvus View Post
You would need a higher resistance series resistor, and one rated to dissipate all of the extra power.
You're right, not sure what I was thinking there.
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Old 11-03-2012, 04:38 PM   #11
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Ummmm, OP said he had a string of them. If in series, the voltage will be split.
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Old 11-03-2012, 05:37 PM   #12
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Quote:
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Ummmm, OP said he had a string of them. If in series, the voltage will be split.
True, though I, and perhaps others here, don't know the OP's level of skill with this kind of thing.
"String" could possibly mean "group," and that could be a bunch in parallel.


At work, it once took me awhile to diagnose what was going on during a tech support call, until I determined that the person on the other end referred to any passive electronic component as "diode." Capacitor, resistor, diode - they were all "diodes" for him. Larger chips, such as ICs or voltage regulators, those were "relays."
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Old 11-03-2012, 06:56 PM   #13
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Quote:
Originally Posted by Jeff7 View Post
True, though I, and perhaps others here, don't know the OP's level of skill with this kind of thing.
"String" could possibly mean "group," and that could be a bunch in parallel.


At work, it once took me awhile to diagnose what was going on during a tech support call, until I determined that the person on the other end referred to any passive electronic component as "diode." Capacitor, resistor, diode - they were all "diodes" for him. Larger chips, such as ICs or voltage regulators, those were "relays."
Intel makes some really fast relays.
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Old 11-03-2012, 09:41 PM   #14
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i have an array of color changing led's that i made up. 10 of them, all run in parallel off a 5v cell charger. they are way brighter then they should be, as they are heavily over driven. but, so far, they dont get hot and they havent burnt out (2 months and counting)
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