White LEDs

[Mike]

Can I light a White LED with just a voltage source such as 3V or do I need one of those White LED drivers?

[wd5gnr]

You need a dropping resistor. Computing the value depends on the voltage of the LED, the battery voltage, and the current you want to drive through the LED. An exercise in ohm's law.<p>If you want to learn more about all of this, check out http://www.al-williams.com/classroom/co ... w.php?id=4.

[Externet]

Hi Mike.
If you have a voltage source of the exact specification for the led Vf, you can connect it directly, no resistor.
If the white led is for example 4.3Vf, your 3V source won't work or will be very dim. You need the 4.3V.
Miguel

[MrAl]

Hi there,<p>With the white Nichia LED you can drive it up
to 30ma normally, but you'll need about 3.75
volts to do that. From two AA cells in series
you'll get about 3.2 volts from new cells
which will drive the LED to about 8ma, but when
the cells voltage drops to about 2.9 or so you'll
only get about 2ma.<p>To get the full 30ma you'll need three cells
in series with a dropping resistor or a switching
boost circuit.<p>With three cells in series the typical dropping
resistor value is about 33 ohms down to as low
as 22 ohms.
Some comercial flashlights drive the LEDs much
higher but they get hot and it reduces their life.<p>
Take care,
Al

[k7elp60]

I have a pair of white LED's connected in parallel running directly off a pair of AA cells.
They work fine. Looking thru the lastest Mouser catalogue I fine the typical Vf for white LED's to be around 3.5 volts.<p>As a side note I have hooked dozens of LED's in parallel and ran them from a fixed voltage source. Even had red, yellow and green in parallel at the same time.<p>I once wired over 150 red LED's in parallel to light up a horse for a parade. Ran the LED's off of 2 battery packs consisting of C cells in series. Had a dropping element to drop the 3V to approximately 2V. The battery packs ran the LED's for several hours.

[cato]

What the heck is a White LED Driver? You gots a part number or a spec sheet link?

[Chris Smith]

White Led drivers are pulsed drivers that make the betteries last longer, and the lights burn brighter.

[Mike]

OK, the reason I asked was since I've seen those White LED drivers, so I thought that I may need one. What I want to do is make a nice indicator for a sub i'm building.<p>If I were to regulate the power down to 5VDC, what size resistor would I need for a 30ma with max 4.5volts I think, but I am not sure.

[Chris Smith]

Voltage in - V of the Led<p>Resistor Value = _____________________
Amperage of the Led<p>So you need to know what the running or forward voltage of the LED is. [4.5v?] <p>And then it will be Resistor = 5 volts - 4.5v?
Over .03 amps = 16 ohms. <p>Assuming the 4.5 volts is your [V_led]

[Mike]

OK, the LED I have is an Agilent Technologies HLMP-CW16-VY000. The specs are here:<p>http://cp.literature.agilent.com/litweb ... 9537EN.pdf<p>DC foreward current: 30ma
peak foreward current: 100ma
reverse voltage: 5V<p>Those are the absolute max ratings.<p>My supply is 5V either 100ma or 1A depending on the chip I have to use (78l05 or 7805)<p>what resistor would I need for this?

[dyarker]

68 or 75 Ohm 1/4W resistor will do

[jwax]

Per the spec sheet, the LED wants 4 volts. You have 5, so one volt needs to be dropped at the resistor. At .03 amp, R=E/I, so 1/.03= 33 ohms. A 1/4 watt or so would suffice.

[dyarker]

Absolute maximum is 30mA. I figured on 20mA typical.

[jollyrgr]

Cato,<p>LED drivers either regulate current and voltage or step up lower voltages to higher voltages to drive the LEDs. Some driver circuits use pulses to increase the voltage and current well beyond the normal bias points to increase the apparent brightness. If you pulse an LED with a high voltage and high current very briefly you will not damage it. But you can get an apparent bright pulse. Do it very fast and it looks like the LED is constantly lit.<p>Back a few months ago there was this thread:<p>http://206.131.241.58/ubb/ultimatebb.ph ... 1&t=001481<p>Check out this circuit:<p>http://www.elecdesign.com/Files/29/5886/Figure_01.gif<p>
I have been able to power white LEDs using two AA batteries. It works just fine but is no where near as bright when powered by three batteries. And I did not use a bias resistor. In other words the LEDs were connected directly to the batteries. Another thing I have done is charge 1 Farad memory capacitors with a hand crank cell phone charger to make a self powered/self charge flashlight. By cranking the handle I charge the capacitor and the charged cap will power the LED for about five minutes of useful light. The first minute or so is very bright.<p>[ September 04, 2004: Message edited by: Jolly Roger ]</p>

[cato]

Thanks Jolly